Tag Archives: Mathematical Physics

Self-accelerated universe induced by repulsive effects as an alternative to dark energy and modified gravities [CL]

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http://arxiv.org/abs/1507.06446


The existence of current-time universe’s acceleration is usually modeled by means of two main strategies. The first makes use of a dark energy barotropic fluid entering \emph{by hand} the energy-momentum tensor of Einstein’s theory. The second lies on extending the Hilbert-Einstein action giving rise to the class of extended theories of gravity. In this work, we propose a third approach, derived as an intrinsic geometrical effect of space-time, which provides repulsive regions under certain circumstances. We demonstrate that the effects of repulsive gravity naturally emerge in the field of a homogeneous and isotropic universe. To this end, we use an invariant definition of repulsive gravity based upon the behavior of the curvature eigenvalues. Moreover, we show that repulsive gravity counterbalances the standard gravitational attraction influencing both late and early times of the universe evolution. This phenomenon leads to the present speed up and to the fast expansion due to the inflationary epoch. In so doing, we are able to unify both dark energy and inflation in a single scheme, showing that the universe changes its dynamics when ${\ddot H\over H}=-2\dot H$, at the repulsion onset time where this condition is satisfied. Further, we argue that the spatial scalar curvature can be taken as vanishing because it does not affect at all the emergence of repulsive gravity. We check the goodness of our approach through two cosmological fits involving the most recent union 2.1 supernova compilation.

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O. Luongo and H. Quevedo
Fri, 24 Jul 15
12/41

Comments: 5 pages, 2 figures

A stochastic quasi-classical wavefunction of the Universe from the third quantization procedure [CL]

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http://arxiv.org/abs/1507.06507


(abbreviated) We study quantized solutions of WdW equation describing a closed FRW universe with a $\Lambda $ term and a set of massless scalar fields. We show that when $\Lambda \ll 1$ in the natural units and the standard $in$-vacuum state is considered, either wavefunction of the universe, $\Psi$, or its derivative with respect to the scale factor, $a$, behave as random quasi-classical fields at sufficiently large values of $a$, when $1 \ll a \ll e^{{2\over 3\Lambda}}$ or $a \gg e^{{2\over 3\Lambda}}$, respectively. Statistical r.m.s value of the wavefunction is proportional to the Hartle-Hawking wavefunction for a closed universe with a $\Lambda $ term. Alternatively, the behaviour of our system at large values of $a$ can be described in terms of a density matrix corresponding to a mixed state, which is directly determined by statistical properties of $\Psi$. It gives a non-trivial probability distribution over field velocities. We suppose that a similar behaviour of $\Psi$ can be found in all models exhibiting copious production of excitations with respect to $out$-vacuum state associated with classical trajectories at large values of $a$. Thus, the third quantization procedure may provide a ‘boundary condition’ for classical solutions of WdW equation.

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P. Ivanov and S. Chernov
Fri, 24 Jul 15
30/41

Comments: To be published in PRD

A geometrical approach to gravitational lensing magnification [CL]

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http://arxiv.org/abs/1507.05099


The standard definition of gravitational lensing magnification is generalized to Lorentzian spacetimes, and it is shown how it can be interpreted geometrically in terms of the van Vleck determinant and the exponential map. This is joint work with Amir B. Aazami (Kavli IPMU, University of Tokyo).

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M. Werner
Tue, 21 Jul 15
41/74

Comments: 4 pages, for the Proceedings of the 14th Marcel Grossmann Meeting, based on gr-qc/1507.02765

The geometry of gravitational lensing magnification [CL]

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http://arxiv.org/abs/1507.02765


We present a definition of unsigned magnification in gravitational lensing valid on arbitrary convex normal neighborhoods of time oriented Lorentzian manifolds. This definition is a function defined at any two points along a null geodesic that lie in a convex normal neighborhood, and foregoes the usual notions of lens and source planes in gravitational lensing. Rather, it makes essential use of the van Vleck determinant, which we present via the exponential map, and Etherington’s definition of luminosity distance for arbitrary spacetimes. We then specialize our definition to spacetimes, like Schwarzschild’s, in which the lens is compact and isolated, and show that our magnification function is monotonically increasing along any geodesic contained within a convex normal neighborhood.

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A. Aazami and M. Werner
Mon, 13 Jul 15
42/48

Comments: 16 pages

Finite-Time Singularities in $k=0$ FLRW Cosmologies [CL]

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http://arxiv.org/abs/1507.02241


In this paper, we consider a spatially flat FLRW cosmological model with matter obeying a barotropic equation of state $p = w \mu$, $-1<w\leq1$, and a cosmological constant, $\Lambda$. We use Osgood’s criterion to establish three cases when such models admit finite-time singularities. The first case is for an arbitrary initial condition, with a negative cosmological constant, and phantom energy $w < -1$. We show that except for a very fine-tuned choice of the initial condition $\theta_{0}$, the universe will develop a finite-time singularity. The second case we consider is for a nonnegative cosmological constant, phantom energy, and the expansion scalar being larger than that of the flat-space de Sitter solution, and show that such models only expand forever for $\Lambda = 0$. In all other cases, the universe model develops a finite-time singularity. The final case we consider is for a nonnegative cosmological constant, a matter source with $-1 < w \leq 1$, and an expansion scalar that is asymptotically that of the de Sitter universe. We show that such models will only expand forever when $\Lambda = 0$, otherwise, they will develop a finite-time singularity. This is significant, since the inflationary epoch is a subset of this domain. However, as we show, the inclusion of a bulk viscosity term in the Einstein field equations eliminates this singularity, and the universe expands forever. This could have interesting implications for the role of bulk viscosity in dynamical models of the universe.

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I. Kohli
Thu, 9 Jul 15
7/50

Comments: arXiv admin note: text overlap with arXiv:1505.07770

Towards relativistic quantum geometry [CL]

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http://arxiv.org/abs/1506.09141


We obtain a gauge-invariant relativistic quantum geometry by using a Weylian-like integrable manifold with a geometric scalar field which provides a gauge-invariant relativistic quantum theory in which the algebra of the Weylian-like field depends on observers. An example for a Reisnn\”er-Nordstr\”om black-hole is studied.

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L. Ridao and M. Bellini
Wed, 1 Jul 15
24/67

Comments: 6 pages, no figures

Exact solutions and spacetime singularities in nonlocal gravity [CL]

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http://arxiv.org/abs/1506.08619


We give here a list of exact classical solutions of a large class of weakly nonlocal theories of gravity, which are unitary and super-renormalizable (or finite) at quantum level. It is explicitly shown that flat and Ricci-flat spacetimes as well as maximally symmetric manifolds are exact solutions of the equation of motion. Therefore, well-known physical spacetimes like Schwarzschild, Kerr, (Anti-) de Sitter serve as solutions for standard matter content. In dimension higher than four we can also have Anti-de Sitter solutions in the presence of positive cosmological constant. We pedagogically show how to obtain these exact solutions. Furthermore, for another version of the theory, written in the Weyl basis, Friedmann-Robertson-Walker (FRW) spacetimes are also exact solutions, when the matter content is given by conformal matter (radiation). We also comment on the presence of singularities and possible resolution of them in finite and conformally invariant theories. “Delocalization” is proposed as a way to solve the black hole singularity problem. In order to solve the problem of cosmological singularities it seems crucial to have a conformally invariant or asymptotically free quantum gravitational theory.

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Y. Li, L. Modesto and L. Rachwal
Tue, 30 Jun 15
66/75

Comments: 33 pages

Shadowing Lemma and Chaotic Orbit Determination [EPA]

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http://arxiv.org/abs/1506.03221


Orbit determination is possible for a chaotic orbit of a dynamical system, given a finite set of observations, provided the initial conditions are at the central time. In a simple discrete model, the standard map, we tackle the problem of chaotic orbit determination when observations extend beyond the predictability horizon. If the orbit is hyperbolic, a shadowing orbit is computed by the least squares orbit determination. We test both the convergence of the orbit determination iterative procedure and the behaviour of the uncertainties as a function of the maximum number $n$ of map iterations observed. When the initial conditions belong to a chaotic orbit, the orbit determination is made impossible by numerical instability beyond a computability horizon, which can be approximately predicted by a simple formula. Moreover, the uncertainty of the results is sharply increased if a dynamical parameter is added to the initial conditions as parameter to be estimated. The uncertainty of the dynamical parameter decreases like $n^a$ with $a<0$ but not large (of the order of unity). If only the initial conditions are estimated, their uncertainty decreases exponentially with $n$. If they belong to a non-chaotic orbit the computational horizon is much larger, if it exists at all, and the decrease of the uncertainty is polynomial in all parameters, like $n^a$ with $a\simeq 1/2$. The Shadowing Lemma does not dictate what the asymptotic behaviour of the uncertainties should be. These phenomena have significant implications, which remain to be studied, in practical problems of orbit determination involving chaos, such as the chaotic rotation state of a celestial body and a chaotic orbit of a planet-crossing asteroid undergoing many close approaches.

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F. Spoto and A. Milani
Thu, 11 Jun 15
15/55

Comments: N/A

Rigorous treatment of the averaging process for co-orbital motions in the planetary problem [EPA]

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http://arxiv.org/abs/1506.02870


We develop a rigorous analytical Hamiltonian formalism adapted to the study of the motion of two planets in co-orbital resonance. By constructing a complex domain of holomorphy for the planetary Hamiltonian, we estimate the size of the transformation that maps this Hamiltonian to its first order averaged over one of the fast angles. After having derived an integrable approximation of the averaged problem, we bound the distance between this integrable approximation and the averaged Hamiltonian. This finally allows to prove rigorous theorems on the behavior of co-orbital motions over a finite but large timescale.

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P. Robutel and L. Niederman
Wed, 10 Jun 15
51/53

Comments: N/A

On solving dynamical equations in general homogeneous isotropic cosmologies with scalaron [CL]

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http://arxiv.org/abs/1506.01664


We study general gauge-dependent dynamical equations describing homogeneous isotropic cosmologies coupled to a scalar field $\psi$ (scalaron). For flat cosmologies ($k=0$), we analyze in detail the previously proposed gauge-independent equation describing the differential, $\chi(\alpha)$, of the map of the metric $\alpha$ to the scalaron $\psi$, which is the main mathematical characteristic (`portrait’) of cosmologies in $\alpha$-version. In a more habitual $\psi$-version, the similar equation for the differential of the inverse map, $\bar{\chi}(\psi)$, can be solved asymptotically or for special scalaron potentials $v(\psi)$.
In the $\alpha$-version the whole dynamical system is explicitly integrable for $k\neq 0$ and any `potential’ $\bar{v}(\alpha)$ replacing $v(\psi)$. There is no \textit{a priori} relation between the two potentials before deriving $\chi$, which depends on the potential itself, though relations between the two pictures can be found in asymptotic regions. An alternative proposal is to specify a cosmology by assuming a characteristic solution or its phase portrait and then finding the potentials from the solutions of the dynamical equations. Our main subject is the mathematical structure of cosmologies, but possible applications of the results are briefly discussed.

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A. Filippov
Fri, 5 Jun 15
39/63

Comments: 27 pages

On Singularities in Cosmic Inflation [CL]

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http://arxiv.org/abs/1505.07770


In this paper, we examine a flat FLRW spacetime with a scalar field potential and show by applying Osgood’s criterion to the Einstein field equations that all such models, irrespective of the particular choice of potential develop finite-time singularities. That is, we show that solutions to the field equations rapidly diverge in finite time. This can have important implications for the role of inflation in cosmological models, since one of the implications of this is that within the inflationary epoch, a singularity develops in finite time, which would call into question the role of inflation in the dynamic evolution of our universe. We further point out that a possible reason for this behaviour is that the solutions to the field equations in such inflationary scenarios do not obey global existence and uniqueness properties, which is a typical characteristic of solutions that diverge in finite time.

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I. Kohli
Fri, 29 May 15
25/68

Comments: For submission to: Classical and Quantum Gravity

A new approach to the analysis of the phase space of f(R)-gravity [CL]

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http://arxiv.org/abs/1505.06015


We propose a new dynamical system formalism for the analysis of f(R) cosmologies. The new approach eliminates the need for cumbersome inversions to close the dynamical system and allows the analysis of the phase space of f(R)-gravity models which cannot be investigated using the standard technique. Differently form previously proposed similar techniques, the new method is constructed in such a way to associate to the fixed points scale factors, which contain four integration constants (i.e. solutions of fourth order differential equations). In this way a new light is shed on the physical meaning of the fixed points. We apply this technique to some f(R) Lagrangians relevant for inflationary and dark energy models.

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S. Carloni
Mon, 25 May 15
19/47

Comments: 32 pages, 10 figures

A 3+1 formalism for quantum electrodynamical corrections to Maxwell equations in general relativity [HEAP]

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http://arxiv.org/abs/1505.05847


Magnetized neutron stars constitute a special class of compact objects harbouring gravitational fields that deviate strongly from the Newtonian weak field limit. Moreover strong electromagnetic fields anchored into the star give rise to non-linear corrections to Maxwell equations described by quantum electrodynamics (QED). Electromagnetic fields close to or above the critical value of $\BQ=4.4\times10^9$~T are probably present in some pulsars and for most of the magnetars. To account properly for emission emanating from the neutron star surface like for instance thermal radiation and its polarization properties, it is important to include general relativistic (GR) effects simultaneously with non-linear electrodynamics. This can be achieved through a 3+1 formalism known in general relativity and that incorporates QED perturbations to Maxwell equations. Starting from the lowest order corrections to the Lagrangian for the electromagnetic field, as given for instance by Born-Infeld or Euler-Heisenberg theory, we derive the non-linear Maxwell equations in general relativity including quantum vacuum effects. We also derive a prescription for the force-free limit and show that these equations can be solved with classical finite volume methods for hyperbolic conservation laws. It is therefore straightforward to include general relativity and quantum electrodynamics in the description of neutron star magnetospheres by using standard classical numerical techniques borrowed from Maxwell and Newton theory. As an application, we show that spin-down luminosity corrections associated to QED effects are negligible with respect to GR corrections.

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J. Petri
Fri, 22 May 15
9/67

Comments: Accepted for publication in Monthly Notices of the Royal Astronomical Society Main Journal

Post-Newtonian effects on the stability of the triangular solution in the three-body problem for general masses [CL]

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http://arxiv.org/abs/1505.04534


Continuing work initiated in earlier publications [Ichita, Yamada and Asada, Phys. Rev. D {\bf 83}, 084026 (2011); Yamada and Asada, Phys. Rev. D {\bf 86}, 124029 (2012)], we examine the post-Newtonian (PN) effects on the stability of the triangular solution in the relativistic three-body problem for general masses. For three finite masses, a condition for stability of the triangular solution is obtained at the first post-Newtonian (1PN) order, and it recovers previous results for the PN restricted three-body problem when one mass goes to zero. The stability regions still exist even at the 1PN order, though the PN triangular configuration for general masses is less stable than the PN restricted three-body case as well as the Newtonian one.

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K. Yamada, T. Tsuchiya and H. Asada
Tue, 19 May 15
2/78

Comments: 23 pages, 4 figures; accepted by PRD

Sharp bounds on the radius of relativistic charged spheres: Guilfoyle's stars saturate the Buchdahl-Andréasson bound [CL]

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http://arxiv.org/abs/1505.03863


Buchdahl, by imposing a few physical assumptions on the matter, i.e., its density is a nonincreasing function of the radius and the fluid is a perfect fluid, and on the configuration, such as the exterior is the Schwarzschild solution, found that the radius $r_0$ to mass $m$ ratio of a star would obey the Buchdahl bound $r_0/m\geq9/4$. He noted that the bound was saturated by the Schwarzschild interior solution, the solution with $\rho_{\rm m}(r)= {\rm constant}$, where $\rho_{\rm m}(r)$ is the energy density of the matter at $r$, when the central central pressure blows to infinity. Generalizations of this bound have been studied. One generalization was given by Andr\’easson by including electrically charged matter and imposing that $p+2p_T \leq\rho_{\rm m}$, where $p$ is the radial pressure and $p_T$ the tangential pressure. His bound is given by $r_0/m\geq9/\left(1+\sqrt{1+3\,q^2/r_0^2}\right)^{2}$, the Buchdahl-Andr\’easson bound, with $q$ being the star’s total electric charge. Following Andr\’easson’s proof, the configuration that saturates the Buchdahl bound is an uncharged shell, rather than the Schwarzschild interior solution. By extension, the configurations that saturate the Buchdahl-Andr\’easson bound are charged shells. One expects then, in turn, that there should exist an electrically charged equivalent to the interior Schwarzschild limit. We find here that this equivalent is provided by the equation $\rho_{\rm m}(r) + {Q^2(r)}/ {\left(8\pi\,r^4\right)}= {\rm constant}$, where $Q(r)$ is the electric charge at $r$. This equation was put forward by Cooperstock and de la Cruz, and Florides, and realized in Guilfoyle’s stars. When the central pressure goes to infinity Guilfoyle’s stars are configurations that saturate the Buchdahl-Andr\’easson bound. It remains to find a proof in Buchdahl’s manner such that these configurations are the limiting configurations of the bound.

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J. Lemos and V. Zanchin
Mon, 18 May 15
16/39

Comments: 10 pages, 3 figures

Equivalent off-diagonal cosmological models and ekpyrotic scenarios in f(R)-modified, massive and Einstein gravity [CL]

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http://arxiv.org/abs/1504.04346


We re-investigate how generic off-diagonal cosmological solutions depending, in general, on all spacetime coordinates can be constructed in massive and f-modified gravity using the anholonomic frame deformation method. There are constructed new classes of locally anisotropic and (in) homogeneous cosmological metrics with open and closed spatial geometries. By resorting such solutions, we show that they describe the late time acceleration due to effective cosmological terms induced by nonlinear off-diagonal interactions, possible modifications of the gravitational action and graviton mass. The cosmological metrics and related St\” uckelberg fields are constructed in explicit form up to nonholonomic frame transforms of the Friedmann-Lama\^{\i}tre-Robertson-Walker (FLRW) coordinates. The solutions include matter, graviton mass and other effective sources modelling nonlinear gravitational and matter fields interactions with polarization of physical constants and deformations of metrics, which may explain dark energy and dark matter effects. However, we argue that it is not obligatory always to modify gravity if we consider effective generalized Einstein equations with nontrivial vacuum and/or non-minimal coupling with matter. Indeed, we state certain conditions when such configurations mimic interesting solutions in general relativity and modifications, for instance, when we can extract the general Painlev\’ e-Gullstrand and FLRW metrics. In a more general context, we elaborate on a reconstruction procedure for off-diagonal cosmological solutions which describe cyclic and ekpyrotic universes. Finally, there are discussed open issues and further perspectives.

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S. Vacaru
Wed, 6 May 15
6/74

Comments: 16 pages, latex2e; it is a regular article variant extended and modified following requests of the Editor and accepted by EPJC; this version contains new results and details comparing to the letter variant arXiv:1304.1080

Self Gravitating Incompressible Fluid in Two Dimensions [EPA]

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http://arxiv.org/abs/1504.02798


In this paper we develop two models for the steady states and evolution of two dimensional isothermal self gravitating and rotating incompressible gas which are based on the hydrodynamic equations for stratified fluid. The first model is for the steady states of the gas while the second addresses the time evolution of the gas subject to some constraints. These models reduce the initial five partial differential equations that govern this system to two for the steady state model and to three for the time dependent model. Analytical and numerical solutions of the model equations are used to study the structure of the resulting steady and time dependent states of the fluid with some possible astrophysical applications.

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M. Humi and Z. Tian
Tue, 14 Apr 15
2/87

Comments: N/A

Comment on the Hojman conservation quantities in Cosmology [CL]

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http://arxiv.org/abs/1503.08466


We comment upon the application of Hojman’s method for the determination of conservation laws in Cosmology, which has been introduced by Capozziello \& Roshan (Phys. Lett. B 726 (2013) 471 (arXiv:1308.3910)), and has been applied recently in the cosmological scenario of a nonminimally coupled scalar field by Paolella \& Capozziello (Phys. Lett. A (2015), in press (arXiv:1503.00098)). We apply the Ansatz, $\phi\left( t\right) =\phi\left( a\left( t\right) \right) $, which was introduced by the cited authors for a minimally-coupled scalar field, and we study the Lie and Noether point symmetries for the reduced equation. We show that under this Ansatz the unknown function of the model cannot be constrained by the requirement of the existence of a conservation law and that the Hojman conservation quantity which arises for the reduced equation is nothing more than the functional form of the Noether conservation law of momentum for the free particle. Finally we show that Hojman’s method for Hamiltonian systems, in which the Hamiltonian function is one of the involved equations of the system, is equivalent with the application of Noether’s Theorem for generalized transformations.

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A. Paliathanasis and P. Leach
Tue, 31 Mar 15
27/71

Comments: 4 pages

Cosmological black holes and white holes with time-dependent mass [CL]

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http://arxiv.org/abs/1502.01003


We consider the causal structure of generalized uncharged McVittie spacetimes with increasing central mass $m (t)$ and positive Hubble factor $H (t)$. Under physically reasonable conditions, namely, a big bang singularity in the past, a positive cosmological constant and an upper limit to the central mass, we prove that the patch of the spacetime described by the cosmological time and areal radius coordinates is always geodesically incomplete, which implies the presence of event horizons in the spacetime. We also show that, depending on the asymptotic behavior of the $m$ and $H$ functions, the generalized McVittie spacetime can have a single black hole, a black-hole/white-hole pair or, differently from classic fixed-mass McVittie, a single white hole. A simple criterion is given to distinguish the different causal structures.

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A. Silva, D. Guariento and C. Molina
Wed, 4 Feb 15
57/59

Comments: 12 pages, 5 figures

A study of the main resonances outside the geostationary ring [CL]

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http://arxiv.org/abs/1501.06273


We investigate the dynamics of satellites and space debris in external resonances, namely in the region outside the geostationary ring. Precisely, we focus on the 1:2, 1:3, 2:3 resonances, which are located at about 66 931.4 km, 87 705.0 km, 55 250.7 km, respectively. Some of these resonances have been already exploited in space missions, like XMM-Newton and Integral.
Our study is mainly based on a Hamiltonian approach, which allows us to get fast and reliable information on the dynamics in the resonant regions. Significative results are obtained even by considering just the effect of the geopotential in the Hamiltonian formulation. For objects (typically space debris) with high area-to-mass ratio the Hamiltonian includes also the effect of the solar radiation pressure. In addition, we perform a comparison with the numerical integration in Cartesian variables, including the geopotential, the gravitational attraction of Sun and Moon, and the solar radiation pressure.
We implement some simple mathematical tools that allows us to get information on the terms which are dominant in the Fourier series expansion of the Hamiltonian around a given resonance, on the amplitude of the resonant islands and on the location of the equilibrium points. We also compute the Fast Lyapunov Indicators, which provide a cartography of the resonant regions, yielding the main dynamical features associated to the external resonances. We apply these techniques to analyze the 1:2, 1:3, 2:3 resonances; we consider also the case of objects with large area-to-mass ratio and we provide an application to the case studies given by XMM-Newton and Integral.

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A. Celletti and C. Gales
Tue, 27 Jan 15
64/79

Comments: 30 pages, 10 figures

Symmetries of Differential equations and Applications in Relativistic Physics [CL]

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http://arxiv.org/abs/1501.05129


In this thesis, we study the one parameter point transformations which leave invariant the differential equations. In particular we study the Lie and the Noether point symmetries of second order differential equations. We establish a new geometric method which relates the point symmetries of the differential equations with the collineations of the underlying manifold where the motion occurs. This geometric method is applied in order the two and three dimensional Newtonian dynamical systems to be classified in relation to the point symmetries; to generalize the Newtonian Kepler-Ermakov system in Riemannian spaces; to study the symmetries between classical and quantum systems and to investigate the geometric origin of the Type II hidden symmetries for the homogeneous heat equation and for the Laplace equation in Riemannian spaces. At last but not least, we apply this geometric approach in order to determine the dark energy models by use the Noether symmetries as a geometric criterion in modified theories of gravity.

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A. Paliathanasis
Thu, 22 Jan 15
2/58

Comments: PhD Thesis; University of Athens (2014); 285 pages N.D.C of Greece this http URL

An Instability of the Standard Model Creates the Anomalous Acceleration Without Dark Energy [CL]

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http://arxiv.org/abs/1412.4001


We introduce a new asymptotic ansatz for spherical perturbations of the Standard Model of Cosmology (SM) which applies during the $p=0$ epoch, and prove that these perturbations trigger instabilities in the SM on the scale of the supernova data. These instabilities create a large, central region of uniform under-density which expands faster than the SM, and this central region of accelerated uniform expansion introduces into the SM {\it precisely} the same range of corrections to redshift vs luminosity as are produced by the cosmological constant in the theory of Dark Energy. A universal behavior is exhibited because all sufficiently small perturbations evolve to a single stable rest point. Moreover, we prove that these perturbations are consistent with, and the instability is triggered by, the one parameter family of self-similar waves which the authors previously proposed as possible time-asymptotic wave patterns for perturbations of the SM at the end of the radiation epoch. Using numerical simulations, we calculate the unique wave in the family that accounts for the same values of the Hubble constant and quadratic correction to redshift vs luminosity as in a universe with seventy percent Dark Energy, $\Omega_{\Lambda}\approx.7$. A numerical simulation of the third order correction associated with that unique wave establishes a testable prediction that distinguishes this theory from the theory of Dark Energy. This explanation for the anomalous acceleration, based on instabilities in the SM together with simple wave perturbations from the radiation epoch that trigger them, provides perhaps the simplest mathematical explanation for the anomalous acceleration of the galaxies that does not invoke Dark Energy.

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J. Smoller, B. Temple and Z. Vogler
Mon, 15 Dec 14
36/53

Comments: N/A

The principle of stationary nonconservative action for classical mechanics and field theories [CL]

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http://arxiv.org/abs/1412.3082


We further develop a recently introduced variational principle of stationary action for problems in nonconservative classical mechanics and extend it to classical field theories. The variational calculus used is consistent with an initial value formulation of physical problems and allows for time-irreversible processes, such as dissipation, to be included at the level of the action. In this formalism, the equations of motion are generated by extremizing a nonconservative action $\mathcal{S}$, which is a functional of a doubled set of degrees of freedom. The corresponding nonconservative Lagrangian contains a potential $K$ which generates nonconservative forces and interactions. Such a nonconservative potential can arise in several ways, including from an open system interacting with inaccessible degrees of freedom or from integrating out or coarse-graining a subset of variables in closed systems. We generalize Noether’s theorem to show how Noether currents are modified and no longer conserved when $K$ is non-vanishing. Consequently, the nonconservative aspects of a physical system are derived solely from $K$. We show how to use the formalism with examples of nonconservative actions for discrete systems including forced damped harmonic oscillators, radiation reaction on an accelerated charge, and RLC circuits. We present examples for nonconservative classical field theories. Our approach naturally allows for irreversible thermodynamic processes to be included in an unconstrained variational principle. We present the nonconservative action for a Navier-Stokes fluid including the effects of viscous dissipation and heat diffusion, as well as an action that generates the Maxwell model for viscoelastic materials, which can be easily generalized to more realistic rheological models. We show that the nonconservative action can be derived as the classical limit of a more complete quantum theory.

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C. Galley, D. Tsang and L. Stein
Wed, 10 Dec 14
43/61

Comments: 42 pages, 4 figures, 1 table

The effect of Poynting-Robertson drag on the triangular Lagrangian points [EPA]

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http://arxiv.org/abs/1412.1630


We investigate the stability of motion close to the Lagrangian equilibrium points L4 and L5 in the framework of the spatial, elliptic, restricted three- body problem, subject to the radial component of Poynting-Robertson drag. For this reason we develop a simplified resonant model, that is based on averaging theory, i.e. averaged over the mean anomaly of the perturbing planet. We find temporary stability of particles displaying a tadpole motion in the 1:1 resonance. From the linear stability study of the averaged simplified resonant model, we find that the time of temporary stability is proportional to beta a1 n1 , where beta is the ratio of the solar radiation over the gravitational force, and a1, n1 are the semi-major axis and the mean motion of the perturbing planet, respectively. We extend previous results (Murray (1994)) on the asymmetry of the stability indices of L4 and L5 to a more realistic force model. Our analytical results are supported by means of numerical simulations. We implement our study to Jupiter-like perturbing planets, that are also found in extra-solar planetary systems.

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C. Lhotka and A. Celletti
Fri, 5 Dec 14
36/56

Comments: 47 pages, 8 figures,

Self-gravitating systems in Extended Gravity [CL]

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http://arxiv.org/abs/1411.3143


Starting from the weak field limit, we discuss astrophysical applications of Extended Theories of Gravity where higher order curvature invariants and scalar fields are considered by generalizing the Hilbert-Einstein action linear in the Ricci curvature scalar $R$. Results are compared to General Relativity in the hypothesis that Dark Matter contributions to the dynamics can be neglected thanks to modified gravity. In particular, we consider stellar hydrostatic equilibrium, galactic rotation curves, and gravitational lensing. Finally, we discuss the weak field limit in the Jordan and Einstein frames pointing out how effective quantities, as gravitational potentials, transform from one frame to the other and the interpretation of results can completely change accordingly.

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A. Stabile and S. Capozziello
Thu, 13 Nov 14
36/49

Comments: Review paper; 59 pages, 15 figures

On the Geometric Structure of Flows I: The Referential Gradient. A Generally Covariant Measure of Flow Geometry [CL]

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http://arxiv.org/abs/1411.2283


Assuming a-priori a smooth generating vector field, we introduce a generally covariant measure of the flow geometry called the referential gradient of the flow. The main result is the explicit relation between the referential gradient and the generating vector field, and is provided for from two equivalent perspectives: a Lagrangian specification with respect to a generalized parameter, and an Eulerian specification making explicit the evolution dynamics. Furthermore, we provide explicit non-trivial conditions which govern the transformation properties of the referential gradient object.

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J. Edmondson
Tue, 11 Nov 14
60/61

Comments: Communications in Mathematical Physics. Submitted 9 Nov 2014

Dark Energy and Dark Matter in Stars Physic [CL]

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http://arxiv.org/abs/1411.0242


We present the basic equations and relations for the relativistic static spherically symmetric stars (SSSS) in the model of minimal dilatonic gravity (MDG) which is {\em locally} equivalent to the f(R) theories of gravity and gives an alternative description of the effects of dark matter and dark energy. The results for the simplest form of the relativistic equation of state (EOS) of neutron matter are represented. Our approach overcomes the well-known difficulties of the physics of SSSS in the f(R) theories of gravity introducing two novel EOS for cosmological energy-pressure densities and dilaton energy-pressure densities, as well as proper boundary conditions.

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P. Fiziev
Wed, 5 Nov 14
41/61

Comments: Latex file, 7 pages, 11 figures. Symposium “Frontiers of Fundamental Physics 14”, Marseille, France. arXiv admin note: substantial text overlap with arXiv:1402.2813

Dynamical symmetries and observational constraints in scalar field cosmology [CL]

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http://arxiv.org/abs/1410.4930


We propose to use dynamical symmetries of the field equations, in order to classify the dark energy models in the context of scalar field (quintessence or phantom) FLRW cosmologies. Practically, symmetries provide a useful mathematical tool in physical problems since they can be used to simplify a given system of differential equations as well as to determine the integrability of the physical system. The requirement that the field equations admit dynamical symmetries results in two potentials one of which is the well known Unified Dark Matter (UDM) potential and another new potential. For each hyperbolic potential we obtain the corresponding analytic solution of the field equations. The proposed analysis suggests that the requirement of the contact symmetry appears to be very competitive to other independent tests used to probe the functional form of a given potential and thus the associated nature of dark energy. Finally, in order to test the viability of the above scalar field models we perform a joint likelihood analysis using some of the latest cosmological data.

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A. Paliathanasis, M. Tsamparlis and S. Basilakos
Tue, 21 Oct 14
7/72

Comments: 20 pages; 4 figures; this paper has been accepted for publication by Phys. Rev. D

Lattice Universe: examples and problems [CL]

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http://arxiv.org/abs/1410.3909


We consider lattice Universes with spatial topologies $T\times T\times T$, $\; T\times T\times R\; $ and $\; T\times R\times R$. In the Newtonian limit of General Relativity, we solve the Poisson equation for the gravitational potential in the enumerated models. In the case of point-like massive sources in the $T\times T\times T$ model, we demonstrate that the gravitational potential has no definite values on the edges joining identical masses in neighboring cells, i.e. at points where masses are absent. Clearly, this is a nonphysical result. The only way to avoid this problem and get a regular solution at any point of the cell is the smearing of these masses over some region. In the cases of $\; T\times T\times R\; $ and $\; T\times R\times R$ topologies, there is no way to get any physically reasonable and nontrivial solution. The only solutions we can get here are the ones which reduce these topologies to the $T\times T\times T$ one.

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M. Brilenkov, M. Eingorn and A. Zhuk
Thu, 16 Oct 14
17/55

Comments: 16 pages, 1 figure

General Relativity and Gravitation: A Centennial Perspective [CL]

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http://arxiv.org/abs/1409.5823


To commemorate the 100th anniversary of general relativity, the International Society on General Relativity and Gravitation (ISGRG) commissioned a Centennial Volume, edited by the authors of this article. We jointly wrote introductions to the four Parts of the Volume which are collected here. Our goal is to provide a bird’s eye view of the advances that have been made especially during the last 35 years, i.e., since the publication of volumes commemorating Einstein’s 100th birthday. The article also serves as a brief preview of the 12 invited chapters that contain in-depth reviews of these advances. The volume will be published by Cambridge University Press and released in June 2015 at a Centennial conference sponsored by ISGRG and the Topical Group of Gravitation of the American Physical Society.

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A. Ashtekar, B. Berger, J. Isenberg, et. al.
Tue, 23 Sep 14
15/60

Comments: 37 pages

A New Solution of Einstein Vacuum Field Equations [CL]

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http://arxiv.org/abs/1409.3758


A new solution of Einstein’s vacuum field equations is discovered which appears as a generalization of the well-known Ozsvath-Schucking solution and explains its source of curvature which has otherwise remained hidden. Curiously, the new solution has a vanishing Kretschmann scalar and is singularity-free despite being curved.
The discovery of the new solution is facilitated by a new insight which reveals that it is always possible to define the source of curvature in a vacuum solution in terms of some dimensional parameters. As the parameters vanish, so does the curvature. The new insight also helps to make the vacuum solutions Machian.

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R. Vishwakarma
Tue, 16 Sep 14
63/63

Comments: Pramana – J. Phys. (in press)

Friedmann's Equations in All Dimensions and Chebyshev's Theorem [CEA]

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http://arxiv.org/abs/1409.3352


This short but systematic work demonstrates a link between Chebyshev’s theorem and the explicit integration in cosmological time $t$ and conformal time $\eta$ of the Friedmann equations in all dimensions and with an arbitrary cosmological constant $\Lambda$. More precisely, it is shown that for spatially flat universes an explicit integration in $t$ may always be carried out, and that, in the non-flat situation and when $\Lambda$ is zero and the ratio $w$ of the pressure and energy density in the barotropic equation of state of the perfect-fluid universe is rational, an explicit integration may be carried out if and only if the dimension $n$ of space and $w$ obey some specific relations among an infinite family. The situation for explicit integration in $\eta$ is complementary to that in $t$. More precisely, it is shown in the flat-universe case with $\Lambda\neq0$ that an explicit integration in $\eta$ can be carried out if and only if $w$ and $n$ obey similar relations among a well-defined family which we specify, and that, when $\Lambda=0$, an explicit integration can always be carried out whether the space is flat, closed, or open. We also show that our method may be used to study more realistic cosmological situations when the equation of state is nonlinear.

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S. Chen, G. Gibbons, Y. Li, et. al.
Fri, 12 Sep 14
7/61

Comments: N/A

Strong shock in the uniformly expanding medium [CEA]

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http://arxiv.org/abs/1408.1918


Propagation of the strong shock in the flat expanding Friedman universe is investigated using methods of dimension and similarity. Exact analytic solution of self-similar equations is obtained, determining dependences of the radius and velocity of the shock wave on time and radius. It is obtained, that in the expanding medium the velocity of shock decreases as $\sim t^{-1/5}$, what is slower than the shock velocity in the static uniform medium $\sim t^{-3/5}$. The radius of the shock wave in the expanding self-gravitating medium increases $\sim t^{4/5}$, more rapidly than the shock wave radius in the uniform non-gravitating medium $\sim t^{2/5}$. So, the shock propagates in the direction of decreasing density with larger speed, that in the static medium, due to accelerating action of the decreasing density, even in the presence of a self-gravitation.

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G. Bisnovatyi-Kogan
Mon, 11 Aug 14
28/55

Comments: 16 pages

Galaxies with Supermassive Binary Black Holes: (II) A Model with Cuspy Galactic Density Profiles [GA]

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http://arxiv.org/abs/1408.0070


The existence and uniqueness of equilibrium points, including Lagrange Points and Jiang-Yeh Points, of a galactic system with supermassive binary black holes embedded in a centrally cuspy galactic halo are investigated herein. Differing from the previous results of non-cuspy galactic profiles that Jiang-Yeh Points only exist under a particular condition, it is found here that the Lagrange Points, L2, L3, L4 and L5, Jiang-Yeh Points, JY1 and JY2, exist under general conditions. The stability analysis shows that L2, L3, JY1 and JY2 are unstable. However, L4 and L5 are only unstable when the galactic total mass is smaller than a critical mass; otherwise they become neutrally stable centers. These results will be important for further studies on the cores of early-type galaxies.

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I. Jiang and L. Yeh
Mon, 4 Aug 14
27/40

Comments: 20 pages, 4 figures, accepted for publication in Astrophysics and Space Science

A fully relativistic radial fall [CL]

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http://arxiv.org/abs/1407.5391


Radial fall has historically played a momentous role. It is one of the most classical problems, the solutions of which represent the level of understanding of gravitation in a given epoch. A {\it gedankenexperiment} in a modern frame is given by a small body, like a compact star or a solar mass black hole, captured by a supermassive black hole. The mass of the small body itself and the emission of gravitational radiation cause the departure from the geodesic path due to the back-action, that is the self-force. For radial fall, as any other non-adiabatic motion, the instantaneous identity of the radiated energy and the loss of orbital energy cannot be imposed and provide the perturbed trajectory. In the first part of this letter, we present the effects due to the self-force computed on the geodesic trajectory in the background field. Compared to the latter trajectory, in the Regge-Wheeler, harmonic and all others smoothly related gauges, a far observer concludes that the self-force pushes inward (not outward) the falling body, with a strength proportional to the mass of the small body for a given large mass; further, the same observer notes an higher value of the maximal coordinate velocity, this value being reached earlier on during infall. In the second part of this letter, we implement a self-consistent approach for which the trajectory is iteratively corrected by the self-force, this time computed on osculating geodesics. Finally, we compare the motion driven by the self-force without and with self-consistent orbital evolution. Subtle differences are noticeable, even if self-force effects have hardly the time to accumulate in such a short orbit.

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A. Spallicci and P. Ritter
Tue, 22 Jul 14
31/45

Comments: To appear in Int. J. Geom. Meth. Mod. Phys

Partial Differential Equations with Random Noise in Inflationary Cosmology [CL]

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http://arxiv.org/abs/1407.4775


Random noise arises in many physical problems in which the observer is not tracking the full system. A case in point is inflationary cosmology, the current paradigm for describing the very early universe, where one is often interested only in the time-dependence of a subsystem. In inflationary cosmology it is assumed that a slowly rolling scalar field leads to an exponential increase in the size of space. At the end of this phase, the scalar field begins to oscillate and transfers its energy to regular matter. This transfer typically involves a parametric resonance instability. This article reviews work which the author has done in collaboration with Walter Craig studying the role which random noise can play in the parametric resonance instability of matter fields in the presence of the oscillatory inflaton field. We find that the particular idealized form of the noise studied here renders the instability more effective. As a corollary, we obtain a new proof of Anderson localization.

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R. Brandenberger
Fri, 18 Jul 14
41/76

Comments: 17 pages, 3 figures, invited contribution to the Fields Institute workshop “Hamiltonian PDEs: Analysis, Computations and Applications”, to be published in the proceedings

On precanonical quantization of gravity [CL]

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http://arxiv.org/abs/1407.3101


Precanonical quantization is based on the mathematical structures of the De Donder-Weyl Hamiltonization of field theories. The resulting formulation of quantum gravity describes the quantum geometry of space-time in terms of operator-valued distances and the transition amplitudes between the values of spin connection at different points of space-time, which obey the covariant precanonical analogue of the Schr\”odinger equation. In the context of quantum cosmology the theory predicts a probability distribution of a cosmological spin-connection field, which may have an observable impact on the large scale structures in the universe.

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I. Kanatchikov
Tue, 15 Jul 14
31/64

Comments: 6pp

Overcoming the Gauge Problem for the Gravitational Self-Force [CL]

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http://arxiv.org/abs/1406.7154


The gravitational waves emitted by binary systems with extreme-mass ratios carry unique astrophysical information that can only be detected by space-based detectors like eLISA. To that end, a very accurate modelling of the system is required. The gravitational self-force program, which has been fully developed in the Lorenz gauge, is the best approach we have so far. However, the computations required would be done more efficiently if we could work in other gauges, like the Regge-Wheeler (RW) one in the case of Schwarzschild black holes. In this letter we present a new scheme, based on the Particle-without-Particle formulation of the field equations, where the gravitational self-force can be obtained from just solving individual wave-type equations like the master equations of the RW gauge. This approach can help to tackle the yet unsolved Kerr case.

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P. Canizares and C. Sopuerta
Mon, 30 Jun 14
19/41

Comments: 5 pages, 1 figure

Curvature fluctuations on asymptotically de Sitter spacetimes via the semiclassical Einstein's equations [CL]

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http://arxiv.org/abs/1406.2223


It has been proposed recently to consider in the framework of cosmology an extension of the semiclassical Einstein’s equations in which the Einstein tensor is considered as a random function. This paradigm yields a hierarchy of equations between the $n$-point functions of the quantum, normal ordered, stress energy-tensor and those associated to the stochastic Einstein tensor. Assuming that the matter content is a conformally coupled massive scalar field on de Sitter spacetime, this framework has been applied to compute the power spectrum of the quantum fluctuations and to show that it is almost scale-invariant. We test the robustness and the range of applicability of this proposal by applying it to a less idealized, but physically motivated, scenario, namely we consider Friedmann-Robertson-Walker spacetimes which behave only asymptotically in the past as a de Sitter spacetime. We show in particular that, under this new assumption and independently from any renormalization freedom, the power spectrum associated to scalar perturbations of the metric behaves consistently with an almost scale-invariant power spectrum.

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C. Dappiaggi and A. Melati
Tue, 10 Jun 14
33/60

Comments: 23 pages

Novel representation of the general Heun's functions [CL]

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http://arxiv.org/abs/1405.6837


In the present article we introduce and study a novel representation of the general Heun’s functions. It is based on the symmetric form of the Heun’s differential equation and hopefully will simplify the solution of the existing basic open problems in the theory of these functions. The novel representation will stimulate also the development of new effective computational methods for calculations with the general Heun’s functions which at present is a quite problematic issue.

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P. Fiziev
Mon, 2 Jun 14
42/56

Comments: 11 pages LaTex file

Regularization of the big bang singularity with a time varying equation of state $w > 1$ [CL]

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http://arxiv.org/abs/1403.2122


We study the classical dynamics of the universe undergoing a transition from contraction to expansion through a big bang singularity. The dynamics is described by a system of differential equations for a set of physical quantities, such as the scale factor $a$, the Hubble parameter $H$, the equation of state parameter $w$, and the density parameter $\Omega$. The solutions of the dynamical system have a singularity at the big bang. We study if these solutions can be uniquely extended through the singularity. In particular, we consider the model in which the contracting universe is dominated by a scalar field with a time varying equation of state $w$, which approaches a constant value $w_c$ near the singularity. We prove that, for $w_c > 1$, the singularity is regularizable only for a discrete set of $w_c$ values that satisfy a coprime number condition. Our result implies that the evolution of a bouncing universe through the big bang singularity does not have a continuous classical limit unless the equation of state is extremely fine-tuned.

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B. Xue and E. Belbruno
Tue, 11 Mar 14
58/66

Recursive structure in the definitions of gauge-invariant variables for any order perturbations [CL]

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http://arxiv.org/abs/1403.1004


The construction of gauge-invariant variables for any order perturbations is discussed. Explicit constructions of the gauge-invariant variables for perturbations to 4th order are shown. From these explicit construction, the recursive structure in the definitions of gauge-invariant variables for any order perturbations is found. Through this recursive structure, the correspondence with the fully non-linear exact perturbations is briefly discussed.

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K. Nakamura
Thu, 6 Mar 14
48/53

A Riccati equation based approach to isotropic scalar field cosmologies with arbitrary self-interaction potentials [CL]

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http://arxiv.org/abs/1402.4363


Gravitationally coupled scalar fields $\phi $, distinguished by the choice of an effective self-interaction potential $V(\phi )$, simulating a temporarily non-vanishing cosmological term, can generate both inflation and late time acceleration. In scalar field cosmological models the evolution of the Hubble function is determined, in terms of the interaction potential, by a Riccati type equation. In the present work we investigate scalar field cosmological models that can be obtained as solutions of the Riccati evolution equation for the Hubble function. Four exact integrability cases of the field equations are presented, representing classes of general solutions of the Riccati evolution equation, and their cosmological properties are investigated in detail.

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T. Harko, F. Lobo and M. Mak
Wed, 19 Feb 14
38/50

Exploring Vacuum Energy in a Two-Fluid Bianchi Type I Universe [CL]

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http://arxiv.org/abs/1402.1967


We use a dynamical systems approach based on the method of orthonormal frames to study the dynamics of a two-fluid, non-tilted Bianchi Type I cosmological model. In our model, one of the fluids is a fluid with bulk viscosity, while the other fluid assumes the role of a cosmological constant and represents nonnegative vacuum energy. We begin by completing a detailed fixed-point analysis of the system which gives information about the local sinks, sources and saddles. We then proceed to analyze the global features of the dynamical system by using topological methods such as finding Lyapunov and Chetaev functions, and finding the $\alpha$- and $\omega$-limit sets using the LaSalle invariance principle. The fixed points found were a flat Friedmann-LeMa\^{\i}tre-Robertson-Walker (FLRW) universe with no vacuum energy, a de Sitter universe, a flat FLRW universe with both vacuum and non-vacuum energy, and a Kasner quarter-circle universe. We also show in this paper that the vacuum energy we observe in our present-day universe could actually be a result of the bulk viscosity of the ordinary matter in the universe, and proceed to calculate feasible values of the bulk viscous coefficient based on observations reported in the Planck data. We conclude the paper with some numerical experiments that shed further light on the global dynamics of the system.

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I. Kohli and M. Haslam
Tue, 11 Feb 14
43/55

Stochastic perturbation of the two-body problem [CL]

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http://arxiv.org/abs/1402.1752


We study the impact of a stochastic perturbation on the classical two-body problem in particular concerning the preservation of first integrals and the Hamiltonian structure. Numerical simulations are performed which illustrate the dynamical behavior of the osculating elements as the semi-major axis, the eccentricity and the pericenter. We also derive a stochastic version of Gauss’s equations in the planar case.

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C. Jacky, P. Frederic and P. Benedicte
Mon, 10 Feb 14
29/49

Non-local-in-time action for the fourth post-Newtonian conservative dynamics of two-body systems [CL]

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http://arxiv.org/abs/1401.4548


We complete the analytical determination, at the 4th post-Newtonian (4PN) approximation, of the conservative dynamics of gravitationally interacting two-point-mass systems. This completion is obtained by resolving the infra-red ambiguity which had blocked a previous 4PN calculation [P.Jaranowski and G.Sch\”afer, Phys. Rev. D 87, 081503(R) (2013)] by taking into account the 4PN breakdown of the usual near-zone expansion due to infinite-range tail-transported temporal correlations found long ago [L.Blanchet and T.Damour, Phys. Rev. D 37, 1410 (1988)]. This leads to a Poincar\’e-invariant 4PN-accurate effective action for two masses, which mixes instantaneous interaction terms (described by a usual Hamiltonian) with a (time-symmetric) non-local-in-time interaction.

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Tue, 21 Jan 14
37/91

Explicit evolution relations with orbital elements for eccentric, inclined, elliptic and hyperbolic restricted few-body problems [EPA]

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http://arxiv.org/abs/1401.4167


Planetary, stellar and galactic physics often rely on the general restricted gravitational N-body problem to model the motion of a small-mass object under the influence of much more massive objects. Here, I formulate the general restricted problem entirely and specifically in terms of the commonly-used orbital elements of semimajor axis, eccentricity, inclination, longitude of ascending node, argument of pericentre, and true anomaly, without any assumptions about their magnitudes. I derive the equations of motion in the general, unaveraged case, as well as specific cases, with respect to both a bodycentric and barycentric origin. I then reduce the equations to three-body systems, and present compact singly- and doubly-averaged expressions which can be readily applied to systems of interest. This method recovers classic Lidov-Kozai and Laplace-Lagrange theory in the test particle limit to any order, but with fewer assumptions, and reveals a complete analytic solution for the averaged planetary pericentre precession in coplanar circular circumbinary systems to at least the first three nonzero orders in semimajor axis ratio. Finally, I show how the unaveraged equations may be used to express resonant angle evolution in an explicit manner that is not subject to expansions of eccentricity and inclination about small nor any other values.

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Mon, 20 Jan 14
19/44

Transient times, resonances and drifts of attractors in dissipative rotational dynamics [CL]

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http://arxiv.org/abs/1401.4378


In a dissipative system the time to reach an attractor is often influenced by the peculiarities of the model and in particular by the strength of the dissipation. In particular, as a dissipative model we consider the spin-orbit problem providing the dynamics of a triaxial satellite orbiting around a central planet and affected by tidal torques. The model is ruled by the oblateness parameter of the satellite, the orbital eccentricity, the dissipative parameter and the drift term. We devise a method which provides a reliable indication on the transient time which is needed to reach an attractor in the spin-orbit model; the method is based on an analytical result, precisely a suitable normal form construction. This method provides also information about the frequency of motion. A variant of such normal form used to parametrize invariant attractors provides a specific formula for the drift parameter, which in turn yields a constraint – which might be of interest in astronomical problems – between the oblateness of the satellite and its orbital eccentricity.

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Mon, 20 Jan 14
22/44

Closed Form Solutions To Bosonic Perturbations In General Relativity [CL]

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http://arxiv.org/abs/1401.3044


We present some results regarding metric perturbations in general relativity and other metric theories of gravity. In particular, using the Newman Penrose variables, we write down and discuss the equations which govern tensor field perturbations of ranks $0, \pm 1$ and $\pm 2$ (scalar,vector,tensor bosonic perturbations) over certain space-times that admit specific background metrics expressible in isotropic coordinates. Armed with these equations, we are able to develop the Hadamard series which can be associated with the fundamental solution of the equations, wherein we introduce an inhomogeneous singularity at the physical space-time point of the perturbing particle. The Hadamard series is then used to generate closed form solutions by making choices for an appropriate ansatz solution. In particular, we solve for the spin-weighted electrostatic potential for the Reissner-Nordstrom black hole and for the fully dynamical potential for the Friedmann-Robertson-Walker cosmological solution.

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Wed, 15 Jan 14
52/67

Topology of the Gauged Kahler Isometry in Minimal Supergravity Models of Inflation [CL]

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http://arxiv.org/abs/1401.1201


In this paper we address the question how to discriminate whether the gauged isometry group G_Sigma of the Kahler manifold Sigma that produces a D-type inflaton potential in a Minimal Supergravity Model is elliptic, hyperbolic or parabolic. We show that the classification of isometries of symmetric cosets can be extended to non symmetric Sigma.s if these manifolds satisfy additional mathematical restrictions. The classification criteria established in the mathematical literature are coherent with simple criteria formulated in terms of the asymptotic behavior of the Kahler potential K(C) = 2 J(C) where the real scalar field C encodes the inflaton field. As a by product of our analysis we show that all phenomenologically admissible potentials for the description of inflation and in particular alpha-attractors are mostly obtained from the gauging of a parabolic isometry. The requirement of regularity of the manifold Sigma poses strong constraints on the alpha-attractors and reduces their space considerably. Curiously there is a unique integrable alpha-attractor corresponding to a particular value of this parameter.

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Wed, 8 Jan 14
19/62

Regular and conformal regular cores for static and rotating solutions [CL]

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http://arxiv.org/abs/1401.0787


Using a new metric for generating rotating solutions, we derive in a general fashion the solution of an imperfect fluid and that of its conformal homolog. We discuss the conditions that the stress-energy tensors and invariant scalars be regular. On classical physical grounds, it is stressed that, conformal fluids used as cores for static or rotating solutions, are exempt from any malicious behavior in that they are finite and defined everywhere.

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Tue, 7 Jan 14
16/65

Rotating analogue black holes: Quasinormal modes and tails, superresonance, and sonic bombs and plants in the draining bathtub acoustic hole [CL]

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http://arxiv.org/abs/1312.7176


The analogy between sound wave propagation and light waves led to the study of acoustic holes, the acoustic analogues of black holes. Many black hole features have their counterparts in acoustic holes. The Kerr metric, the rotating metric for black holes in general relativity, has as analogue the draining bathtub metric, a metric for a rotating acoustic hole. Here we report on the progress that has been made in the understanding of features, such as quasinormal modes and tails, superresonance, and instabilities when the hole is surrounded by a reflected mirror, in the draining bathtub metric. Given then the right settings one can build up from these instabilities an apparatus that stores energy in the form of amplified sound waves. This can be put to wicked purposes as in a bomb, or to good profit as in a sonic plant.

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Tue, 31 Dec 13
20/49

Time-analyticity of Lagrangian particle trajectories in ideal fluid flow [CL]

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http://arxiv.org/abs/1312.6320


It is known that the Eulerian and Lagrangian structures of fluid flow can be drastically different; for example, ideal fluid flow can have a trivial (static) Eulerian structure, while displaying chaotic streamlines. Here we show that ideal flow with limited spatial smoothness (an initial vorticity that is just a little better than continuous), nevertheless has time-analytic Lagrangian trajectories before the initial limited smoothness is lost. For proving such results we use a little-known Lagrangian formulation of ideal fluid flow derived by Cauchy in 1815 in a manuscript submitted for a prize of the French Academy. This formulation leads to simple recurrence relations among the time-Taylor coefficients of the Lagrangian map from initial to current fluid particle positions; the coefficients can then be bounded using elementary methods. We first consider various classes of incompressible fluid flow, governed by the Euler equations, and then turn to a case of compressible flow of cosmological relevance, governed by the Euler-Poisson equations.

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Wed, 25 Dec 13
21/23

Galactic dynamics in MOND — Existence of equilibria with finite mass and compact support [CL]

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http://arxiv.org/abs/1312.3765


We consider a self-gravitating collisionless gas where the gravitational interaction is modeled according to MOND (modified Newtonian dynamics). For the resulting modified Vlasov-Poisson system we establish the existence of spherically symmetric equilibria with compact support and finite mass. In the standard situation where gravity is modeled by Newton’s law the latter properties only hold under suitable restrictions on the prescribed microscopic equation of state. Under the MOND regime no such restrictions are needed.

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Tue, 17 Dec 13
29/78

Spherically symmetric equilibria for self-gravitating kinetic or fluid models in the non-relativistic and relativistic case – A simple proof for finite extension [CL]

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http://arxiv.org/abs/1203.2590


We consider a self-gravitating collisionless gas as described by the Vlasov-Poisson or Einstein-Vlasov system or a self-gravitating fluid ball as described by the Euler-Poisson or Einstein-Euler system. We give a simple proof for the finite extension of spherically symmetric equilibria, which covers all these models simultaneously. In the Vlasov case the equilibria are characterized by a local growth condition on the microscopic equation of state, i.e., on the dependence of the particle distribution on the particle energy, at the cut-off energy E_0, and in the Euler case by the corresponding growth condition on the equation of state p=P(\rho) at \rho=0. These purely local conditions are slight generalizations to known such conditions.

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Mon, 16 Dec 13
6/49

Adiabatic regularization and particle creation for scalar and spin one-half fields [CL]

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http://arxiv.org/abs/1311.4958


The extension of the adiabatic regularization method to spin-$1/2$ fields requires a self-consistent adiabatic expansion of the field modes. We provide here the details of such expansion, which differs from the WKB ansatz that works well for scalars, to firmly establish the generalization of the adiabatic renormalization scheme to spin-$1/2$ fields. We also provide a general overview of the adiabatic method to analyze particle creation and perform renormalization of relevant expectation values. We focus on the computation of particle production in de Sitter spacetime and obtain an analytic expression of the renormalized stress-energy tensor for Dirac fermions.

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Thu, 21 Nov 13
21/50

Relativistic $\langle σv_{\text{rel}} \rangle}$ in the calculation of relics abundances: a closer look [CEA]

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http://arxiv.org/abs/1311.4508


After clarifying the relation between the invariant relativistic relative velocity $v_{\texttt{r}}$, the M\o{}ller velocity $\bar{v}$, and the non-relativistic relative velocity $v_r$, the relativistic thermally averaged cross section times relative velocity $\langle \sigma v_\texttt{rel}\rangle$ that appears in the density evolution equation for thermal relics is reformulated only in terms the relativistic relative velocity $v_\texttt{r}$. Considering the annihilation of dark matter into a particle-antiparticle pair $f\bar{f}$, in the cases $m_f=0$, $m_f=m$ and $m_f \gg m$, we find that the coefficients of the expansion of $\langle \sigma v_{\texttt{r}}\rangle$ in powers of the relative velocity admit an exact analytical representation in terms of the Meijer $G$ functions that can be reduced to combinations of modified Bessel functions of the second kind.

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Tue, 19 Nov 13
62/75

Probability distribution for the relative velocity of colliding particles in a relativistic classical gas [CEA]

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http://arxiv.org/abs/1311.4494


We find the probability density function $\mathcal{P}(v_{\texttt{r}})$ of the relativistic relative velocity for two colliding particles in a non-degenerate relativistic gas. The distribution reduces to Maxwell distribution for the relative velocity in the non-relativistic limit. We find an exact formula for the mean value $\langle v_{\texttt{r}}\rangle$. The mean velocity tends to the Maxwell’s value in the non-relativistic limit and to the velocity of light in the ultra-relativistic limit. At a given temperature $T$, when at least for one of the two particles the ratio of the rest energy over the thermal energy $m c^2/k_B T$ is smaller than 40 the Maxwell distribution is inadequate.

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Tue, 19 Nov 13
64/75

Hidden Superconformal Symmetry of the Cosmological Evolution [CL]

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http://arxiv.org/abs/1311.3326


In the superconformal formulation of supergravity, the standard supergravity action appears as a result of spontaneous symmetry breaking when the conformal compensator scalar field, the conformon, acquires a nonzero value, giving rise to the Planck mass. After that, many symmetries of the original theory become well hidden, and therefore they are often ignored. However, recent developments demonstrated that superconformal invariance is more than just a tool: it plays an important role in generalizing previously existing formulations of supergravity and developing new classes of inflationary models. In this paper we describe hidden superconformal symmetry of the cosmological evolution. In this formulation, inflation can be equivalently described as the conformon instability, and creation of the universe `from nothing’ can be interpreted as spontaneous symmetry breaking due to emergence of a classical conformon field. We develop a general formalism that allows to describe the cosmological evolution simultaneously with the evolution of the conformon. We find a set of gauge invariant physical observables, including the superconformally invariant generalizations of the square of the Weyl tensor, which are necessary for invariant description of the cosmological singularities.

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Fri, 15 Nov 13
26/56

Galaxies with Supermassive Binary Black Holes: (I) A Possible Model for the Centers of Core Galaxies [GA]

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http://arxiv.org/abs/1311.1311


The dynamics of galactic systems with central binary black holes is studied. The model is a modification from the restricted three body problem, in which a galactic potential is added as an external potential. Considering the case with an equal mass binary black holes, the conditions of existence of equilibrium points, including Lagrange Points and additional new equilibrium points, i.e. Jiang-Yeh Points, are investigated. A critical mass is discovered to be fundamentally important. That is, Jiang-Yeh Points exist if and only if the galactic mass is larger than the critical mass. The stability analysis is performed for all equilibrium points. The results that Jiang-Yeh Points are unstable could lead to the core formation in the centers of galaxies.

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Thu, 7 Nov 13
23/60

Periodic orbits in cosmological billiards: the Selberg trace formula for asymptotic Bianchi IX universes, evidence for scars in the wavefunction of the quantum universe and large-scale structure anisotropies of the present universe [CL]

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http://arxiv.org/abs/1311.0488


The Selberg trace formula is specified for cosmological billiards in $4=3+1$ spacetime dimensions. The spectral formula is rewritten as an exact sum over the initial conditions for the Einstein field equations for which periodic orbits are implied. For this, a suitable density of measure invariant under the billiard maps has been defined, within the statistics implied by the BKL paradigm. The trace formula has also been specified for the stochastic limit of the dynamics, where the sum over initial conditions has been demonstrated to be equivalent to a sum over suitable symmetry operations on the generators of the groups that define the billiard dynamics, and acquires different features for the different statistical maps.
Evidence for scars at the quantum regime is provided. The validity of the Selberg trace formula at the classical level and in the quantum regime enforces the validity of the semiclassical descriptions of these systems, thus offering further elements for the comparison of quantum-gravity effects and the present observed structure of the universe. This procedure also constitutes a new approach in hyperbolic geometry for the application of the Selberg trace formula for a chaotic system whose orbits are associated to precise statistical distributions, for both billiard tables corresponding to the desymmetrized fundamental domain and to that a a congruence subgroup of it.

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Tue, 5 Nov 13
31/73

Integrable Scalar Cosmologies II. Can they fit into Gauged Extended Supergavity or be encoded in N=1 superpotentials? [CL]

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http://arxiv.org/abs/1310.5340


The question whether the integrable one-field cosmologies classified in a previous paper by Fre, Sagnotti and Sorin can be embedded as consistent one-field truncations into Extended Gauged Supergravity or in N=1 supergravity gauged by a superpotential without the use of D-terms is addressed in this paper. The answer is that such an embedding is very difficult and rare but not impossible. Indeed we were able to find two examples of integrable models embedded in Supergravity in this way. Both examples are fitted into N=1 Supergravity by means of a very specific and interesting choice of the superpotential W(z). The question whether there are examples of such an embedding in extended Gauged Supergravity remains open. In the present paper, relying on the embedding tensor formalism we classified all gaugings of the N=2 STU model, confirming, in the absence on hypermultiplets, the uniqueness of the stable de Sitter vacuum found several years ago by Fre, Trigiante and Van Proeyen and excluding the embedding of any integrable cosmological model. A detailed analysis of the space of exact solutions of the first Supergravity embedded integrable cosmological model revealed several new features worth an in depth consideration. When the scalar potential has an extremum at a negative value, the universe necessarily collapses into a Big Crunch notwithstanding its spatial flatness. The causal structure of these universes is quite different from that of the closed, positive curved, universe: indeed in this case the particle and event horizons do not coincide and develop complicated patterns. The cosmological consequences of this unexpected mechanism deserve careful consideration.

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Date added: Tue, 22 Oct 13

Axial Symmetric Kahler manifolds, the D-map of Inflaton Potentials and the Picard-Fuchs Equation [CL]

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http://arxiv.org/abs/1310.5278


In this paper we provide a definition of the D-map, namely of the mathematical construction implicitly utilized by supergravity that associates an axial symmetric Kahler surface to every positive definite potential function V(phi). The properties of the D-map are discussed in general. Then the D-map is applied to the list of integrable cosmological potentials classified by us in a previous publication with A. Sagnotti. Several interesting geometrical and analytical properties of the manifolds in the image of this D-map are discovered and illustrated. As a by-product of our analysis we demonstrate the existence of (integrable) Starobinsky-like potentials that can be embedded into supergravity. Some of them follow from constant curvature Kahler manifolds. In the quest for a microscopic interpretation of inflaton dynamics we present the Ariadne’s thread provided by a new mathematical concept that we introduce under the name of axial symmetric descendants of one dimensional special Kahler manifolds. By means of this token we define a clearcut algorithm that to each potential function V(phi) associates a unique 4th order Picard-Fuchs equation of restricted type. Such an equation encodes information on the chiral ring of a superconformal field theory to be sought for, unveiling in this way a microscopic interpretation of the inflaton potential. We conjecture that the physical mechanism at the basis of the transition from a special manifold to its axial symmetric descendant is probably the construction of an Open String descendant of a Closed String model.

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Date added: Tue, 22 Oct 13

Quantum Vacuum Instability of 'Eternal' de Sitter Space [CL]

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http://arxiv.org/abs/1310.1963


The Euclidean or Bunch-Davies O(4,1) invariant ‘vacuum’ state of quantum fields in global de Sitter space is shown to be unstable to small perturbations, even for a massive free field with no self-interactions. There are perturbations of this state with arbitrarily small energy density at early times that is exponentially blueshifted in the contracting phase of ‘eternal’ de Sitter space, and becomes large enough to disturb the classical geometry through the semiclassical Einstein eqs. at later times. In the closely analogous case of a constant, uniform electric field, a time symmetric state equivalent to the de Sitter invariant one is constructed, which is also not a stable vacuum state under perturbations. The role of a quantum anomaly in the growth of perturbations and symmetry breaking is emphasized in both cases. In de Sitter space, the same results are obtained either directly from the renormalized stress tensor of a massive scalar field, or for massless conformal fields of any spin, more directly from the effective action and stress tensor associated with the conformal trace anomaly. The anomaly stress tensor shows that states invariant under the O(4) subgroup of the de Sitter group are also unstable to perturbations of lower spatial symmetry, implying that both the O(4,1) isometry group and its O(4) subgroup are broken by quantum fluctuations. Consequences of this result for cosmology and the problem of vacuum energy are discussed.

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Date added: Fri, 18 Oct 13