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

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

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

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

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)

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

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

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

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

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

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!

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

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

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