Tag Archives: High Energy Physics – Theory

Gravitational lens on an optical constant-curvature background: Its application to Weyl gravity model [CL]

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http://arxiv.org/abs/2304.02219


We describe the gravitational lens on a constant-curvature background by using an optical constant-curvature (OCC) approach that allows to explicitly take into account a global geometry of the background space. First, light rays are curves in the space described by an optical metric. The OCC approach focuses on the case that the optical metric for the background spacetime has a constant curvature, for which the exact lens equation on an OCC background [Phys. Rev. D 105, 084022 (2022)] can be used. As a concrete example, next we discuss the gravitational lens in Mannheim-Kazanas (MK) solution, which include Rindler and de Sitter terms. By fully taking into account a background dependence of the light deflection, the deflection angle of light consistent with the OCC approach is well defined at large distance. In the OCC approach, finally, we examine the global behavior of the deflection angle and the gravitational lens observables in the Weyl gravity.

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K. Takizawa and H. Asada
Thu, 6 Apr 23
52/76

Comments: 11 pages, 4 figures

Causality bounds on scalar-tensor EFTs [CL]

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http://arxiv.org/abs/2304.01259


We compute the causality/positivity bounds on the Wilson coefficients of scalar-tensor effective field theories. Two-sided bounds are obtained by extracting IR information from UV physics via dispersion relations of scattering amplitudes, making use of the full crossing symmetry. The graviton $t$-channel pole is carefully treated in the numerical optimization, taking into account the constraints with fixed impact parameters. It is shown that the typical sizes of the Wilson coefficients can be estimated by simply inspecting the dispersion relations. We carve out sharp bounds on the leading coefficients, particularly, the scalar-Gauss-Bonnet couplings, and discuss how some bounds vary with the leading $(\partial\phi)^4$ coefficient and as well as phenomenological implications of the causality bounds.

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D. Hong, Z. Wang and S. Zhou
Thu, 6 Apr 23
53/76

Comments: 72 pages, 15 figures

Borel resummation of secular divergences in stochastic inflation [CL]

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http://arxiv.org/abs/2304.02592


We make use of Borel resummation to extract the exact time dependence from the divergent series found in the context of stochastic inflation. Correlation functions of self-interacting scalar fields in de Sitter spacetime are known to develop secular IR divergences via loops, and the first terms of the divergent series have been consistently computed both with standard techniques for curved spacetime quantum field theory and within the framework of stochastic inflation. We show that Borel resummation can be used to interpret the divergent series and to correctly infer the time evolution of the correlation functions. In practice, we adopt a method called Borel–Pad\'{e} resummation where we approximate the Borel transformation by a Pad\'{e} approximant. We also discuss the singularity structures of Borel transformations and mention possible applications to cosmology.

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M. Honda, R. Jinno, L. Pinol, et. al.
Thu, 6 Apr 23
71/76

Comments: 40 pages, 12 figures

Oscillon formation during inflationary preheating with general relativity [CL]

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http://arxiv.org/abs/2304.01673


We study the non-perturbative evolution of inflationary fluctuations during preheating using fully non-linear general-relativistic field-theory simulations. We choose a single-field inflationary model that is consistent with observational constraints and start the simulations at the end of inflation with fluctuations both in the field and its conjugate momentum. Gravity enhances the growth of density perturbations, which then collapse and virialize, forming long-lived stable oscillon-like stars that reach compactnesses $\mathcal{C}\equiv GM/R \sim 10^{-3}-10^{-2}$. We find that $\mathcal{C}$ increases for larger field models, until it peaks due to the interplay between the overdensity growth and Hubble expansion rates. Whilst gravitational effects can play an important role in the formation of compact oscillons during preheating, the objects are unlikely to collapse into primordial black holes without an additional enhancement of the initial inflationary fluctuations.

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J. Aurrekoetxea, K. Clough and F. Muia
Wed, 5 Apr 23
14/62

Comments: 7 pages. 4 figures. Comments welcome! Movie: this https URL

Black hole superradiant instability for massive spin-2 fields [CL]

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http://arxiv.org/abs/2304.01265


Due to coherent superradiant amplification, massive bosonic fields can trigger an instability in spinning black holes, tapping their energy and angular momentum and forming macroscopic Bose-Einstein condensates around them. This phenomenon produces gaps in the mass-spin distribution of astrophysical black holes, a continuous gravitational-wave signal emitted by the condensate, and several environmental effects relevant for gravitational-wave astronomy and radio images of black holes. While the spectrum of superradiantly unstable mode is known in great detail for massive scalar (spin-0) and vector (spin-1) perturbations, so far only approximated results were derived for the case of massive tensor (spin-2) fields, due to the nonseparability of the field equations. Here, solving a system of ten elliptic partial differential equations, we close this program and compute the spectrum of the most unstable modes of a massive spin-2 field for generic black-hole spin and boson mass, beyond the hydrogenic approximation and including the unique dipole mode that dominates the instability in the spin-2 case. We find that the instability timescale for this mode is orders of magnitude shorter than for any other superradiant mode, yielding much stronger constraints on massive spin-2 fields. These results pave the way for phenomenological studies aimed at constraining beyond Standard Model scenarios, ultralight dark matter candidates, and extensions to General Relativity using gravitational-wave and electromagnetic observations, and have implications for the phase diagram of vacuum solutions of higher-dimensional gravity.

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O. Dias, G. Lingetti, P. Pani, et. al.
Wed, 5 Apr 23
24/62

Comments: 5+1 pages, 2+1 figures

The dipolar death of massive gravity [CL]

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http://arxiv.org/abs/2304.01252


We study extreme-mass-ratio systems in theories admitting the Schwarzschild solution and propagating a massive graviton. We show that, in addition to small corrections to the quadrupolar and higher-order modes, a dipolar mode is excited in these theories and we quantify its excitation. While LIGO-Virgo-KAGRA observations are not expected to impose meaningful constraints in the dipolar sector, future observations by the Einstein Telescope or by LISA, together with bounds from dispersion relations, can rule out theories of massive gravity admitting vacuum General Relativistic backgrounds. For the bound to be circumvented, one needs to move away from Ricci-flat solutions, and enter a territory where constraints based on wave propagation and dispersion relations are not reliable.

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V. Cardoso, F. Duque, A. Maselli, et. al.
Wed, 5 Apr 23
48/62

Comments: 4 pages plus Supplemental Material

Stochastic Multiple Fields Inflation: Diffusion Dominated Regime [CEA]

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http://arxiv.org/abs/2304.00577


We study multiple fields inflation in diffusion dominated regime using stochastic $\delta N$ formalism. The fields are under pure Brownian motion in a dS background with boundaries in higher dimensional field space. This setup can be realized towards the final stages of the ultra slow-roll setup where the classical drifts fall off exponentially and the perturbations are driven by quantum kicks. We consider both symmetric and asymmetric boundaries with absorbing and reflective boundary conditions and calculate the average number of e-folds, the first crossing probabilities and the power spectrum. We study the primordial black holes (PBHs) formation in this setup and calculate the mass fraction and the contribution of PBHs in dark matter energy density for various higher dimensional field spaces.

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K. Asadi, A. Nassiri-Rad and H. Firouzjahi
Tue, 4 Apr 23
34/111

Comments: 40 pages, 15 figures

Primordial Stochastic Gravitational Wave Backgrounds from a Sharp Feature in Three-field Inflation [CEA]

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http://arxiv.org/abs/2304.00065


The detection of a primordial stochastic gravitational wave background has the potential to reveal unprecedented insights into the early universe, and possibly into the dynamics of inflation. Generically, UV-complete inflationary models predict an abundance of light scalars, so any inflationary stochastic background may well be formed in a model with several interacting degrees of freedom. The stochastic backgrounds possible from two-field inflation have been well-studied in the literature, but it is unclear how similar they are to the possibilities from many-field inflation. In this work, we study stochastic backgrounds from more-than-two field inflation for the first time, focusing on the scalar-induced background produced by a brief turn in three-field space. We find an analytic expression for the enhancement in the power spectrum as a function of the turn rate and the torsion, and show that unique signatures of three-field dynamics are possible in the primordial power spectrum and gravitational wave spectrum. We confirm our analytic results with a suite of numerical simulations and find good agreement in the shape and amplitude of the power spectra. We also comment on the detection prospects in LISA and other future detectors. We do not expect the moderately large growth of the inflationary perturbations necessary for detection to cause a breakdown of perturbation theory, but this must be verified on a case-by-case basis for specific microphysical models to make a definitive claim.

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V. Aragam, S. Paban and R. Rosati
Tue, 4 Apr 23
35/111

Comments: 28 pages, 7 figures

Spontaneous Human Combustion rules out all standard candidates for Dark Matter [CL]

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http://arxiv.org/abs/2304.00319


We argue that the reported cases of Spontaneous Human Combustion (SHC) are most likely due to the impact of the human body with an extremely high energy particle like cosmic rays or Dark Matter. Normal and antimatter cosmic rays and classical weakly-interacting massive particles (WIMPs) with energies of GeV to ZeV can be easily ruled out due to their inability to dump enough energy into a small region of human tissue, leaving as the single remaining candidate massive Dark Matter particles. While primordial Black Holes would appear to be very good candidates for inducing the SHC phenomenon, we show that the estimated local Dark Matter density requires that the particles have masses of $\sim 10$\,kg, clearly ruling out this candidate. All of the other classic DM candidates — from scalar and pseudo-scalar spin 1/2 and spin 2 gauge singlets to nuclearitic strange quark “bowling balls” — can be ruled out. Axions tailored to solve the CP-problem also cannot be invoked, no matter what mass is considered. The only particles left are massive mega-axions (MaMAs), for which there are an infinite number of possible string models.

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F. Hessman and J. Wheeler
Tue, 4 Apr 23
88/111

Comments: N/A

Topological densities in Einstein-scalar-Gauss-Bonnet gravity [CL]

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http://arxiv.org/abs/2304.00322


The present work is devoted to studying the background dynamical evolution of a scalar field in Einstein-Gauss-Bonnet gravity in maximally symmetric space-time. This study is useful for giving meaning to the presence of two Gauss-Bonnet vacua, instead of using the spherically symmetric bubbles of the “true” vacuum expand in the “false” vacuum. The theory admits two possible effective cosmological constants, which lead to two maximally symmetric vacuum solutions. The first solution corresponds to the dynamics of dark energy. When there is matter, the second solution describes dark matter. In Einstein-Gauss-Bonnet gravity, we establish the expression of the topological mass spectrum which depends on the golden ratio and its inverse. In the Schwarzschild limit, the topological density corresponds to the standard model radiation energy density. We find the mass loss rate which gives the evolution of mass over time.

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M. Bousder and Z. Sakhi
Tue, 4 Apr 23
96/111

Comments: 19 pages, 1 figure

Energy flow in Ultra High Energy Cosmic Ray interactions as a probe of thermalization and potential solution to the Muon puzzle [CL]

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http://arxiv.org/abs/2304.00294


Indicators that illustrate the formation of a strongly interacting thermalized matter of partons have been observed in high-multiplicity proton-proton, proton-nucleus, and nucleus-nucleus collisions at RHIC and LHC energies. Strangeness enhancement in such ultra-relativistic heavy-ion collisions is considered to be a consequence of this thermalized phase, known as quark-gluon plasma (QGP). Simultaneously, proper modeling of hadronic energy fraction in interactions of ultra-high energy cosmic rays (UHECR) has been proposed as a solution for the muon puzzle. These interactions have center-of-mass collision energies in the order of LHC or higher, indicating that the possibility of a thermalized partonic state cannot be overlooked in UHECR-air interactions. This work investigates the hadronic energy fraction and strangeness enhancement to explore QGP-like phenomena in UHECR-air interactions using various high-energy hadronic models. A thermalized system with statistical hadronization is considered through the EPOS LHC model, while PYTHIA 8, QGSJET II-04, and SYBILL 2.3d consider string fragmentation in the absence of any thermalization. We have found that EPOS LHC gives a better description of strangeness enhancement as compared to other models. We conclude that adequately treating all the relevant effects and further retuning the models is necessary to explain the observed effects.

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R. Scaria, S. Deb, C. Singh, et. al.
Tue, 4 Apr 23
109/111

Comments: 7 pages and 6 figures. Submitted for publication

Scale-Invariant Model for Gravitational Waves and Dark Matter [CEA]

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http://arxiv.org/abs/2303.18122


We have conducted a revised analysis of the first-order phase transition that is associated with symmetry breaking in a classically scale-invariant model that has been extended with a new $SU(2)$ gauge group. By incorporating recent developments in the understanding of supercooled phase transitions, we were able to calculate all of its features and significantly limit the parameter space. We were also able to predict the gravitational wave spectra generated during this phase transition and found that this model is well-testable with LISA. Additionally, we have made predictions regarding the relic dark matter abundance. Our predictions are consistent with observations but only within a narrow part of the parameter space. We have placed significant constraints on the supercool dark matter scenario by improving the description of percolation and reheating after the phase transition, as well as including the running of couplings. Finally, we have also analyzed the renormalization-scale dependence of our results.

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A. Karam, M. Kierkla and B. Świeżewska
Mon, 3 Apr 23
13/53

Comments: 18+9 pages, 9 figures, invited talk given at CORFU2022, summarizing the results of arXiv:2210.07075

The cosmological constant is probably still zero [CL]

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http://arxiv.org/abs/2303.17723


We consider a wide class of four-dimensional effective field theories in which gravity is coupled to multiple four-forms and their dual scalar fields, with membrane sources charged under the corresponding three-form potentials. Four-form flux, quantised in units of the membrane charges, generically generates a landscape of vacua with a range of values for the cosmological constant that is scanned through membrane nucleation. We list various ways in which the landscape can be made sufficiently dense to be compatible with observations of the current vacuum without running into the empty universe problem. Further, we establish the general criteria required to ensure the absolute stability of the Minkowski vacuum under membrane nucleation and the longevity of those vacua that are parametrically close by. This selects the current vacuum on probabilistic grounds and can even be applied in the classic model of Bousso and Polchinski, albeit with some mild violation of the membrane weak gravity conjecture. We present other models where the membrane weak gravity conjecture is not violated but where the same probabilistic methods can be used to tackle the cosmological constant problem.

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Y. Liu, A. Padilla and F. Pedro
Mon, 3 Apr 23
24/53

Comments: 29 pages

What can a detected photon with a given gravitational redshift tell us about the maximum density of a compact star? [CL]

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http://arxiv.org/abs/2303.17630


Far away observers can in principle bound from below the dimensionless maximum-density parameter $\Lambda\equiv4\pi R^2\rho_{\text{max}}$ of a compact star by measuring the gravitational redshift factor $z\equiv\nu_{\text{e}}/\nu_{\infty}-1$ of photons that were emitted from the {\it surface} of the star: $\Lambda\geq{3\over2}[1-(1+z)^{-2}]$ [here $R$ is the radius of the star and ${\nu_{\text{e}},\nu_{\infty}}$ are respectively the frequency of the emitted light as measured at the location of the emission and by asymptotic observers]. However, if photons that were created somewhere {\it inside} the star can make their way out and reach the asymptotic observers, then the measured redshift parameter $z$ may not determine uniquely the surface properties of the star, thus making the above bound unreliable. In the present compact paper we prove that in these cases, in which the creation depth of a detected photon is not known to the far away observers, the empirically measured redshift parameter can still be used to set a (weaker) lower bound on the dimensionless density parameter of the observed star: $\Lambda\geq{3\over2}[1-(1+z)^{-2/3}]$.

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S. Hod
Mon, 3 Apr 23
50/53

Comments: 5 pages

Electromagnetic fields in compact binaries: a post-Newtonian approach [CL]

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http://arxiv.org/abs/2303.17536


Galactic binaries, and notably double white dwarfs systems, will be a prominent source for the future LISA and Einstein Telescope detectors. Contrarily to the black holes observed by the current LIGO-Virgo-KAGRA network, such objects bear intense magnetic fields, that are naturally expected to leave some imprints on the gravitational wave emission. The purpose of this work is thus to study those imprints within the post-Newtonian (PN) framework, particularly adapted to double white dwarfs systems. To this end, we construct an effective action that takes into account the whole electromagnetic structure of a star, and then specify it to dipolar order. With this action at hand, we compute the acceleration and Noetherian quantities for generic electric and magnetic dipoles, at a relative 2PN order. Finally, focusing on physically relevant systems, we show that the magnetic effects on the orbital frequency, energy and angular momentum is significant, confirming previous works conclusions.

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Q. Henry, F. Larrouturou and C. Poncin-Lafitte
Fri, 31 Mar 23
11/70

Comments: 23 pages, no figure, supplementary material attached

Charged Gauss-Bonnet black holes supporting non-minimally coupled scalar clouds: Analytic treatment in the near-critical regime [CL]

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http://arxiv.org/abs/2303.16926


Recent numerical studies have revealed the physically intriguing fact that charged black holes whose charge-to-mass ratios are larger than the critical value $(Q/M){\text{crit}}=\sqrt{2(9+\sqrt{6})}/5$ can support hairy matter configurations which are made of scalar fields with a non-minimal negative coupling to the Gauss-Bonnet invariant of the curved spacetime. Using {\it analytical} techniques, we explore the physical and mathematical properties of the composed charged-black-hole-nonminimally-coupled-linearized-massless-scalar-field configurations in the near-critical $Q/M\gtrsim (Q/M){\text{crit}}$ regime. In particular, we derive an analytical resonance formula that describes the charge-dependence of the dimensionless coupling parameter $\bar\eta_{\text{crit}}=\bar\eta_{\text{crit}}(Q/M)$ of the composed Einstein-Maxwell-nonminimally-coupled-scalar-field system along the {\it existence-line} of the theory, a critical border that separates bald Reissner-Nordstr\”om black holes from hairy charged-black-hole-scalar-field configurations. In addition, it is explicitly shown that the large-coupling $-\bar\eta_{\text{crit}}(Q/M)\gg1$ analytical results derived in the present paper for the composed Einstein-Maxwell-scalar theory agree remarkably well with direct numerical computations of the corresponding black-hole-field resonance spectrum.

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S. Hod
Fri, 31 Mar 23
48/70

Comments: 8 pages

Primordial black holes and stochastic inflation beyond slow roll: I — noise matrix elements [CEA]

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http://arxiv.org/abs/2303.17375


Primordial Black Holes (PBHs) may form in the early Universe, from the gravitational collapse of large density perturbations, generated by large quantum fluctuations during inflation. Since PBHs form from rare over-densities, their abundance is sensitive to the tail of the primordial probability distribution function (PDF) of the perturbations. It is therefore important to calculate the full PDF of the perturbations, which can be done non-perturbatively using the ‘stochastic inflation’ framework. In single field inflation models generating large enough perturbations to produce an interesting abundance of PBHs requires violation of slow roll. It is therefore necessary to extend the stochastic inflation formalism beyond slow roll. A crucial ingredient for this are the stochastic noise matrix elements of the inflaton potential. We carry out analytical and numerical calculations of these matrix elements for a potential with a feature which violates slow roll and produces large, potentially PBH generating, perturbations. We find that the transition to an ultra slow-roll phase results in the momentum induced noise terms becoming larger than the field noise whilst each of them falls exponentially for a few e-folds. The noise terms then start rising with their original order restored, before approaching constant values which depend on the nature of the slow roll parameters in the post transition epoch. This will significantly impact the quantum diffusion of the coarse-grained inflaton field, and hence the PDF of the perturbations and the PBH mass fraction.

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S. Mishra, E. Copeland and A. Green
Fri, 31 Mar 23
53/70

Comments: 47 pages, 8 figures

Exact solutions and cosmological constraints in fractional cosmology [CL]

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http://arxiv.org/abs/2303.16409


This paper investigates exact solutions of cosmological interest in fractional cosmology. Given $\mu$, the order of the fractional derivative, and $w$, the matter equation of state, we present particular exact power-law solutions. We discuss the exact general solution of the system obtained by solving a Riccati Equation, where the solution for the scale factor is a combination of power-law. Using cosmological data, we estimate the free parameters $(\alpha_0, \mu)$, where $H_{0}=100\frac{\text{km/s}}{\text{Mpc}}h$, and $\alpha_0:=t_0 H_0 = \frac{1}{6} \left(9 -2 \mu +\sqrt{8 \mu (2 \mu -9)+105}\right)(1+ 2 \epsilon_0)$, is the current age parameter. The joint analysis with data from SNe Ia + OHD leads to $h=0.684_{-0.027}^{+0.031}$, $\mu=1.840_{-0.773}^{+1.446}$ and $\epsilon_0=\left(1.213_{-1.057}^{+0.482}\right)\times 10^{-2}$, where the best-fit values are calculated at $3\sigma$ CL. On the other hand, these best-fit values lead to an age of the Universe with a value of $t_0=\alpha_0/H_0=25.62_{-4.46}^{+6.89}\;\text{Gyrs}$, a current deceleration parameter of $q_{0}=-0.37_{-0.11}^{+0.08}$, both at $3\sigma$ CL, and a current matter density parameter of $\Omega_{m,0}=0.531_{-0.260}^{+0.195}$ at $1\sigma$ CL. Finding a Universe roughly twice older as the one of $\Lambda$CDM is a distinction of Fractional Cosmology. Focusing our analysis on these results, we can conclude that the region in which $\mu>2$ is not ruled out by observations. This region of a parameter is relevant because, in the absence of matter, fractional cosmology gives a power-law solution $a(t)= \left(t/t_0\right)^{\mu-1}$, which is accelerated for $\mu>2$. We present a fractional origin model that leads to an accelerated state without appealing to $\Lambda$ or Dark Energy.

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E. González, G. Leon and G. Fernandez-Anaya
Thu, 30 Mar 23
21/66

Comments: 51 pages, 10 figures

Scalar induced gravitational waves in modified teleparallel gravity theories [CL]

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http://arxiv.org/abs/2303.16695


Primordial black holes (PBHs) forming out of the collapse of enhanced cosmological perturbations provide access to the early Universe through their associated observational signatures. In particular, enhanced cosmological perturbations collapsing to form PBHs are responsible for the generation of a stochastic gravitational-wave background (SGWB) induced by second-order gravitational interactions, usually called scalar induced gravitational waves (SIGWs). This SGWB is sensitive to the underlying gravitational theory; hence it can be used as a novel tool to test the standard paradigm of gravity and constrain possible deviations from general relativity. In this work, we study the aforementioned GW signal within modified teleparallel gravity theories, developing a formalism for the derivation of the GW spectral abundance within any form of gravitational action. At the end, working within viable $f(T,\phi)$ models without matter-gravity couplings, and accounting for the effect of mono-parametric $f(T)$ gravity at the level of the source and the propagation of the tensor perturbations, we show that the respective GW signal is indistinguishable from that within GR. Interestingly, we find that in order to break the degeneracy between different $f(T)$ theories through the portal of SIGWs one should necessarily consider non-minimal matter-gravity couplings at the level of the gravitational action.

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C. Tzerefos, T. Papanikolaou, E. Saridakis, et. al.
Thu, 30 Mar 23
64/66

Comments: 16 pages without appendices (24 in total), 2 figures

Synchronous coordinates and gauge-invariant observables in cosmological spacetimes [CL]

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http://arxiv.org/abs/2303.16218


We consider the relational approach to construct gauge-invariant observables in cosmological perturbation theory using synchronous coordinates. We construct dynamical synchronous coordinates as non-local scalar functionals of the metric perturbation in the fully non-linear theory in an arbitrary gauge. We show that the observables defined in this dynamical coordinate system are gauge-independent, and that the full perturbed metric has the expected form in these coordinates. Our construction generalises the familiar synchronous gauge in linearised gravity, widely used in cosmological perturbation theory, to the non-linear theory. We also work out the expressions for the gauge-invariant Einstein equations, sourced either by an ideal fluid or a scalar field up to second order in perturbation theory, and give explicit expressions for the Hubble rate — as measured by synchronous observers or by observers co-moving with the matter field — up to that order. Finally, we consider quantised linear perturbations around Minkowski and de Sitter backgrounds, and compute the two-point function of the gauge-invariant metric perturbation in synchronous coordinates, starting with two-point function in a general linear covariant gauge. Although the gauge-fixed two-point function contains gauge modes, we show that the resulting gauge-invariant two-point function only contains the physical tensor modes and it is thus positive, i.e., it has a spectral representation.

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M. Fröb and W. Lima
Thu, 30 Mar 23
66/66

Comments: 52 pages. Comments are welcome

Squeezed bispectrum and one-loop corrections in transient constant-roll inflation [CEA]

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http://arxiv.org/abs/2303.16035


In canonical single-field inflation, the production of primordial black holes (PBH) requires a transient violation of the slow-roll condition. The transient ultra slow-roll inflation is an example of such scenario, and more generally, one can consider the transient constant-roll inflation. We investigate the squeezed bispectrum in the transient constant-roll inflation, and find that the Maldacena’s consistency relation holds for a sufficiently long-wavelength mode, whereas it is violated for modes around the peak scale for the non-attractor case. We also demonstrate how the one-loop corrections are modified compared to the case of the transient ultra slow-roll inflation, focusing on representative one-loop terms orgiginating from a time derivative of the second slow-roll parameter in the cubic action. We find that the perturbativity requirement on those terms does not rule out the production of PBH from the transient constant-roll inflation. Therefore, it is a simple counterexample of the recently claimed no-go theorem of PBH production from single-field inflation.

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H. Motohashi and Y. Tada
Wed, 29 Mar 23
46/73

Comments: 26 pages, 14 figures

Scalar Love numbers and Love symmetries of 5-dimensional Myers-Perry black hole [CL]

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http://arxiv.org/abs/2303.16036


The near-zone “Love” symmetry resolves the naturalness issue of black hole Love number vanishing with $\text{SL}\left(2,\mathbb{R}\right)$ representation theory. Here, we generalize this proposal to $5$-dimensional asymptotically flat and doubly spinning (Myers-Perry) black holes. We consider the scalar response of Myers-Perry black holes and extract its static scalar Love numbers. In agreement with the naturalness arguments, these Love numbers are, in general, non-zero and exhibit logarithmic running unless certain resonant conditions are met; these conditions include new cases with no previously known analogs. We show that there exist two near-zone truncations of the equations of motion that exhibit enhanced $\text{SL}\left(2,\mathbb{R}\right)$ Love symmetries that explain the vanishing of the static scalar Love numbers in the resonant cases. These Love symmetries can be interpreted as local $\text{SL}\left(2,\mathbb{R}\right)\times\text{SL}\left(2,\mathbb{R}\right)$ near-zone symmetries spontaneously broken down to global $\text{SL}\left(2,\mathbb{R}\right)\times U\left(1\right)$ symmetries by the periodic identification of the azimuthal angles. We also discover an infinite-dimensional extension of the Love symmetry into $\text{SL}\left(2,\mathbb{R}\right)\ltimes\hat{U}\left(1\right)_{\mathcal{V}}^2$ that contains both Love symmetries as particular subalgebras, along with a family of $\text{SL}\left(2,\mathbb{R}\right)$ subalgebras that reduce to the exact near-horizon Myers-Perry black hole isometries in the extremal limit. Finally, we show that the Love symmetries acquire a geometric interpretation as isometries of subtracted (effective) black hole geometries that preserve the internal structure of the black hole and interpret these non-extremal $\text{SL}\left(2,\mathbb{R}\right)$ structures as remnants of the enhanced isometry of the near-horizon extremal geometries.

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P. Charalambous and M. Ivanov
Wed, 29 Mar 23
48/73

Comments: 45+16 pages, 3 Figures

Shadows in dyonic Kerr-Sen black holes [CL]

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http://arxiv.org/abs/2303.14513


Black holes with dyonic charges in Einstein-Maxwell-dilaton-axion supergravity theory are revisited in the context of black hole shadows. We consider static as well as rotating (namely the dyonic Kerr-Sen) black holes. The matter stress-energy tensor components, sourced by the Maxwell, axion and dilaton fields satisfy the standard energy conditions. The analytical expressions for the horizon and the shadow radius of the static spacetimes demonstrate their dependence on $P^2+Q^2$ ($P$, $Q$ the magnetic and electric charges, respectively) and the mass parameter $M$. The shadow radius lies in the range $2M <R_{shadow}<3\sqrt{3} M$ and there is no stable photon orbit outside the horizon. Further, shadows cast by the rotating dyonic Kerr-Sen black holes are also studied and compared graphically with their Kerr-Newman and Kerr-Sen counterparts. Deviation of the shadow boundary is prominent with the variation of the magnetic charge, for the relatively slowly rotating dyonic Kerr-Sen spacetimes. We test any possible presence of a magnetic monopole charge in the backdrop of recent EHT observations for the supermassive black holes M87$^$ and Sgr A$^$. Deviation from circularity of the shadow boundary ($\Delta C$) and deviation of the average shadow radius from the Schwarzschild shadow radius (quantified as the fractional deviation parameter $\delta$) are the two observables used here. Observational bound on $\Delta C$ (available only for M87$^$) is satisfied for all theoretically allowed regions of parameter space and thus cannot constrain the parameters. The observational bound on $\delta$ available for Sgr A$^$ translates into an upper limit on any possible magnetic monopole charge linked to Sgr A$^*$ and is given as $P\lesssim 0.873\, M$. Such a constraint on $P$ is however expected to be far more stringent for other astrophysical tests.

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S. Jana and S. Kar
Tue, 28 Mar 23
2/81

Comments: 23 pages, 8 figures

Swampland Criteria and Constraints on Inflation in a $f(R,T)$ Gravity Theory [CL]

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http://arxiv.org/abs/2303.14724


In this paper, we worked in the framework of an inflationary $f(R,T)$ theory, in the presence of a canonical scalar field. More specifically, the $f(R,T)=\gamma R+2\kappa\alpha T$ gravity. The values of the dimensionless parameters $\alpha$ and $\gamma$ are taken to be $\alpha \geq 0$ and $0 < \gamma \leq 1$. The motivation for that study was the striking similarities between the slow-roll parameters of the inflationary model used in this work and the ones obtained by the rescaled Einstein-Hilbert gravity inflation $f(R)=\alpha R$. We examined a variety of potentials to determine if they agree with the current Planck Constraints. In addition, we checked whether these models satisfy the Swampland Criteria and we specified the exact region of the parameter space that produces viable results for each model. As we mention in Section IV the inflationary $f(R,T)$ theory used in this work can not produce a positive $n_T$ which implies that the stochastic gravitational wave background will not be detectable.

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V. Oikonomou, K. Revis, I. Papadimitriou, et. al.
Tue, 28 Mar 23
22/81

Comments: IJMPD Accepted

Decay of ALP Condensates via Gravitation-Induced Resonance [CL]

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http://arxiv.org/abs/2303.14800


Oscillating scalar field condensates induce small amplitude oscillations of the Hubble parameter which can induce a decay of the condensate due to a parametric resonance instability [1]. We show that this instability can lead to the decay of the coherence of the condensate of axion-like particle (ALP) fields during the radiation phase of standard cosmology for rather generic ALP parameter values, with possible implications for certain experiments aiming to search for ALP candidates. As an example, we study the application of this instability to the QCD axion. We also study the magnitude of the induced entropy fluctuations.

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R. Brandenberger, V. Kamali and R. Ramos
Tue, 28 Mar 23
37/81

Comments: 9 pages, 2 figures

Inflationary Dynamics and Swampland Criteria for Modified Gauss-Bonnet Gravity Compatible with GW170817 [CL]

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http://arxiv.org/abs/2303.14594


In this article we present an alternative formalism for the inflationary phenomenology of rescaled Einstein-Gauss-Bonnet models which are in agreement with the GW170817 event. By constraining the propagation velocity of primordial tensor perturbations, an approximate form for the time derivative of the scalar field coupled to the Gauss-Bonnet density is extracted. In turn, the overall degrees of freedom decrease and similar to the case of the canonical scalar field, only one scalar function needs to be designated, while the other is extracted from the continuity equation of the scalar field. We showcase explicitly that the slow-roll indices can be written in a closed form as functions of three dimensionless parameters, namely $x=\frac{1}{2\alpha}\bigg(\frac{\kappa\xi’}{\xi”}\bigg)^2$, $\beta=8H^2\xi”$ and $\gamma=\frac{\xi’\xi”’}{\xi”^2}$ and in turn, we prove that the Einstein-Gauss-Bonnet model can in fact produce a blue-tilted tensor spectral index if the condition $\beta\geq1$ is satisfied, which is possible only for Einstein-Gauss-Bonnet models with $\xi”(\phi_k)>0$. Afterwards, a brief comment on the running of the spectral indices is made where it is shown that $a_{\mathcal{S}}(k_)$ and $a_{\mathcal{T}}(k_)$ in the constrained case are approximately of the order $\mathcal{O}(10^{-3})$, if not smaller. Last but not least, we examine the conditions under which the Swampland criteria are satisfied. We connect the tracking condition related to scalar field theories with the present models, and we highlight the important feature of the models we propose that the tracking condition can be satisfied only if the Swampland criteria are simultaneously satisfied, however the cases with $\xi\sim1/V$ and $\xi\sim V$ are excluded, as they cannot describe the inflationary era properly.

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S. Odintsov, V. Oikonomou and F. Fronimos
Tue, 28 Mar 23
41/81

Comments: PRD Accepted, abstract reduced due to arXiv limitations

Constraint on Early Dark Energy from Isotropic Cosmic Birefringence [CEA]

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http://arxiv.org/abs/2303.15369


Polarization of the cosmic microwave background (CMB) is sensitive to new physics violating parity symmetry, such as the presence of a pseudoscalar “axionlike” field. Such a field may be responsible for early dark energy (EDE), which is active prior to recombination and provides a solution to the so-called Hubble tension. The EDE field coupled to photons in a parity-violating manner would rotate the plane of linear polarization of the CMB and produce a cross-correlation power spectrum of $E$- and $B$-mode polarization fields with opposite parities. In this paper, we fit the $EB$ power spectrum predicted by the photon-axion coupling of the EDE model with a potential $V(\phi)\propto [1-\cos(\phi/f)]^3$ to polarization data from Planck. We find that the unique shape of the predicted $EB$ power spectrum is not favored by the data and obtain a first constraint on the photon-axion coupling constant, $g=(0.04\pm 0.16)M_{\text{Pl}}^{-1}$ (68% CL), for the EDE model that best fits the CMB and galaxy clustering data. This constraint is independent of the miscalibration of polarization angles of the instrument or the polarized Galactic foreground emission. Our limit on $g$ may have important implications for embedding EDE in fundamental physics, such as string theory.

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J. Eskilt, L. Herold, E. Komatsu, et. al.
Tue, 28 Mar 23
47/81

Comments: 7 pages, 3 figures, 1 table. The stacked EB power spectrum is publicly available at this https URL

Non-Gaussianity in rapid-turn multi-field inflation [CEA]

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http://arxiv.org/abs/2303.14156


We show that theories of inflation with multiple, rapidly turning fields can generate large amounts of non-Gaussianity. We consider a general theory with two fields, an arbitrary field-space metric, and a potential that supports sustained, rapidly turning field trajectories. Our analysis accounts for non-zero field cross-correlation and does not fix the power spectra of curvature and isocurvature perturbations to be equal at horizon crossing. Using the $\delta N$ formalism, we derive a novel, analytical formula for bispectrum generated from multi-field mixing on super-horizon scales. Rapid-turn inflation can produce a bispectrum with several potentially large contributions that are not necessarily of the local shape. We exemplify the applicability of our formula with a fully explicit model and show that the new contributions indeed can generate a large amplitude of local non-Gaussianity, $f_{\rm NL}^{\rm loc}\sim {\cal O}(1)$. These results will be important when interpreting the outcomes of future observations.

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O. Iarygina, M. Marsh and G. Salinas
Mon, 27 Mar 23
28/59

Comments: 35 pages, 6 figures

A test of Einstein's equivalence principle in ngEHT observations [CL]

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http://arxiv.org/abs/2303.13271


We show that Event Horizon Telescope (EHT) observations allow us to test the fundamental principles of General Relativity (GR). GR is based on the universality of gravity and Einstein’s equivalence principle (EEP). However, EEP is not a basic principle of physics but an empirical fact. Non-Minimal Coupling (NMC) of electromagnetic fields violates EEP, and their effects manifest in the strong-gravity regime. Hence, EHT provides an opportunity to test NMC in the strong-gravity regime. We show that, to the leading order in the spin parameter, NMC of the electromagnetic field modifies the black hole image in two ways: First, for one polarization mode, the horizon casts a shadow of radius \emph{greater than} $\sqrt{27} GM/c^2$ on the image of the source. For the other polarization mode, it is \emph{smaller than} $\sqrt{27} GM/c^2$. Second, the brightness and the position of the lensing ring are affected by the non-minimal coupling. The lensing ring is more prominent for one polarization mode than the other. Finally, we discuss the constraints on the NMC constant from future ngEHT observations.

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J. Johnson, S. Jana and S. Shankaranarayanan
Fri, 24 Mar 23
3/56

Comments: 18 pages, 4 figures

Tidal Peeling Events: low-eccentricity tidal disruption of a star by a stellar-mass black hole [HEAP]

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http://arxiv.org/abs/2303.12846


Close encounters between stellar-mass black holes (BHs) and stars occur frequently in dense star clusters and in the disks of active galactic nuclei (AGNs). Recent studies have shown that in highly eccentric close encounters, the star can be tidally disrupted by the BH (micro-tidal disruption event, or micro-TDE), resulting in rapid mass accretion and possibly bright electromagnetic signatures. Here we consider a scenario in which the star might approach the stellar-mass BH in a gradual, nearly circular inspiral, under the influence of dynamical friction on a circum-binary gas disk or three-body interactions in a star cluster. We perform hydro-dynamical simulations of this scenario using the smoothed particle hydrodynamics code PHANTOM. We find that the mass of the star is slowly stripped away by the BH. We call this gradual tidal disruption a “tidal-peeling event”, or a TPE. Depending on the initial distance and eccentricity of the encounter, TPEs might exhibit significant accretion rates and orbital evolution distinct from those of a typical (eccentric) micro-TDE.

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C. Xin, Z. Haiman, R. Perna, et. al.
Fri, 24 Mar 23
16/56

Comments: N/A

On the oscillations of the inflaton field of the simplest $α$-attractor T-model [CL]

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http://arxiv.org/abs/2303.13008


In this work, we consider homogeneous oscillations of the inflaton field after inflation. In particular, we obtain an analytical result for the (average) equation of state for the oscillating inflaton field for the simplest $\alpha$-attractor T-model. The result is useful for the study of its post-inflationary evolution. The most dramatic possibility is that during inflaton field oscillation, the (average) equation of state is that of a cosmological constant. This implies the end of slow-roll inflation in this model could be the beginning of oscillating inflation.

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C. Lin
Fri, 24 Mar 23
17/56

Comments: 9 pages, 3 figures

Scattering and conversion of electromagnetic and gravitational waves by Reissner-Nordström black holes: the Regge pole description [CL]

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http://arxiv.org/abs/2303.12656


We investigate the problem of scattering and conversion of monochromatic planar gravitational and electromagnetic waves impinging upon a Reissner-Nordstr\”om black hole using a Regge pole description, i.e., a complex angular momentum approach. For this purpose, we first compute numerically the Regge pole spectrum for various charge-to-mass ratio configurations. We then derive an asymptotic expressions for the lowest Regge poles, and by considering Bohr-Sommerfeld-type quantization conditions, obtain the spectrum of weakly damped quasinormal frequencies from the Regge trajectories. Next, we construct the scattering and conversion amplitudes as well as the total differential cross sections for different processes using both a complex angular momentum representation and a partial wave expansion method. Finally, we provide an analytical approximation of the scattering and conversion cross sections of different processes from asymptotic expressions for the lowest Regge poles and the associated residues based on the correspondence Regge poles, “surface waves” propagating close to the photon (graviton) sphere. This allows us to extract the physical interpretation encoded in the partial wave expansions in the high-frequency regime (i.e., in the short-wavelength regime), and to describe semiclassically with very good agreement both black hole glory and a large part of the orbiting oscillations, thus unifying these two phenomena from a purely wave point of view.

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M. Hadj
Thu, 23 Mar 23
12/67

Comments: N/A

Do the CMB Temperature Fluctuations Conserve Parity? [CEA]

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http://arxiv.org/abs/2303.12106


Observations of the Cosmic Microwave Background (CMB) have cemented the notion that the large-scale Universe is both statistically homogeneous and isotropic. But is it invariant also under mirror reflections? To probe this we require parity-sensitive statistics: for scalar observables, the simplest is the four-point function. We make the first measurements of the parity-odd CMB trispectrum, focusing on the large-scale ($2<\ell<510$) temperature anisotropies measured by Planck. This is facilitated by new maximum-likelihood estimators for binned correlators, which account for mask convolution and leakage between even- and odd-parity components, and achieve optimal variances within $\approx 20\%$. We perform a blind test for parity violation by comparing a $\chi^2$ statistic from Planck to theoretical expectations, using two suites of simulations to account for the possible likelihood non-Gaussianity and residual foregrounds. We find consistency at the $\approx 0.5\sigma$ level, yielding no evidence for parity violation, with roughly $250\times$ the squared sensitivity of large scale structure measurements (according to mode-counting arguments), and with the advantage of linear physics, Gaussian statistics, and accurate mocks. The measured trispectra can be used to constrain physical models of inflationary parity violation, including Ghost Inflation, Cosmological Collider scenarios, and Chern-Simons gauge fields. Considering eight such models, we find no evidence for new physics, with a maximal detection significance of $2.0\sigma$. These results suggest that the recent parity excesses seen in the BOSS galaxy survey are not primordial in origin. Tighter constraints can be wrought by including smaller scales (though rotational invariance washes out the flat-sky limit) and adding polarization data.

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O. Philcox
Thu, 23 Mar 23
16/67

Comments: 7+12 pages, 4+5 figures, submitted to Phys. Rev. Lett. Code available at this https URL

Impact of the Hubble tension on the $r$-$n_s$ contour [CEA]

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http://arxiv.org/abs/2303.12345


The injection of early dark energy (EDE) before the recombination, a possible resolution of the Hubble tension, will not only shift the scalar spectral index $n_s$ towards $n_s=1$, but also be likely to tighten the current upper limit on tensor-to-scalar ratio $r$. In this work, with the latest CMB datasets (Planck PR4, ACT, SPT and BICEP/Keck), as well as BAO and SN, we confirm this result, and discuss its implication on inflation. We also show that if we happen to live with EDE, how the different inflation models currently allowed would be distinguished by planned CMB observations, such as CMB-S4 and LiteBIRD.

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J. Jiang, G. Ye and Y. Piao
Thu, 23 Mar 23
44/67

Comments: 13 pages, 5 figures

Gauge invariance on the light-cone: curvature perturbations and radiative degrees of freedom [CL]

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http://arxiv.org/abs/2303.11743


We derive the expressions on the observed light-cone for some relevant cosmological gauge invariant variables, such as the Mukhanov-Sasaki variable and $E$- and $B$- modes of the tensor perturbations. Since the structure of the light-cone does not reflect in a direct way the FLRW symmetries, we develop a formalism which is coordinate independent and classifies the perturbations according to their helicities. Even though we work with linear perturbations, our formalism can be readily extended to non-linear theory and put the basis to study the evolution of cosmological perturbations, since the early- until the the late-time Universe, directly along the observed light-cone.

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G. Fanizza, G. Marozzi and M. Medeiros
Wed, 22 Mar 23
10/68

Comments: 34 pages, 1 table

One-loop Corrections in Power Spectrum in Single Field Inflation [CEA]

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http://arxiv.org/abs/2303.12025


We revisit the one-loop correction in curvature perturbation power spectrum in models of single field inflation which undergo a phase of ultra slow-roll (USR) inflation. We include the contributions from both the cubic and quartic interaction Hamiltonians and calculate the one-loop corrections on the spectrum of the CMB scale modes from the small scale modes which leave the horizon during the USR phase. It is shown that the amplitude of one-loop corrections depends on the sharpness of the transition from the USR phase to the final slow-roll phase. For an arbitrarily sharp transition, the one-loop correction becomes arbitrarily large, invalidating the perturbative treatment of the analysis. We speculate that for a mild transition, the large one-loop corrections are washed out during the subsequent evolution after the USR phase. The implications for primordial black holes formation are briefly reviewed.

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H. Firouzjahi
Wed, 22 Mar 23
14/68

Comments: 26 pages, 1 Figure

Relic gravitons and high-frequency detectors [CL]

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http://arxiv.org/abs/2303.11928


Cosmic gravitons are expected in the MHz-GHz regions that are currently unreachable by the operating wide-band interferometers and where various classes of electromechanical detectors have been proposed through the years. The minimal chirp amplitude detectable by these instruments is often set on the basis of the sensitivities reachable by the detectors currently operating in the audio band. By combining the observations of the pulsar timing arrays, the limits from wide-band detectors and the other phenomenological bounds we show that this requirement is far too generous and even misleading since the actual detection of relic gravitons well above the kHz would demand chirp and spectral amplitudes that are ten or even fifteen orders of magnitude smaller than the ones currently achievable in the audio band, for the same classes of stochastic sources. We then examine more closely the potential high-frequency signals and show that the sensitivity in the chirp and spectral amplitudes must be even smaller than the ones suggested by the direct and indirect constraints on the cosmic gravitons. We finally analyze the high-frequency detectors in the framework of Hanbury-Brown Twiss interferometry and argue that they are actually more essential than the ones operating in the audio band (i.e. between few Hz and few kHz) if we want to investigate the quantumness of the relic gravitons and their associated second-order correlation effects. We suggest, in particular, how the statistical properties of thermal and non-thermal gravitons can be distinguished by studying the corresponding second-order interference effects.

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M. Giovannini
Wed, 22 Mar 23
22/68

Comments: 42 pages, 13 figures

Bimetric-Affine Quadratic Gravity [CL]

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http://arxiv.org/abs/2303.11353


Bimetric gravity, is a theory of gravity that posits the existence of two interacting and dynamical metric tensors. The spectrum of bimetric gravity consists of a massless and a massive spin-2 particle. The form of the interactions between the two metrics $g_{\mu\nu}$ and $f_{\mu\nu}$ is constrained by requiring absence of the so called Boulware-Deser ghost. In this work we extend the original bimetric theory to its bimetric-affine counterpart, in which the associated two connections, $\Gamma_{ \mu\,\,\,\nu}^{\,\,\,\rho}(g)$ and $\widetilde{\Gamma}_{ \mu\,\,\,\nu}^{\,\,\,\rho}(f)$, are treated as independent variables. We examine in detail the case of an additional quadratic in the Ricci scalar curvature term $\mathcal{R}^2(g)$ and we find that this theory is free of ghosts for a wide range of the interaction parameters, not excluding the possibility of a Dark Matter interpretation of the massive spin-2 particle.

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I. Gialamas and K. Tamvakis
Wed, 22 Mar 23
41/68

Comments: 10 pages, 2 figures

Revisiting Puffy Dark Matter with Novel Insights: Partial Wave Analysis [CL]

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http://arxiv.org/abs/2303.11058


We present a comprehensive study on the self-interaction cross-section of puffy dark matter (DM) particles, which have a significant intrinsic size compared to their Compton wavelength. For such puffy DM self-interaction cross-section in the resonant and classical regimes, our study demonstrates the significance of the Yukawa potential and the necessity of partial wave analysis: (i) Due to the finite-size effect of puffy DM particles, the new Yukawa potential of puffy DM is found to enlarge the Born-effective regime for the self-interaction cross-section, compared with the point-like DM; (ii) Our partial wave analysis shows that depending on the value of the ratio between $R_{\chi}$ (radius of a puffy DM particle) and $1/m_{\phi}$ (force range), the three regimes (Born-effective, resonant and classical) for puffy DM self-interaction cross-section can be very different from the point-like DM; (iii) We find that to solve the small-scale anomalies via self-interacting puffy DM, the Born-effective and the resonant regimes exist for dwarf galaxies, while for the cluster and Milky Way galaxy the non-Born regime is necessary.

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W. Wang, W. Xu, J. Yang, et. al.
Tue, 21 Mar 23
35/68

Comments: 17page, 8figure

Compact stellar structures in Weyl geometric gravity [CL]

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http://arxiv.org/abs/2303.10339


We consider the structure and physical properties of specific classes of neutron, quark, and Bose-Einstein Condensate stars in the conformally invariant Weyl geometric gravity theory. The basic theory is derived from the simplest conformally invariant action, constructed, in Weyl geometry, from the square of the Weyl scalar, the strength of the Weyl vector, and a matter term, respectively. The action is linearized in the Weyl scalar by introducing an auxiliary scalar field. To keep the theory conformally invariant the trace condition is imposed on the matter energy-momentum tensor. The field equations are derived by varying the action with respect to the metric tensor, Weyl vector field and scalar field. By adopting a static spherically symmetric interior geometry, we obtain the field equations, describing the structure and properties of stellar objects in Weyl geometric gravity. The solutions of the field equations are obtained numerically, for different equations of state of the neutron and quark matter. More specifically, constant density stellar models, and models described by the stiff fluid, radiation fluid, quark bag model, and Bose-Einstein Condensate equations of state are explicitly constructed numerically in both general relativity and Weyl geometric gravity, thus allowing an in depth comparison between the predictions of these two gravitational theories. As a general result it turns out that for all the considered equations of state, Weyl geometric gravity stars are more massive than their general relativistic counterparts. As a possible astrophysical application of the obtained results we suggest that the recently observed neutron stars, with masses in the range of 2$M_{\odot}$ and 3$M_{\odot}$, respectively, could be in fact conformally invariant Weyl geometric neutron or quark stars.

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Z. Haghani and T. Harko
Tue, 21 Mar 23
36/68

Comments: 20 pages, 15 figures, accepted for publication in PRD

Primordial Gravitational Waves in non-Minimally Coupled Chromo-Natural Inflation [CEA]

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http://arxiv.org/abs/2303.10718


We consider inflation driven by an axion-like particle coupled to an SU(2) gauge sector via a Chern-Simons term. Known as chromo-natural inflation, this scenario is in tension with CMB observations. In order to remedy this fact and preserve both the symmetries and the intriguing gravitational wave phenomenology exhibited by the model, we explore the non-minimal coupling of the axion-inflaton to the Einstein tensor. We identify regions of parameter space corresponding to a viable cosmology at CMB scales. We also highlight the possibility of a large and chiral gravitational wave signal at small scales. This is of particular interest for gravitational wave interferometers.

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E. Dimastrogiovanni, M. Fasiello, M. Michelotti, et. al.
Tue, 21 Mar 23
64/68

Comments: 27 pages, 13 figures

Ultraviolet Sensitivity in Higgs-Starobinsky Inflation [CL]

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http://arxiv.org/abs/2303.09866


The general scalar-tensor theory that includes all the dimension-four terms has parameter regions that can produce successful inflation consistent with cosmological observations. This theory is in fact the same as the Higgs-Starobinsky inflation, when the scalar is identified with the Standard Model Higgs boson. We consider possible dimension-six operators constructed from non-derivative terms of the scalar field and the Ricci scalar as perturbations. We investigate how much suppression is required for these operators to avoid disrupting the successful inflationary predictions. To ensure viable cosmological predictions, the suppression scale for the sixth power of the scalar should be as high as the Planck scale. For the other terms, much smaller scales are sufficient.

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S. Lee, T. Modak, K. Oda, et. al.
Mon, 20 Mar 23
29/51

Comments: 23 pages, 12 figures

Holographic dark energy satisfying the energy conditions in Lovelock gravity [CL]

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http://arxiv.org/abs/2303.09563


In this paper, we show that the holographic dark energy density hides in the solutions of Lovelock gravity for black holes. Using the obtained mass and temperature we find density equations. We propose a physical interpretation of the rescaled Lovelock couplings as a topological mass that describes the Lovelock branch. In addition to this, we present new solutions that satisfy the energy conditions according to the Lovelock coupling and the horizon curvatures. This work can be extended to the equation of the state {\omega}{{\Lambda}} of dark energy in third-order Lovelock gravity. We show that the value “-1” represents a stable equilibrium of {\omega}{{\Lambda}}.

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M. Bousder, E. Salmani, A. Fatimy, et. al.
Mon, 20 Mar 23
34/51

Comments: 16 pages, 2 figures

Optimal Estimation of the Binned Mask-Free Power Spectrum, Bispectrum, and Trispectrum on the Full Sky [CEA]

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http://arxiv.org/abs/2303.08828


We derive optimal estimators for the two-, three-, and four-point correlators of statistically isotropic scalar fields defined on the sphere, such as the Cosmic Microwave Background temperature fluctuations, allowing for arbitrary (linear) masking and inpainting schemes. In each case, we give the optimal unwindowed estimator (obtained via a maximum-likelihood prescription, with an associated Fisher deconvolution matrix), and an idealized form, and pay close attention to their efficient computation. For the trispectrum, we include both parity-even and parity-odd contributions, as allowed by symmetry. The estimators can include arbitrary weighting of the data (and remain unbiased), but are shown to be optimal in the limit of inverse-covariance weighting and Gaussian statistics. The normalization of the estimators is computed via Monte Carlo methods, with the rate-limiting steps (involving spherical harmonic transforms) scaling linearly with the number of bins. An accompanying code package, PolyBin, implements these estimators in Python, and we demonstrate the estimators’ efficacy via a suite of validation tests.

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O. Philcox
Fri, 17 Mar 23
10/67

Comments: 33 pages, 10 figures, code available at this https URL

Novel exact ultra-compact and ultra-sparse hairy black holes emanating from regular and phantom scalar fields [CL]

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http://arxiv.org/abs/2303.09116


In the framework of a simple gravitational theory that contains a scalar field minimally coupled to gravity, we investigate the emergence of analytic black-hole solutions with non-trivial scalar hair of secondary type. Although it is possible for one to obtain asymptotically (A)dS solutions using our setup, in the context of the present work, we are solely interested in asymptotically flat solutions. At first, we study the properties of static and spherically symmetric black-hole solutions emanating from both regular and phantom scalar fields. We find that the regular-scalar-field-induced solutions are solutions describing ultra-compact black holes, while the phantom scalar fields generate ultra-sparse black-hole solutions. The latter are black holes that can be potentially of very low density since, contrary to ultra-compact ones, their horizon radius is always greater than the horizon radius of the corresponding Schwarzschild black hole of the same mass. Then, we generalize the above static solutions to slowly rotating ones and compute their angular velocities explicitly. Finally, the study of the axial perturbations of the derived solutions takes place, in which we show that there is always a region in the parameter space of the free parameters of our theory that allows the existence of both ultra-compact and ultra-sparse black holes.

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A. Bakopoulos and T. Nakas
Fri, 17 Mar 23
12/67

Comments: 22 pages, 6 figures

Swampland criteria for rescaled Einstein-Hilbert gravity with string corrections [CL]

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http://arxiv.org/abs/2301.08126


In this work we examine the Swampland criteria for a specific class of rescaled $f(R)$ gravitational models, that are capable of unifying the primordial era of the Universe with the late-time era with the inclusion of string corrections. In particular, we develop separately the theoretical framework of Gauss-Bonnet and Chern-Simons theories considering that, the rescale parameter is constrained to reside in the area $0<\alpha<1$. As showcased, in the context of the aforementioned theories, a viable inflationary phenomenology consistent with the latest Planck data can be obtained for both cases for a wide variety of values. The Swampland criteria which where examined are satisfied, not necessarily simultaneously, for small values of the rescale parameter, which is in agreement with the case of a canonical scalar field with absent string corrective terms. The Gauss-Bonnet model is also further constrained, in order to obtain a propagation velocity of tensor perturbations which coincides with that of light, according to the recent observations from the GW170817. As a result of this assumption the degrees of freedom of the theory are reduced. An interesting feature which arises from the overall phenomenology is that, due to the inclusion of string corrections the tensor spectral index of primordial perturbations is now capable of obtaining a positive value which is not possible in the case of the canonical scalar field. Last but not least, the power-law model which is known to be incompatible with observations is now rendered viable by including a parity violating term and as showcased, it satisfies the Swampland criteria as well.

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A. Gitsis, K. Revis, S. Venikoudis, et. al.
Fri, 20 Jan 23
4/72

Comments: N/A

The Scale of Supersymmetry Breaking and the Dark Dimension [CL]

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http://arxiv.org/abs/2301.07719


We argue for a relation between the supersymmetry breaking scale and the measured value of the dark energy density $\Lambda$. We derive it by combining two quantum gravity consistency swampland constraints, which tie the dark energy density $\Lambda$ and the gravitino mass $M_{3/2}$, respectively, to the mass scale of a light Kaluza-Klein tower and, therefore, to the UV cut-off of the effective theory. Whereas the constraint on $\Lambda$ has recently led to the Dark Dimension scenario, with a prediction of a single mesoscopic extra dimension of the micron size, we use the constraint on $M_{3/2}$ to infer the implications of such a scenario for the scale of supersymmetry breaking. We find that a natural scale for supersymmetry signatures is $M={\cal O}\left(\Lambda^{1/8}\right)={\cal O}({\rm TeV})$. This mass scale is within reach of LHC and of the next generation of hadron colliders. Finally, we discuss possible string theory and effective supergravity realizations of the Dark Dimension scenario with broken supersymmetry.

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L. Anchordoqui, I. Antoniadis, N. Cribiori, et. al.
Fri, 20 Jan 23
17/72

Comments: 18 pages

Continuous Spectrum on Cosmological Collider [CL]

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http://arxiv.org/abs/2301.07920


We study the effects of a massive field with a continuous spectrum (continuum isocurvaton) on the inflationary bispectrum in the squeezed limit. As a concrete example, we extend the quasi-single field inflation model to include a continuum isocurvaton with a well-motivated spectral density from extra dimensions and focus on a contribution to the bispectrum with a single continuum isocurvaton exchange. In contrast to the usual case without the continuous spectrum, the amplitude of the bispectrum has a damping feature in the deep squeezed limit, which can be strong evidence for the continuous spectrum.

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S. Aoki
Fri, 20 Jan 23
45/72

Comments: 23 pages, 3 figures

Redshift evolution of cosmic birefringence in CMB anisotropies [CEA]

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http://arxiv.org/abs/2301.07971


We study the imprints of a cosmological redshift-dependent pseudoscalar field $\phi$ on the rotation of cosmic microwave background (CMB) linear polarization generated by a coupling $ \phi F^{\mu\nu} \tilde F_{\mu \nu}$. We show how either phenomenological or theoretically motivated redshift dependence of the pseudoscalar field, such as those in models of Early Dark Energy, Quintessence or axion-like dark matter, lead to CMB polarization and temperature-polarization power spectra which exhibit a multipole dependence which goes beyond the widely adopted approximation in which the redshift dependence of the linear polarization angle is neglected. Because of this multipole dependence, the isotropic birefringence effect due to a general coupling $\phi F^{\mu\nu} \tilde F_{\mu \nu}$ is not degenerate with a systematic calibration angle uncertainty. By taking this multipole dependence into account, we calculate the parameters of these phenomenological and theoretical redshift dependence of the pseudoscalar field which can be detected by future CMB polarization experiments on the basis of a $\chi^2$ analysis for a Wishart likelihood. As a final example of our approach, we compute by Markov Chain MonteCarlo (MCMC) the minimal coupling $g_\phi$ in Early Dark Energy which could be detected by future experiments, with or without marginalizing on a systematic rotation angle uncertainty.

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M. Galaverni, F. Finelli and D. Paoletti
Fri, 20 Jan 23
53/72

Comments: 18 pages, 16 figures

Parametric resonance in abelian and non-abelian gauge fields via space-time oscillations [CL]

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http://arxiv.org/abs/2301.07456


We study the evolution of abelian electromagnetic as well as non-abelian gauge fields, in the presence of space-time oscillations. In the non-abelian case, we consider linear approximation, to analyse the time evolution of the field modes. In both abelian and non-abelian, the mode equations, show the presence of the same parametric resonant spatial modes. The large growth of resonant modes induces large fluctuations in physical observables including those that break the $CP-$symmetry. We also evolve small random fluctuations of fields, using numerical simulations in $2+1$ dimensions. These simulations help study non-linear effects $vs$ the gauge coupling, in the non-abelian case. Our results show that there is an increase in energy density with the coupling, at late times. These results suggest that gravitational waves may excite non-abelian gauge fields more efficiently than electromagnetic fields. Also, gravitational waves in the early Universe and from the merger of neutron stars, black holes etc. may enhance $CP-$violation and generate an imbalance in chiral charge distributions, magnetic fields etc.

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S. Dave, S. Digal and V. Mamale
Thu, 19 Jan 23
78/100

Comments: 19 pages, 16 figures

Implication of island for inflation and primordial perturbations [CL]

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http://arxiv.org/abs/2301.07403


It is usually thought that the efolds number of inflation must be bounded by its de Sitter entropy, otherwise we will have an information paradox. However, in light of the island rule for computing the entanglement entropy, we show that such a bound might be nonexistent, while the information flux of primordial perturbation modes the observer after inflation is able to detect follows a Page curve. In corresponding eternally inflating spacetime, it seems that our inflating patch must be accompanied with a neighbourly collapsed patch (eventually developing into a black hole) so that its Hawking radiation might be just our primordial perturbations. Accordingly, the perturbation spectrum we observed will present a “Page-like” suppression at large scale.

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Y. Piao
Thu, 19 Jan 23
79/100

Comments: 22 pages, 7 figures

Backreaction in cosmic screening approach [CL]

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http://arxiv.org/abs/2301.06886


We investigate the backreaction of nonlinear perturbations on the global evolution of the Universe within the cosmic screening approach. To this end, we have considered the second-order scalar perturbations. An analytical study of these perturbations followed by a numerical evaluation shows that, first, the corresponding average values have a negligible backreaction effect on the Friedmann equations and, second, the second-order correction to the gravitational potential is much less than the first-order quantity. Consequently, the expansion of perturbations into orders of smallness in the cosmic screening approach is correct.

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M. Eingorn, B. O’Briant, A. Diouf, et. al.
Thu, 19 Jan 23
91/100

Comments: 11 pages, 2 figures

On the Interaction between Ultralight Bosons and Quantum-Corrected Black Holes [CL]

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http://arxiv.org/abs/2301.06840


Both ultralight dark matter and exploring the quantum nature of black holes are all topics of great interest in gravitational wave astronomy at present. The superradiant instability allows an exotic compact object (ECO) to be surrounded by an ultralight boson cloud, which leads to the emission of gravitational waves and further triggers rich dynamical effects. In this paper, we study the gravitational effects of superradiant instabilities by calculating the energy fluxes of gravitational waves emitted from ultralight scalar dark matter fields by solving the Teukolsky equation in the background of a massive ECO phenomenologically described by a Kerr geometry with a reflective boundary condition at its physical boundary. We find that both the amplitude and phase of the reflectivity will either suppress or enhance the energy flux of GWs by several orders of magnitude if $M\mu \gtrsim 0.5$ where $M$ and $\mu$ are the mass of ECO and boson, respectively. However, the modifications to energy flux are negligible if $M \mu \lesssim 0.5$. Our results suggest that reflectivity will play a significant role in the near-horizon physics of ECO.

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R. Guo, C. Yuan and Q. Huang
Wed, 18 Jan 23
3/133

Comments: 20 pages, 6 figures

Closed-Form Formulae for Inflation Correlators [CL]

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http://arxiv.org/abs/2301.07047


We derive exact and closed-form expressions for a large class of two-point and three-point inflation correlators with the tree-level exchange of a single massive particle. The intermediate massive particle is allowed to have arbitrary mass, spin, chemical potential, and arbitrary nonderivative or derivative couplings to external inflaton modes. We also allow the coupling coefficients to have arbitrary complex power dependences on the conformal time. Our results feature closed-form expressions involving only familiar special functions and without any infinite sums. This is achieved by an improved bootstrap method with a suitable change of variables. Our results cover a wide range of cosmological collider models and can be directly used for future phenomenological studies. Our results can also be used as basic building blocks for constructing more complicated inflation correlators.

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Z. Qin and Z. Xianyu
Wed, 18 Jan 23
9/133

Comments: 41 pages

Solving both $H_0$ and $σ_8$ tensions in $f(T)$ gravity [CL]

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http://arxiv.org/abs/2301.06881


We report how to alleviate both the $H_0$ and $\sigma_8$ tensions simultaneously within $f(T)$ gravity. In particular, we consider the parametrization $f(T)=-T-2\Lambda/M_P^2+\alpha T^\beta$, where two out of the three parameters are independent. This model can efficiently fit observations solving the two tensions. To our knowledge, this is the first time where a modified gravity theory can alleviate both $H_0$ and $\sigma_8$ tensions simultaneously, hence, offering an additional argument in favor of gravitational modification.

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E. Saridakis
Wed, 18 Jan 23
16/133

Comments: 10 pages, 3 figures, Invited talk at the 16th Marcel Grossmann meeting, based on arXiv:1909.06388 . Published at the proceedings of MG16

$H_0$ Tension in Torsion-based Modified Gravity [CEA]

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http://arxiv.org/abs/2301.06328


The rising concern in the Hubble constant tension ($H_0$ tension) of the cosmological models motivates the scientific community to search for alternative cosmological scenarios that could resolve the $H_0$ tension. In this regard, we aim to work on a torsion-based modified theory of gravity which is an alternative description to the coherence model. We find an analytic solution of the Hubble parameter using a linear Lagrangian function of torsion $T$ and trace of energy-momentum tensor $\mathcal{T}$ for the dust case. Further, we constrain the cosmological and model parameters; to do that, we use Hubble and Pantheon samples and Markov Chain Monte Carlo (MCMC) simulation through Bayesian statistics. We obtain the values of Hubble constant as $H_0= 69.9\pm 6.8$ km$s^{-1}$ Mp$c^{-1}$, $H_0= 70.3\pm 6.3$ km$s^{-1}$ Mp$c^{-1}$, and $H_0= 71.4\pm 6.3$ km$s^{-1}$ Mp$c^{-1}$ at confidence level (CL), for Hubble, Pantheon, and their combine analysis, respectively. These outputs of $H_0$ for our model align with the recent observational measurements of $H_0$. In addition, we test the $Om$ diagnostic to check our model’s dark energy profile.

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S. Mandal, S. Mishra and P. Sahoo
Wed, 18 Jan 23
28/133

Comments: Comments are welcome

Ekpyrotic Inflation: Transient Acceleration after Non-minimal M-flation Preheating [CL]

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http://arxiv.org/abs/2301.06598


Light massive preheat fields acquire a non-vanishing dispersion during parametric resonance from their quantum particle production. This in turn will modify the inflaton potential, which in some cases can induce a transient period of acceleration. We illustrate this phenomenon in the setup of non-supersymmetric non-minimal M-flation (non-$\mathbb{M}$-flation) which has some motivations from the brane compactifications in string theory. Implementing a lattice simulation by the LATTICEEASY code, we compute the potential correction term in our scenario and show that the modified term indeed causes the universe to make a transition from the decelerated expansion to a temporary phase of acceleration. The correction term reduces to some extent the number density of the particles generated during preheating, but the efficiency of preheating remains still enough to have successful particle production after inflation. We also compute the spectrum of the gravitational waves (GWs) generated during preheating in our setup by using the LATTICEEASY code. Although the peak frequency remains almost the same, the inclusion of the correction term reduces the amplitude of the gravitational spectrum by almost one order of magnitude.

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A. Ashoorioon and K. Rezazadeh
Wed, 18 Jan 23
43/133

Comments: 21 pages, 13 figures

A unified effective approach to cosmological perturbations [CL]

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http://arxiv.org/abs/2301.05679


A unified model independent effective description of cosmological perturbations is derived in terms of two effective quantities, playing the role of effective propagation speeds of curvature perturbations and gravitational waves, encoding the effects of the interaction of perturbations at any order, and inducing a modification of the friction term of the perturbations propagation equation. The approach can be applied to dark energy, modified gravity, dark matter, for fields of arbitrary number and spin, and is particularly suitable for model independent analysis of observational data. The structure of the effective actions and equations is the same for scalar and tensor perturbations.
The effective actions can be written as the Klein-Gordon action in terms of an appropriately defined effective metric, dependent on the effective speed. In this geometrical interpretation, the effective metric emerges as the result of the interaction and self-interaction of perturbations, hinting to possible connections with emergent gravity.
As an example of an application we find that the effective speeds of curvature perturbations and gravitational waves can be frequency and polarization dependent even for a minimally coupled scalar field in general relativity, when higher order terms effects are computed, going beyond the quadratic action, or in axion inflation. We discuss the relation between the effective speed and quantum correlators.

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A. Romano
Wed, 18 Jan 23
73/133

Comments: N/A

Stability of electrically charged stars, regular black holes, quasiblack holes, and quasinonblack holes [CL]

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http://arxiv.org/abs/2301.06563


The stability of a class of electrically charged fluid spheres under radial perturbations is studied. Among these spheres there are regular stars, overcharged tension stars, regular black holes, quasiblack holes, and quasinonblack holes, all of which have a Reissner-Nordstr\”om exterior. We formulate the dynamical perturbed equations by following the Chandrasekhar approach and investigate the stability against radial perturbations through numerical methods. It is found that (i) under certain conditions that depend on the adiabatic index of the radial perturbation, there are stable charged stars and stable tension stars; (ii) also depending on the adiabatic index there are stable regular black holes; (iii) quasiblack hole configurations formed by, e.g., charging regular pressure stars or by discharging regular tension stars, can be stable against radial perturbations for reasonable values of the adiabatic index; (iv) quasinonblack holes are unstable against radial perturbations.

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A. Masa, J. Lemos and V. Zanchin
Wed, 18 Jan 23
105/133

Comments: 38 pages, 15 figures, 18 tables

Astrophysical consequences of dark matter for photon orbits and shadows of supermassive black holes [CL]

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http://arxiv.org/abs/2301.06373


We consider Kerr black holes (BHs) surrounded by perfect dark fluid matter (PFDM), with an additional parameter ($k$) because of PFDM, apart from mass ($M$) and rotation parameter ($a$) — the rotating PFDM BHs. We analyze the photon orbits around PFDM BHs and naked singularities (NSs) and emphasise the effect of PFDM on photon boomerangs. Interestingly, the azimuthal oscillations first increase and then decrease for retrograde orbits, whereas they first decrease and then increase for prograde orbits, with increasing $k$. Unlike in the Kerr NSs, photon boomerangs can form around rotating PFDM NSs. We use the Event Horizon Telescope (EHT) observational results for Schwarzschild shadow deviations of M87* and Sgr A*, $\delta_{M87^}=-0.01\pm0.17$ and $\delta_{Sgr A^} = -0.08^{+0.09}{-0.09}~\text{(VLTI)},-0.04^{+0.09}{-0.10}~\text{(Keck)}$, to report the upper bounds on the PFDM parameter: $0\leq k\leq 0.0792M$ and $k^{max}\in[0.0507M, 0.0611M]$ respectively. Together with the EHT bounds on the shadows of Sgr A$^$ and M87$^$, our analysis concludes that a substantial part of the rotating PFDM BH parameter space agrees with the EHT observations. Thus, one must consider the possibility of the rotating PFDM BHs being strong candidates for the astrophysical BHs.

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A. Anjum, M. Afrin and S. Ghosh
Wed, 18 Jan 23
126/133

Comments: 17 pages, 16 figures, 6 tables

Non-Gaussianity in the cosmic microwave background from loop quantum cosmology [CEA]

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http://arxiv.org/abs/2301.05406


Primordial non-Gaussianity has set strong constraints on models of the early universe. Studies have shown that Loop Quantum Cosmology (LQC), which is an attempt to extend inflationary scenario to planck scales, leads to a strongly scale dependent and oscillatory non-Gaussianity. In particular, the non-Gaussianity function $f_{_{\rm NL}}(k_1,\, k_2,\, k_3)$ generated in LQC, though similar to that generated during slow roll inflation at small scales, is highly scale dependent and oscillatory at large wavelengths. In this work, we investigate the imprints of such a primordial bispectrum in the bispectrum of Cosmic Microwave Background (CMB). Inspired by earlier works, we propose an analytical template for the primordial bispectrum in LQC and compute the corresponding reduced bispectra of temperature and electric polarisation and their three-point cross-correlations. We show that CMB bispectra generated in LQC is consistent with the observations from Planck. We conclude with a discussion of our results and its implications to LQC.

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R. K and V. Sreenath
Mon, 16 Jan 23
32/50

Comments: 17 pages, 8 figures

Non-Gaussianity in the cosmic microwave background from loop quantum cosmology [CEA]

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http://arxiv.org/abs/2301.05406


Primordial non-Gaussianity has set strong constraints on models of the early universe. Studies have shown that Loop Quantum Cosmology (LQC), which is an attempt to extend inflationary scenario to planck scales, leads to a strongly scale dependent and oscillatory non-Gaussianity. In particular, the non-Gaussianity function $f_{_{\rm NL}}(k_1,\, k_2,\, k_3)$ generated in LQC, though similar to that generated during slow roll inflation at small scales, is highly scale dependent and oscillatory at large wavelengths. In this work, we investigate the imprints of such a primordial bispectrum in the bispectrum of Cosmic Microwave Background (CMB). Inspired by earlier works, we propose an analytical template for the primordial bispectrum in LQC and compute the corresponding reduced bispectra of temperature and electric polarisation and their three-point cross-correlations. We show that CMB bispectra generated in LQC is consistent with the observations from Planck. We conclude with a discussion of our results and its implications to LQC.

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R. K and V. Sreenath
Mon, 16 Jan 23
41/50

Comments: 17 pages, 8 figures

Tidal deformations of a binary system induced by an external Kerr black hole [CL]

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http://arxiv.org/abs/2301.04879


The dynamics of a binary system moving in the background of a black hole is affected by tidal forces. In this work, for the Kerr black hole, we derive the electric and magnetic tidal moments at quadrupole order, where the latter are computed for the first time in full generality. We make use of these moments in the scenario of a hierarchical triple system made of a Kerr black hole and an extreme-mass ratio binary system consisting of a Schwarzschild black hole and a test particle. We study how the secular dynamics of the test particle in the binary system is distorted by the presence of tidal forces from a much larger Kerr black hole. Our treatment includes strong gravitational effects beyond the post-Newtonian approximation both for the binary system and for the tidal forces since the binary system is allowed to be close to the event horizon of the Kerr black hole. We compute the shifts in the physical quantities for the secular dynamics of the test particle and show that they are gauge-invariant. In particular, we apply our formalism to the innermost stable circular orbit for the test particle and to the case of the photon sphere. Our results are relevant for the astrophysical situation in which the binary system is in the vicinity of a supermassive black hole.

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F. Camilloni, G. Grignani, T. Harmark, et. al.
Fri, 13 Jan 23
2/72

Comments: 27 pages, 3 figures

Running vacuum in FLRW spacetime: The dynamics of $ρ_{\rm vac}(H)$ from the quantized matter fields [CL]

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http://arxiv.org/abs/2301.05205


Phenomenological work in the last few years has provided significant support to the idea that the vacuum energy density (VED) is a running quantity with the cosmological evolution and that this running helps to alleviate the cosmological tensions afflicting the $\Lambda$CDM. On the theoretical side, recent devoted studies have shown that the properly renormalized $\rho_{\rm vac}$ in FLRW spacetime adopts the “running vacuum model” (RVM) form. While in three previous studies by two of us (CMP and JSP) such computations focused solely on scalar fields non-minimally coupled to gravity, in the present work we compute the spin-$1/2$ fermionic contributions and combine them both. The calculation is performed using a new version of the adiabatic renormalization procedure based on subtracting the UV divergences at an off-shell renormalization point $M$. The quantum scaling of $\rho_{\rm vac}$ with $M$ turns into cosmic evolution with the Hubble rate, $H$. As a result the “cosmological constant” $\Lambda$ appears in our framework as the nearly sustained value of $8\pi G(H)\rho_{\rm vac}(H)$ around (any) given epoch $H$, where $G(H)$ is the gravitational coupling, which is also running, although very mildly (logarithmically). We find that the VED evolution at present reads $\delta \rho_{\rm vac}(H)\sim \nu_{\rm eff} m_{\rm Pl}^2 \left(H^2-H_0^2 \right)\ (|\nu_{\rm eff}|\ll 1)$. The coefficient $\nu_{\rm eff}$ receives contributions from all the quantized fields, bosons and fermions. Remarkably, there also exist higher powers ${\cal O}(H^{6})$ which can trigger inflation in the early universe. Finally, the equation of state (EoS) of the vacuum receives also quantum corrections from bosons and fermion fields, shifting its value from -1. The remarkable consequence is that the EoS of the quantum vacuum may nowadays effectively look like quintessence.

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C. Moreno-Pulido, J. Peracaula and S. Cheraghchi
Fri, 13 Jan 23
58/72

Comments: LaTex, 67 pages

Shadows of Kerr-Vaidya-like black holes [CL]

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http://arxiv.org/abs/2301.04967


In this work, we study the shadow boundary curves of rotating time-dependent black hole solutions which have well-defined Kerr and Vaidya limits. These solutions are constructed by applying the Newman-Janis algorithm to a spherically symmetric seed metric conformal to the Vaidya solution with a mass function that is linear in Eddington-Finkelstein coordinates. Equipped with a conformal Killing vector field, this class of solution exhibits separability of null geodesics, thus allowing one to develop an analytic formula for the boundary curve of its shadow. We find a simple power law describing the dependence of the mean radius and asymmetry factor of the shadow on the accretion rate. Applicability of our model to recent Event Horizon Telescope observations of M87${}^$ and Sgr A${}^$ is also discussed.

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H. Tan
Fri, 13 Jan 23
64/72

Comments: 22 pages

Ghost and Laplacian Instabilities in the Teleparallel Horndeski Gravity [CL]

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http://arxiv.org/abs/2301.04457


Teleparallel geometry offers a platform on which to build up theories of gravity where torsion rather than curvature mediates gravitational interaction. The teleparallel analogue of Horndeski gravity is an approach to teleparallel geometry where scalar-tensor theories are considered in this torsional framework. Being teleparallel gravity of lower order in dynamics, this turns out to be more general than metric Horndeski gravity. In other words, the class of teleparallel Horndeski gravity models is much broader than the standard metric one. In this work, we explore constraints on this wide range of models coming from ghost and Laplacian instabilities. The aim is to limit pathological branches of the theory by fundamental considerations. It is possible to conclude that a very large class of models results physically viable.

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S. Capozziello, M. Caruana, J. Said, et. al.
Thu, 12 Jan 23
5/68

Comments: N/A

Reconstruction Methods and the Amplification of the Inflationary Spectrum [CL]

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http://arxiv.org/abs/2301.04477


We analyze the consequences of different evolutions of the Hubble parameter on the spectrum of scalar inflationary perturbations. The analysis is restricted to inflationary phases described by a transient evolution, when uncommon features arise in the inflationary spectra which may lead to an amplitude enhancement. We then discuss how the spectrum is, respectively, amplified or blue-tilted in the presence or absence of a growing solution of the Mukhanov-Sasaki equation. The cases of general relativity with a minimally coupled inflaton and that of induced gravity are considered explicitly. Finally, some remarks on constant roll inflation are discussed.

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L. Chataignier, A. Kamenshchik, A. Tronconi, et. al.
Thu, 12 Jan 23
41/68

Comments: N/A

Inflation and Primordial Black Holes [CEA]

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http://arxiv.org/abs/2301.03600


We review conceptual aspects of inflationary scenarios able to produce primordial black holes, by amplifying the size of curvature fluctuations to the level required for triggering black hole formation. We identify general mechanisms to do so, both for single and multiple field inflation. In single field inflation, the spectrum of curvature fluctuations is enhanced by pronounced gradients of background quantities controlling the cosmological dynamics, which can induce brief phases of non–slow-roll inflationary evolution. In multiple field inflation, the amplification occurs through appropriate couplings with additional sectors, characterized by tachyonic instabilities that enhance the size of their fluctuations. As representative examples, we consider axion inflation, and two-field models of inflation with rapid turns in field space. We develop our discussion in a pedagogical manner, by including some of the most relevant calculations, and by guiding the reader through the existing theoretical literature, emphasizing general themes common to several models.

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O. Özsoy and G. Tasinato
Wed, 11 Jan 23
59/80

Comments: 58 Pages, 19 Figures + Appendices, Comments are welcome

Hubble tension as a guide for refining the early Universe: Cosmologies with explicit local Lorentz and diffeomorphism violation [CL]

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http://arxiv.org/abs/2301.03883


This paper is dedicated to assessing modified cosmological settings based on the gravitational Standard-Model Extension (SME). Our analysis rests upon the Hubble tension (HT), which is a discrepancy between the observational determination of the Hubble parameter via data from the Cosmic Microwave Background (CMB) and Type Ia supernovae, respectively. While the latter approach is model-independent, the former highly depends on the model used to describe the physics of the early Universe. Motivated by the HT, we take into account two recently introduced cosmological models as test frameworks of the pre-CMB era. These settings involve local Lorentz and diffeomorphism violation parameterized by nondynamical SME background fields $s_{00}$ and $s^{ij}$, respectively. We aim at explaining the tension in the measured results of the cosmic expansion rate in early and late epochs by resorting to these two modified cosmologies as potential descriptions of the pre-CMB era. As long as the HT does not turn out to be a merely systematic effect, it can serve as a criterion for exploring regions of the parameter space in certain pre-CMB new-physics candidates such as SME cosmologies. By setting extracted limits on SME coefficients into perspective with already existing bounds in the literature, we infer that none of the aforementioned models are suitable pre-CMB candidates for fixing the HT. In this way, new physics arising from the particular realizations of Lorentz and diffeomorphism violation studied in this paper does not explain the HT. Our paper exemplifies how to exploit this discrepancy as a novel possibility of refining our description of the early Universe.

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M. Khodadi and M. Schreck
Wed, 11 Jan 23
63/80

Comments: 14 pages (two columns)

Swampland Revisited [CL]

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http://arxiv.org/abs/2301.03464


The transcendental expectation of string theory is that the nature of the fundamental forces, particle spectra and masses, together with coupling constants, is uniquely determined by mathematical and logical consistency, non-empirically, that is by pure reason. However pluralism triumphed with the explosive emergence of the multiverse. String theorists have extended a long-sought dream (their unique and final theory) to a landscape or a happy caparnaum. Proponents of string theory try to qualify their arguments via swampland conjectures while cosmologists retreat to their telescopes. We review the current status of the string theory swampland.

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M. Cass\e’ and J. Silk
Tue, 10 Jan 23
70/93

Comments: N/A

TCC bounds on the static patch of de Sitter space [CL]

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http://arxiv.org/abs/2301.02741


Recently, Pei-Ming Ho and Hikaru Kawai have argued that treating particles as wave packets can lead to a shutdown of Hawking radiation at late times near the horizon of black holes. This shutdown arises from viewing quantum field theory near the black hole horizon as an effective field theory, and imposing an appropriate UV cutoff. We show that this effect is also present in the static patch of de Sitter space, leading to a shutdown of Gibbons-Hawking radiation near the de Sitter horizon. Assuming this effect is due to the breakdown of effective field theory, we obtain a bound $t \lesssim H^{-1} \ln (H^{-1} M_P)$ on the time scale of validity of effective field theory in de Sitter space, which matches with the predictions of the Trans-Planckian Censorship Conjecture.

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M. Blamart, S. Laliberte and R. Brandenberger
Tue, 10 Jan 23
72/93

Comments: N/A

Irregular universe in the Nieh-Yan modified teleparallel gravity [CL]

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http://arxiv.org/abs/2301.02847


The Nieh-Yan modified teleparallel gravity is a model which modifies the general relativity equivalent teleparallel gravity by a coupling between the Nieh-Yan density and an axion-like field. This model predicts parity violations in the gravitational waves if the axion-like field has a non-trivial background, and more importantly it is ghost free and avoids the pathologies presented in other parity-violating gravity models. The cosmological dynamics and perturbations of the Nieh-Yan modified teleparallel gravity have been investigated in detail, but all these previous investigations rely on the symmetry requirement that in the background universe both the metric and affine connection are homogeneous and isotropic. In this paper we relax the symmetry constraint on the connection and leave it arbitrary at the beginning, after all the cosmological principle only needs the metric of the background spacetime to meet the symmetry requirement. We find a new flat universe solution for the Nieh-Yan modified teleparallel gravity, for which the background dynamics itself is unchanged but the perturbations around it present a new feature that the scalar and tensor perturbations are coupled together at the linear level. The implications of this peculiar feature in primordial perturbations from inflation are also discussed.

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M. Li and H. Rao
Tue, 10 Jan 23
78/93

Comments: 20 pages, 1 figures, irregular universe, teleparallel gravity

Analytical coordinate time at first post-Newtonian order [CL]

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http://arxiv.org/abs/2301.02472


In this letter, we exploit the Damour-Deruelle solution to derive the analytical expression of the coordinate time in terms of the polar angle. This formula has advantageous applications in both pulsar timing and gravitational-wave theory.

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V. Falco, E. Battista and J. Antoniadis
Mon, 9 Jan 23
2/59

Comments: 5 pages; 4 figures; letter accepted on Europhysics Letters (EPL)

Identifying modified theories of gravity using binary black-hole ringdowns [CL]

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http://arxiv.org/abs/2301.02267


Black-hole spectroscopy, that is, measuring the characteristic frequencies and damping times of different modes in a black-hole ringdown, is a powerful probe for testing deviations from the general theory of relativity (GR). In this work, we present a comprehensive study on its ability to identify deviations from the spectrum of a Kerr black hole in GR. Specifically, we investigate the performance of black hole spectroscopy on a diverse set of theoretically motivated as well as phenomenologically modified spectra. We find that while the signal-to-noise ratio $\rho_{\rm RD}$ in the ringdown required to identify a modification to the GR Kerr black hole spectrum depends on the details of the modifications, a modification that introduces $\sim 1 \%$ shift in the fundamental mode frequencies can typically be distinguished with $\rho_{\rm RD} \in [150,500]$. This range of $\rho_{\rm RD}$ is feasible with the next-generation detectors, showing a promising science case for black hole spectroscopy.

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C. Pacilio and S. Bhagwat
Mon, 9 Jan 23
10/59

Comments: v1: 14 pages, 4 figures

Phase Transitions and Critical Phenomena for the FRW Universe in an Effective Scalar-Tensor Theory [CL]

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http://arxiv.org/abs/2301.01938


We find phase transitions and critical phenomena of the FRW (Friedmann-Robertson-Walker) universe in the framework of an effective scalar-tensor theory that belongs to the Horndeski class. We identify the thermodynamic pressure (generalized force) $P$ of the FRW universe in this theory with the work density $W$ of the perfect fluid, which is a natural definition directly read out from the first law of thermodynamics. We derive the thermodynamic equation of state $P=P(V, T)$ for the FRW universe in this theory and make a thorough discussion of its $P$-$V$ phase transitions and critical phenomena. We calculate the critical exponents, and show that they are the same with the mean field theory, and thus obey the scaling laws.

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H. Abdusattar, S. Kong, H. Zhang, et. al.
Fri, 6 Jan 23
38/55

Comments: 12 pages, 1 figures

WIMPs, FIMPs, and Inflaton phenomenology via reheating, CMB and $Δ N_{eff}$ [CL]

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http://arxiv.org/abs/2301.01641


In this paper, we extensively analyzed the reheating dynamics after inflation and looked into its possible implication on dark matter (DM) and inflaton phenomenology. We studied the reheating through various possible channels of inflaton going into massless scalars (bosonic reheating) and fermions (fermionic reheating) via non-gravitational and gravity-mediated decay processes. We further include the finite temperature effect on the decay process. Along with their precise roles in governing the dynamics, we compared the relative importance of different temperature-corrected decay channels in the gradual process of reheating depending on the reheating equation of state (EoS), which is directly related to inflaton potential. Particularly, the universal gravitational decay of inflaton is observed to play a very crucial role in the reheating process for a large range of inflaton decay parameters. For our study, we consider typical $\alpha$-attractor inflationary models. We further establish the intriguing connection among those different inflaton decay channels and the CMB power spectrum that can have profound implications in building up a unified model of inflation, reheating, and DM. We analyze both fermion and scalar DM with different physical processes being involved, such as gravitational scattering, thermal bath scattering, and direct inflaton decay. Gravitational decay can again be observed to play a crucial role in setting the maximum limit on DM mass that has already been observed earlier in the literature [52]. Depending on the coupling strength, we have analyzed in detail the production of both FIMP and WIMP-like DM during reheating and their detailed phenomenological implications from the perspective of various cosmological and laboratory experiments.

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M. Haque, D. Maity and R. Mondal
Thu, 5 Jan 23
4/51

Comments: 50 pages, 17 figures

Addressing Cosmological Tensions by Non-Local Gravity [CEA]

Posted on by

http://arxiv.org/abs/2301.01503


Alternative cosmological models have been under deep scrutiny in recent years, aiming to address the main shortcomings of the $\Lambda$CDM model. Moreover, as the accuracy of cosmological surveys improved, new tensions have risen between the model-dependent analysis of the Cosmic Microwave Background and lower redshift probes. Within this framework, we review two quantum-inspired non-locally extended theories of gravity, whose main cosmological feature is a geometrically driven accelerated expansion. The models are especially investigated in light of the Hubble and growth tension, and promising features emerge for the Deser–Woodard one. On the one hand, the cosmological analysis of the phenomenological formulation of the model shows a lowered growth of structures but an equivalent background with respect to $\Lambda$CDM. On the other hand, the study of the lensing features at the galaxy cluster scale of a new formulation of non-local cosmology, based on Noether symmetries, makes room for the possibility of alleviating both the $H_0$ and $\sigma_8$ tension. However, the urgent need for a screening mechanism arises for this non-local theory of gravity.

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F. Bouchè, S. Capozziello and V. Salzano
Thu, 5 Jan 23
17/51

Comments: 19 pages, 2 figures. Published in Universe as part of the Special Issue “Modified Gravity Approaches to the Tensions of {\Lambda}CDM”

Instability of the cosmological DBI-Galileon in the non-relativistic limit [CL]

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http://arxiv.org/abs/2301.01723


The DBI-Galileon model is a tensor-scalar theory of gravity which finds its foundation as the most general theory of the dynamics of a 4D brane embedded in a 5D bulk. It is of particular interest as it provides a few free parameters with a physical meaning, such as the cosmological constant which is there related to the brane tension. Most studies of this model have been performed assuming a maximally symmetric geometry for the 5D bulk, in which it has been shown that the theory reduces to various types of Galileon. In contrast, the general case for the geometry of the bulk provides a different covariantization of the Galileon model than the covariant Galileon: the DBI-Galileon. From the tight constraints on the gravitational waves speed, we are naturally led to consider the non-relativistic limit of the model where the kinetic energy of the brane is small compared to its tension, that we study in the context of late-time cosmology. The DBI-Galileon in the non-relativistic limit is simply an expansion around General Relativity (GR) which can be expressed as a shift-symmetric Horndeski theory. We developed the description of this theory at the background and perturbation level. However, by studying the scalar and tensor perturbations around a flat FLRW background, we found that they contain a ghost degree of freedom leading to fatal instability of the vacuum for every combination of the free parameters. As a lesson, we emphasized which of the Horndeski terms competes to avoid this instability in more general cases.

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C. Leloup, L. Heitz and J. Neveu
Thu, 5 Jan 23
28/51

Comments: N/A

Black Hole Greybody Factors from Korteweg-de Vries Integrals: Computation [CL]

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http://arxiv.org/abs/2301.01096


It has recently been shown that the dynamics of perturbed non-rotating black holes (BHs) admits an infinite number of symmetries that are generated by the flow of the Korteweg-de Vries (KdV) equation. These symmetries lead to an infinite number of conserved quantities that can be obtained as integrals of differential polynomials in the potential appearing in the gauge-invariant master equations describing the BH perturbations, the KdV integrals. These conserved quantities are the same for all the possible potentials, which means that they are invariant under Darboux transformations, and they fully determine the BHs transmission amplitudes, or greybody factors, via a moment problem. In this paper we introduce a new semi-analytical method to obtain the greybody factors associated with BH scattering processes by solving the moment problem using only the KdV integrals. The method is based on the use of Pad\’e approximants and we check it first by comparing with results from the case of a P\”oschl-Teller potential, for which we have analytical expressions for the greybody factors. Then, we apply it to the case of a Schwarzschild BH and compare with results from computations based on the Wentzel-Kramers-Brillouin (WKB) approximation. It turns out that the new method provides accurate results for the BH greybody factors for all frequencies. The method is also computationally very efficient.

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M. Lenzi and C. Sopuerta
Wed, 4 Jan 23
3/43

Comments: 21 pages, 13 figures, 26 plots

The Primordial Black Hole Formation from Single-Field Inflation is Not Ruled Out [CEA]

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http://arxiv.org/abs/2301.00599


A standard scenario to form primordial black holes in the early universe is based on a phase of ultra-slow-roll in single-field inflation when the amplitude of the short scale modes is enhanced compared to the CMB plateau. Based on general arguments, we show that the loop corrections to the large-scale linear power spectrum from the short modes are small and conclude that the scenario is not ruled out.

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A. Riotto
Tue, 3 Jan 23
12/49

Comments: 9 pages, LaTeX file

Search for echoes on the edge of quantum black holes [CL]

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http://arxiv.org/abs/2301.00025


I perform an unprecedented template-based search for stimulated emission of Hawking radiation (or Boltzmann echoes) by combining the gravitational wave data from 65 binary black hole merger events observed by the LIGO/Virgo collaboration. With a careful Bayesian inference approach, I found no statistically significant evidence for this signal in either of the 3 Gravitational Wave Transient Catalogs GWTC-1, GWTC-2 and GWTC-3. However, the data cannot yet conclusively rule out the presence of Boltzmann echoes either, with the Bayesian evidence ranging within 0.3-1.6 for most events, and a common (non-vanishing) echo amplitude for all mergers being disfavoured at only 2:5 odds. The only exception is GW190521, the most massive and confidently detected event ever observed, which shows a positive evidence of 9.2 for stimulated Hawking radiation. An optimal combination of posteriors yields an upper limit of $A < 0.42$ (at $90\%$ confidence level) for a universal echo amplitude, whereas $A \sim 1$ was predicted in the canonical model. The next generation of gravitational wave detectors such as LISA, Einstein Telescope, and Cosmic Explorer can draw a definitive conclusion on the quantum nature of black hole horizons.

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J. Abedi
Tue, 3 Jan 23
26/49

Comments: 6 pages, 4 figures

Gravitational collapse and odd-parity black hole perturbations in Minimal Theory of Bigravity [CL]

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http://arxiv.org/abs/2301.00498


We investigate dynamical properties of static and spherically symmetric systems in the self-accelerating branch of the Minimal Theory of Bigravity (MTBG). In the former part, we study the gravitational collapse of pressure-less dust and find special solutions, where, in both the physical and fiducial sectors, the exterior and interior spacetime geometries are given by the Schwarzschild spacetimes and the Friedmann-Lema\^itre-Robertson-Walker universes dominated by pressure-less dust, respectively, with specific time slicings. In the spatially-flat case, under a certain tuning of the initial condition, we find exact solutions of matter collapse in which the two sectors evolve independently. In the spatially-closed case, once the matter energy densities and the Schwarzschild radii are tuned between the two sectors, we find exact solutions that correspond to the Oppenheimer-Snyder model in GR. In the latter part, we study odd-parity perturbations of the Schwarzschild-de Sitter solutions written in the spatially-flat coordinates. For the higher-multipole modes $\ell\geq2$, we find that in general the system reduces to that of four physical modes, where two of them are dynamical and the remaining two are shadowy, i.e. satisfying only elliptic equations. In the case that the ratio of the lapse functions between the physical and fiducial sectors are equal to a constant determined by the parameters of the theory, the two dynamical modes are decoupled from each other but sourced by one of the shadowy modes. Otherwise, the two dynamical modes are coupled to each other and sourced by the two shadowy modes. On giving appropriate boundary conditions to the shadowy modes as to not strongly back-react/influence the dynamics of the master variables, in the high frequency and short wavelength limits, we show that the two dynamical modes do not suffer from ghost or gradient instabilities.

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M. Minamitsuji, A. Felice, S. Mukohyama, et. al.
Tue, 3 Jan 23
32/49

Comments: 27 pages

Ultimate Black Hole Recoil: What the maximum high energy collisions kick is? [CL]

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http://arxiv.org/abs/2301.00018


We performed a series of 1341 full numerical simulations of high energy collision of black holes to search for the maximum recoil velocity after their merger. We consider equal mass binaries with opposite spins pointing along their orbital plane and perform a search of spin orientations, impact parameters, and initial linear momenta to find the maximum recoil for a given spin magnitude $s$. This spin sequence for $s=0.4, 0.7, 0.8, 0.85, 0.9$ is then extrapolated to the extreme case, $s=1$, to obtain an estimated maximum recoil velocity of $26,677\pm 470$ km/s, thus nearly $9\%$ the speed of light.

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J. Healy and C. Lousto
Tue, 3 Jan 23
49/49

Comments: 6 figures, 1 Table

Mimetic K-essence [CL]

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http://arxiv.org/abs/2212.14867


We propose a new non-trivial way to combine mimetic dark matter with the mimetic formulation of unimodular gravity. This yields a Weyl-invariant higher-derivative scalar-vector-tensor theory. We demonstrate that on-shell its behavior mimics GR with an additional k-essence scalar. The overall scale of the k-essence arises as an integration constant — a global degree of freedom. Interestingly, we find that the resulting fluid cannot make transition through ultra-relativistic equation of state. We develop a method to find a mimetic theory corresponding to any eligible k-essence and identify, which k-essences can or cannot be reproduced this way. Finally, we show that abandoning the Weyl symmetry of the setup allows us to obtain both unimodular gravity and mimetic dark matter simultaneously, from one conformal redefinition of the metric.

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P. Jiroušek, K. Shimada, A. Vikman, et. al.
Mon, 2 Jan 23
13/44

Comments: 16 pages + references, 1 figure, Happy New Year!

Non-existence of quantum black hole horizons in the improved dynamics approach [CL]

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http://arxiv.org/abs/2212.14535


In this paper, we study the quantum geometric effects near the locations that black hole horizons used to appear in the classical theory within the framework of the improved dynamic approach, in which the two polymerization parameters of the Kantowski-Sachs spacetime are functions of the phase variables. Our detailed analysis shows that the effects are so strong that black hole horizons of the effective quantum theory do not exist any longer, and the corresponding Kantowski-Sachs model now describes the entire spacetime of the trapped region, instead of being only the internal region of a black hole, as it is usually expected in loop quantum gravity.

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W. Gan, X. Kuang, Z. Yang, et. al.
Mon, 2 Jan 23
20/44

Comments: revtex4-2, 3 figures, and two tables

Tunneling wavefunction proposal with loop quantum geometry effects [CL]

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http://arxiv.org/abs/2212.14065


In Vilenkin’s tunneling wavefunction proposal our expanding universe is born via a tunneling through a barrier from nothing at the zero scale factor. We explore the viability of this proposal for the spatially closed FLRW model with a positive cosmological constant including quantum gravity modifications in the Planck regime. Our setting is the effective spacetime description of loop quantum cosmology (LQC) which is known to replace the big bang singularity with a bounce due to the holonomy modifications. Due to the bounce, the barrier potential of the Wheeler-DeWitt theory is replaced by a step like potential which makes the tunneling proposal incompatible. But for a complete picture of singularity resolution, inverse scale factor modifications from quantum geometry must be included which play an important role at very small scale factors in the spatially closed models. We show that with inclusion of inverse scale factor modifications the resulting potential is again a barrier potential. The universe at the vanishing scale factor is dynamically non-singular and in an Einstein static like phase. We show that quantum geometric effects in LQC provide a non-singular completion of Vilenkin’s tunneling proposal. We also find that quantum geometric effects result in a possibility of a tunneling to a quantum cyclic universe albeit for a very large value of cosmological constant determined by the quantum geometry.

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M. Motaharfar and P. Singh
Mon, 2 Jan 23
27/44

Comments: 19 pages, 6 figures

Critical collapse for the Starobinsky $R^2$ model [CL]

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http://arxiv.org/abs/2212.14805


We study gravitational collapse for the Starobinsky $R^2$ model, a particular example of an $f(r)$ theory, in a spherically symmetric spacetime. We add a massless scalar field as matter content to the spacetime. We work in the Einstein frame, where an additional scalar field arises due to the conformal transformation. As in general relativity, depending on the initial data, we found that the gravity scalar field and the physical scalar field can collapse, forming a black hole, in which the final solution is the Schwarzschild metric. We found the threshold of black hole formation through a fine-tuning method and studied critical collapse near this regime.

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Y. Baez
Mon, 2 Jan 23
30/44

Comments: 15 pages, 3 figures

On the structural stability of a simple cosmological model in $R+α R^{2}$ theory of gravity [CL]

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http://arxiv.org/abs/2212.14843


Theory of gravity with a quadratic contribution of scalar curvature is investigated in terms of dynamical system approach. The simplest Friedmann-Robertson-Walker metric is used to formulate dynamics in Jordan frame as well as in conformally transformed Einstein frame. We show that in both frames there are stable de Sitter states for which the Hubble function expansion naturally gives terms corresponding to non-substantial dark matter. Using invariant centre manifold we show that in the Einstein frame there is a zero measure set of initial conditions leading from unstable to stable de Sitter state. Additionally, the initial de Sitter state is plunged with a parallelly propagated singularity. We show that the Jordan frame and the Einstein frame formulation of the theory are physically nonequivalent.

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O. Hrycyna
Mon, 2 Jan 23
35/44

Comments: 18 pages, 3 figs

Particle production during inflation: constraints expected from redshifted 21 cm observations from the epoch of reionization [CEA]

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http://arxiv.org/abs/2212.14064


We examine a type of features in the primordial scalar power spectrum, namely, the bump-like feature(s) that arise as a result of burst(s) of particle production during inflation. The latest CMB observations by Planck 2018 can accommodate such features. In the near future, observations of redshifted 21 cm signal from the epoch of reionization can put additional constraints on inflation models by exploiting the expected tomographic information across a wide range of co-moving wave-numbers. In this work, we study the potential of upcoming observational data from SKA-Low to constrain the parameters of the primordial power spectrum with bump-like features. We use simulated mock data expected from SKA-Low, with uncertainties estimated from different foreground removal models, and constrain the parameters of primordial features within a Bayesian framework. We study two scenarios: in the first scenario, where the astrophysical parameters relevant to the evolution of the 21 cm signal are known, we find that 21 cm power spectra do have the potential to probe the primordial bump-like features. As the input amplitude of the bump is decreased below roughly 10% of the amplitude of the primordial power spectrum without the bump, the uncertainties in the recovered values for both amplitude and location of the bump are found to increase, and the recovered values of the location of the bump also get increasingly more biased towards higher values. Hence, it becomes harder to constrain these parameters. In the second scenario, we analyze the achievable constraints on primordial features when two of the astrophysical parameters, namely, minimum halo mass and ionizing efficiency are uncertain. We find that the effect of the bump on the shape and the amplitude of the 21 cm power spectrum is distinct from the impact of varying the astrophysical parameters, and hence they may potentially be distinguished.

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S. Naik, P. Chingangbam and K. Furuuchi
Mon, 2 Jan 23
37/44

Comments: 26 pages

Hidden Symmetries, Rapid Turns and Cosmic Acceleration [CL]

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http://arxiv.org/abs/2212.14127


Hidden symmetries provide a powerful tool for finding exact solutions in multifield cosmological models. We review how, using such symmetries, one can find inflationary solutions in two-field models, which lead to the generation of primordial black holes. We also discuss an exact solution in a two-field cosmological model, which describes dark energy. This solution is obtained with the use of a hidden symmetry, although the latter is broken by a constant term in the scalar potential. All of the above solutions are characterized by field-space trajectories with rapid turns.

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L. Anguelova
Mon, 2 Jan 23
44/44

Comments: 12 pages, contribution to the proceedings of the international conference BPU11, Belgrade, 2022

The effective Equation of State in Palatini $f(R)$ cosmology [CL]

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http://arxiv.org/abs/2212.13825


We investigate how the cosmological Equation of State can be used for scrutinizing extended theories of gravity, in particular, the Palatini $f(R)$ gravity. Specifically, the approach consists, at first, in investigating the effective Equation of State produced by a given model. Then, the inverse problem can also be considered in view of determining which models are compatible with a given effective Equation of State. We consider and solve some cases and show that, for example, power-law models are (the only models) capable of transforming barotropic Equations of State into effective barotropic ones. Moreover, the form of Equation of State is preserved (only) for $f(R)=R$, as expected. In this perspective, modified Equations of State are a feature capable of distinguishing Extended Gravity with respect to General Relativity. We also investigate quadratic and non-homogeneous effective Equations of State showing, in particular, that they contain the Starobinsky model and other ones.

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S. S.Camera, S. S.Capozziello, L. L.Fatibene, et. al.
Thu, 29 Dec 22
1/47

Comments: 19 pages

Imprint of galactic rotation curves and metric fluctuations on the recombination era anisotropy [CEA]

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http://arxiv.org/abs/2212.13942


In applications of the conformal gravity theory it has been shown that a scale of order 105 Mpc due to large scale inhomogeneities such as clusters of galaxies is imprinted on the rotation curves of galaxies. Here we show that this same scale is imprinted on recombination era anisotropies in the cosmic microwave background. We revisit an analysis due to Mannheim and Horne, to show that in the conformal gravity theory the length scale of metric signals that originate in the primordial nucleosynthesis era at $10^{9\circ}$K can fill out the entire recombination era sky. Similarly, the length scale of acoustic signals that originate at $10^{13\circ}$K can also fill out the entire recombination era sky. We show that the amplitudes of metric fluctuations that originate in the nucleosynthesis era can grow by a factor of $10^{12}$ at recombination, and by a factor of $10^{18}$ at the current time. In addition we find that without any period of exponential expansion a length scale as small as $10^{-33}$ cm can grow to the size of the recombination sky if it begins to grow at a temperature of order $10^{33}$ degrees.

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P. Mannheim
Thu, 29 Dec 22
10/47

Comments: 16 pages

$α$-attractor inflation: Models and Predictions [CEA]

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http://arxiv.org/abs/2212.13363


The $\alpha$-attractor models are some of the most interesting models of inflation from the point of view of upcoming observations in cosmology and are also attractive from the point of view of supergravity. We confront representative models of exponential and polynomial $\alpha$-attractors with the latest cosmological data (Planck’ 18+BICEP2/Keck array) to obtain predictions and best-fit values of model parameters. The analysis is done by making use of ModeChord and CosmoMC plugged together via PolyChord.

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S. Bhattacharya, K. Dutta, M. Gangopadhyay, et. al.
Thu, 29 Dec 22
16/47

Comments: 7 pages, 6 figures

On the Schwarzschild-de Sitter metric of nonlocal de Sitter gravity [CL]

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http://arxiv.org/abs/2212.13896


Earlier constructed a simple nonlocal de Sitter gravity model has a cosmological solution in a very good agreement with astronomical observations. In this paper, we continue the investigation of the nonlocal de Sitter model of gravity, focusing on finding an appropriate solution for the Schwarzschild-de Sitter metric. We succeeded to solve the equations of motion in a certain approximation. The obtained approximate solution is of particular interest for examining the possible role of non-local de Sitter gravity in describing the effects in galactic dynamics that are usually attributed to dark matter.

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I. Dimitrijevic, B. Dragovich, Z. Rakic, et. al.
Thu, 29 Dec 22
38/47

Comments: 10 pages

Bouncing Cosmology in VCDM [CL]

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http://arxiv.org/abs/2212.13561


We construct an asymmetric bouncing scenario within the VCDM model, a modified gravity theory with two local physical degrees of freedom. The scenario is exempt of any ghost or gradient instability, and avoids any singularity problem, ad-hoc matching conditions or anisotropic stress issue (BKL instability). It moreover succeeds in generating the cosmological perturbations compatible with the observations. The scalar spectral index can be adapted by the choice of the equation of state of the matter sector and the form of the VCDM potential leading to an almost scale-invariant power spectrum. Satisfying the CMB bounds on the tensor-to-scalar ratio leads to a blue tensor spectrum.

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A. Ganz, P. Martens, S. Mukohyama, et. al.
Thu, 29 Dec 22
39/47

Comments: 14 pages, 8 figures

The effective Equation of State in Palatini $f(R)$ cosmology [CL]

Posted on by

http://arxiv.org/abs/2212.13825


We investigate how the cosmological Equation of State can be used for scrutinizing extended theories of gravity, in particular, the Palatini $f(R)$ gravity. Specifically, the approach consists, at first, in investigating the effective Equation of State produced by a given model. Then, the inverse problem can also be considered in view of determining which models are compatible with a given effective Equation of State. We consider and solve some cases and show that, for example, power-law models are (the only models) capable of transforming barotropic Equations of State into effective barotropic ones. Moreover, the form of Equation of State is preserved (only) for $f(R)=R$, as expected. In this perspective, modified Equations of State are a feature capable of distinguishing Extended Gravity with respect to General Relativity. We also investigate quadratic and non-homogeneous effective Equations of State showing, in particular, that they contain the Starobinsky model and other ones.

Read this paper on arXiv…

S. S.Camera, S. S.Capozziello, L. L.Fatibene, et. al.
Thu, 29 Dec 22
40/47

Comments: 19 pages

On the Schwarzschild-de Sitter metric of nonlocal de Sitter gravity [CL]

Posted on by

http://arxiv.org/abs/2212.13896


Earlier constructed a simple nonlocal de Sitter gravity model has a cosmological solution in a very good agreement with astronomical observations. In this paper, we continue the investigation of the nonlocal de Sitter model of gravity, focusing on finding an appropriate solution for the Schwarzschild-de Sitter metric. We succeeded to solve the equations of motion in a certain approximation. The obtained approximate solution is of particular interest for examining the possible role of non-local de Sitter gravity in describing the effects in galactic dynamics that are usually attributed to dark matter.

Read this paper on arXiv…

I. Dimitrijevic, B. Dragovich, Z. Rakic, et. al.
Thu, 29 Dec 22
41/47

Comments: 10 pages

$α$-attractor inflation: Models and Predictions [CEA]

Posted on by

http://arxiv.org/abs/2212.13363


The $\alpha$-attractor models are some of the most interesting models of inflation from the point of view of upcoming observations in cosmology and are also attractive from the point of view of supergravity. We confront representative models of exponential and polynomial $\alpha$-attractors with the latest cosmological data (Planck’ 18+BICEP2/Keck array) to obtain predictions and best-fit values of model parameters. The analysis is done by making use of ModeChord and CosmoMC plugged together via PolyChord.

Read this paper on arXiv…

S. Bhattacharya, K. Dutta, M. Gangopadhyay, et. al.
Thu, 29 Dec 22
44/47

Comments: 7 pages, 6 figures

The causality road from dynamical triangulations to quantum gravity that describes our Universe [CL]

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http://arxiv.org/abs/2212.13109


It is shown how one, guided by causality, starting from so-called dynamical triangulations, is led to a candidate of quantum gravity that describes our Universe. This theory is based on W- and Jordan algebras. It explains how our Universe was created, how cosmic inflation began and ended, how the topology and the geometry of our Universe was formed, and what was the origin of Big Bang energy. The theory also leads to a modified Friedmann equation which explains the present accelerating expansion of our Universe without appealing to the cosmological constant.

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Y. Watabiki
Tue, 27 Dec 22
17/30

Comments: This is a contribution to the Handbook of Quantum Gravity which will be published in 2023. It will appear as a chapter in the section of the handbook denoted “Causal Dynamical Triangulations”

Constraints on the scalar inflation from preheating of LATTICEEASY [CEA]

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http://arxiv.org/abs/2212.12851


In this paper, we study scalar inflation in detail by applying the preheating of LATTICEEASY simulation. In general, scalar inflation potential with non-minimal coupling can be approximated to the quartic potential inflation. We observe that the evolutionary trend of this potential is independent of the coupling coefficient, and theoretical predictions for the scalar spectral index $n_s$ and tensor-to-scalar power ratio $r$ are also independent of the coefficient, which implies that the coefficients of this model will not be bounded by the Planck observations. Fortunately, the properties of preheating after inflation provide a feasible scheme to study those coefficients. For the preheating process, the evolution of particle number density, scale factor, and energy density can be restored and tracked by applying LATTICEEASY simulation, then the parameters energy ratio $\gamma$ and the e-folding number of preheating $N_{pre}$ will be deduced, and the $n_s$ and $r$ can be further predicted. We have tested the scalar inflation model by the preheating nature of LATTICEEASY based on the analytical relationship between preheating and inflation models.

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W. Cheng and T. Qin
Tue, 27 Dec 22
22/30

Comments: 13 pages, 6 figures