Showing papers on "Friedmann–Lemaître–Robertson–Walker metric published in 2014"
TL;DR: The approach provides a unified treatment of the linear cosmological perturbations about a FLRW universe, obtained by a systematic expansion of the general action up to quadratic order, and shows that the behaviour of these linear perturbation is generically characterized by five time-dependent functions.
Abstract: We review and extend a novel approach that we recently introduced, to describe general dark energy or scalar-tensor models. Our approach relies on an Arnowitt-Deser-Misner (ADM) formulation based on the hypersurfaces where the underlying scalar field is uniform. The advantage of this approach is that it can describe in the same language and in a minimal way a vast number of existing models, such as quintessence, F(R) theories, scalar tensor theories, their Horndeski extensions and beyond. It also naturally includes Horava–Lifshitz theories. As summarized in this review, our approach provides a unified treatment of the linear cosmological perturbations about a Friedmann-Lemaitre-Robertson-Walker (FLRW) universe, obtained by a systematic expansion of our general action up to quadratic order. This shows that the behavior of these linear perturbations is generically characterized by five time-dependent functions. We derive the full equations of motion in the Newtonian gauge. In the Horndeski case, we obtain the equation of state for dark energy perturbations in terms of these functions. Our unifying description thus provides the simplest and most systematic way to confront theoretical models with current and future cosmological observations.
266 citations
TL;DR: In this article, the cosmological effects of the quantum loops of massless particles, which lead to temporal non-localities in the equations of motion governing the scale factor a(t), are discussed.
Abstract: We discuss cosmological effects of the quantum loops of massless particles, which lead to temporal non-localities in the equations of motion governing the scale factor a(t). For the effects discussed here, loops cause the evolution of a(t) to depend on the memory of the curvature in the past with a weight that scales initially as 1/(t-t'). As one of our primary examples we discuss the situation with a large number of light particles, such that these effects occur in a region where gravity may still be treated classically. However, we also describe the effect of quantum graviton loops and the full set of Standard Model particles. We show that these effects decrease with time in an expanding phase, leading to classical behavior at late time. In a contracting phase, within our approximations the quantum results can lead to a bounce-like behavior at scales below the Planck mass, avoiding the singularities required classically by the Hawking-Penrose theorems. For conformally invariant fields, such as the Standard Model with a conformally coupled Higgs, this result is purely non-local and parameter independent.
120 citations
TL;DR: In this article, the authors consider modified gravity which may describe the early-time inflation and/or late-time cosmic acceleration of the universe and discuss the properties of F(R), F(G), string-inspired and scalar-Einstein-Gauss-Bonnet gravities.
Abstract: We consider modified gravity which may describe the early-time inflation and/or late-time cosmic acceleration of the universe. In particular, we discuss the properties of F(R), F(G), string-inspired and scalar-Einstein–Gauss–Bonnet gravities, including their FRW equations and fluid or scalar-tensor description. Simplest accelerating cosmologies are investigated and possibility of unified description of the inflation with dark energy is described. The cosmological reconstruction program which permits to get the requested universe evolution from modified gravity is developed. As some extension, massive F(R) bigravity which is ghost-free theory is presented. Its scalar-tensor form turns out to be the easiest formulation. The cosmological reconstruction method for such bigravity is presented. The unified description of inflation with dark energy in F(R) bigravity turns out to be possible.
118 citations
TL;DR: In this paper, the authors study cosmological perturbations of Friedmann-Robertson-Walker solutions in ghost free massive bigravity, including also a second matter sector, and find that subhorizon exponential instabilities are unavoidable and they lead to a premature departure from the perturbative regime of cosmologically perturbation.
Abstract: Cosmological perturbations of Friedmann-Robertson-Walker solutions in ghost free massive bigravity, including also a second matter sector, are studied in detail. At early time, we find that subhorizon exponential instabilities are unavoidable and they lead to a premature departure from the perturbative regime of cosmological perturbations.
106 citations
TL;DR: In this paper, the authors present a new test of the validity of the Friedmann-Lema-tre-Robertson-Walker (FLRW) metric, based on comparing the distance from redshift 0 to $z_1$ and from 0 to z_2$ to the distance between 0 to 2$ to provide a model-independent measurement of spatial curvature.
Abstract: We present a new test of the validity of the Friedmann-Lema\^{\i}tre-Robertson--Walker (FLRW) metric, based on comparing the distance from redshift 0 to $z_1$ and from $z_1$ to $z_2$ to the distance from $0$ to $z_2$. If the universe is described by the FLRW metric, the comparison provides a model-independent measurement of spatial curvature. The test is kinematic and relies on geometrical optics, it is independent of the matter content of the universe and the applicability of the Einstein equation on cosmological scales. We apply the test to observations, using the Union2.1 compilation of supernova distances and Sloan Lens ACS Survey galaxy strong lensing data. The FLRW metric is consistent with the data, and the spatial curvature parameter is constrained to be $-1.22<\Omega_{K0}<0.63$, or $-0.08<\Omega_{K0}<0.97$ with a prior from the cosmic microwave background and the local Hubble constant, though modelling of the lenses causes significant systematic uncertainty.
105 citations
TL;DR: In this paper, it was shown that the large matter inhomogeneities that occur on small scales cannot produce significant effects on large scales, so satisfies Einstein's equation with the averaged stress energy tensor of matter as its source.
Abstract: Extremely well! In the ΛCDM model, the spacetime metric, gab, of our Universe is approximated by an FLRW metric, , to about one part in 104 or better on both large and small scales, except in the immediate vicinity of very strong field objects, such as black holes. However, derivatives of gab are not close to derivatives of , so there can be significant differences in the behavior of geodesics and huge differences in curvature. Consequently, observable quantities in the actual Universe may differ significantly from the corresponding observables in the FLRW model. Nevertheless, as we shall review here, we have proven general results showing that—within the framework of our approach to treating backreaction—the large matter inhomogeneities that occur on small scales cannot produce significant effects on large scales, so satisfies Einsteinʼs equation with the averaged stress–energy tensor of matter as its source. We discuss the flaws in some other approaches that have suggested that large backreaction effects may occur. As we also will review here, with a suitable 'dictionary,' Newtonian cosmologies provide excellent approximations to cosmological solutions to Einsteinʼs equation (with dust and a cosmological constant) on all scales. Our results thereby provide strong justification for the mathematical consistency and validity of the ΛCDM model within the context of general relativistic cosmology.
103 citations
TL;DR: In this paper, the cosmological reconstruction of the evolution background in the FRW universe is studied, where the Ricci scalar and trace of the energy-momentum tensor corresponding to the background is reconstructed.
Abstract: We study the cosmological reconstruction of $$f(R,T)$$
gravity (where $$R$$
and $$T$$
denote the Ricci scalar and trace of the energy–momentum tensor) corresponding to the evolution background in FRW universe. It is shown that any cosmological evolution including $$\Lambda $$
cold dark matter, phantom or non-phantom eras and possible phase transition from decelerating to accelerating can be reproduced in this theory. We propose some specific forms of Lagrangian in the perspective of de Sitter and power law expansion history. Finally, we formulate the perturbed evolution equations and analyze the stability of some important solutions.
98 citations
TL;DR: In this article, the authors derived an exact solution of an isotropic and homogeneous universe governed by gravity, and showed that the solution constrains the torsion scalar to be a constant function at all time.
Abstract: In the present work we derive an exact solution of an isotropic and homogeneous Universe governed by $$f(T)$$
gravity. We show how the torsion contribution to the FRW cosmology can provide a unique origin for both early and late acceleration phases of the Universe. The three models (
$$k=0, \pm 1$$
) show a built-in inflationary behavior at some early Universe time; they restore suitable conditions for the hot Big bang nucleosynthesis to begin. Unlike the standard cosmology, we show that even if the Universe initially started with positive or negative sectional curvatures, the curvature density parameter enforces evolution to a flat Universe. The solution constrains the torsion scalar $$T$$
to be a constant function at all time $$t$$
, for the three models. This eliminates the need for dark energy (DE). Moreover, when the continuity equation is assumed for the torsion fluid, we show that the flat and closed Universe models violate the conservation principle, while the open one does not. The evolution of the effective equation of state (EoS) of the torsion fluid implies a peculiar trace from a quintessence-like DE to a phantom-like one crossing a matter and radiation EoS in between; then it asymptotically approaches a de Sitter fate.
87 citations
TL;DR: In this paper, a consistent, general, and covariant formulation for a single fluid is presented, where the interaction is a natural implication of the imperfectness of the fluids, which makes it difficult to construct a robust formulation of interacting fluids, namely the construction of a Lagrangian whose variation would give rise to the interacting equations.
Abstract: Coupling dark energy and dark matter through an effective fluid description is a very common procedure in cosmology; however, it always remains in comoving coordinates in the special FLRW space. We construct a consistent, general, and covariant formulation, where the interaction is a natural implication of the imperfectness of the fluids. This imperfectness makes difficult the final step towards a robust formulation of interacting fluids, namely the construction of a Lagrangian whose variation would give rise to the interacting equations. Nevertheless, we present a formal solution to this problem for a single fluid, through the introduction of an effective metric.
85 citations
TL;DR: In this article, it was shown that the large matter inhomogeneities that occur on small scales cannot produce significant effects on large scales, so $g_{ab}^{(0)$ satisfies Einstein's equation with the averaged stress-energy tensor of matter as its source.
Abstract: Extremely well! In the $\Lambda$CDM model, the spacetime metric, $g_{ab}$, of our universe is approximated by an FLRW metric, $g_{ab}^{(0)}$, to about 1 part in $10^4$ or better on both large and small scales, except in the immediate vicinity of very strong field objects, such as black holes. However, derivatives of $g_{ab}$ are not close to derivatives of $g_{ab}^{(0)}$, so there can be significant differences in the behavior of geodesics and huge differences in curvature. Consequently, observable quantities in the actual universe may differ significantly from the corresponding observables in the FLRW model. Nevertheless, as we shall review here, we have proven general results showing that---within the framework of our approach to treating backreaction---the large matter inhomogeneities that occur on small scales cannot produce significant effects on large scales, so $g_{ab}^{(0)}$ satisfies Einstein's equation with the averaged stress-energy tensor of matter as its source. We discuss the flaws in some other approaches that have suggested that large backreaction effects may occur. As we also will review here, with a suitable "dictionary," Newtonian cosmologies provide excellent approximations to cosmological solutions to Einstein's equation (with dust and a cosmological constant) on all scales. Our results thereby provide strong justification for the mathematical consistency and validity of the $\Lambda$CDM model within the context of general relativistic cosmology.
82 citations
TL;DR: In this article, a new method for calculating CMB anisotropies in a non-flat Friedmann universe, relying on a very stable algorithm for the calculation of hyperspherical Bessel functions, that can be pushed to arbitrary precision levels.
Abstract: We present a new method for calculating CMB anisotropies in a non-flat Friedmann universe, relying on a very stable algorithm for the calculation of hyperspherical Bessel functions, that can be pushed to arbitrary precision levels. We also introduce a new approximation scheme which gradually takes over in the flat space limit and leads to significant reductions of the computation time. Our method is implemented in the Boltzmann code CLASS. It can be used to benchmark the accuracy of the CAMB code in curved space, which is found to match expectations. For default precision settings, corresponding to 0.1% for scalar temperature spectra and 0.2% for scalar polarisation spectra, our code is two to three times faster, depending on curvature. We also simplify the temperature and polarisation source terms significantly, so the different contributions to the C-l's are easy to identify inside the code.
TL;DR: In this article, an extended Chaplygin gas equation of state for which it recovers barotropic fluid with quadratic EO of state was proposed, where numerical method was used to investigate the behavior of some cosmological parameters such as scale factor, Hubble expansion parameter, energy density, and deceleration parameter.
Abstract: We propose extended Chaplygin gas equation of state for which it recovers barotropic fluid with quadratic equation of state. We use numerical method to investigate the behavior of some cosmological parameters such as scale factor, Hubble expansion parameter, energy density, and deceleration parameter. We also discuss the resulting effective equation of state parameter. Using density perturbations we investigate the stability of the theory.
TL;DR: The cosmology of an extended version of Horndeski theories with second-order equations of motion on the flat Friedmann-Lemaitre-Robertson-Walker (FLRW) background was studied in this paper.
Abstract: We study the cosmology of an extended version of Horndeski theories with second-order equations of motion on the flat Friedmann-Lemaitre-Robertson-Walker (FLRW) background. In addition to a dark energy field $\chi$ associated with the gravitational sector, we take into account multiple scalar fields $\phi_I$ ($I=1,2\cdots,N-1$) characterized by the Lagrangians $P^{(I)}(X_I)$ with $X_I=\partial_{\mu}\phi_I\partial^{\mu}\phi_I$. These additional scalar fields can model the perfect fluids of radiation and non-relativistic matter. We derive propagation speeds of scalar and tensor perturbations as well as conditions for the absence of ghosts. The theories beyond Horndeski induce non-trivial modifications to all the propagation speeds of $N$ scalar fields, but the modifications to those for the matter fields $\phi_I$ are generally suppressed relative to that for the dark energy field $\chi$. We apply our results to the covariantized Galileon with an Einstein-Hilbert term in which partial derivatives of the Minkowski Galileon are replaced by covariant derivatives. Unlike the covariant Galileon with second-order equations of motion in general space-time, the scalar propagation speed square $c_{s1}^2$ associated with the field $\chi$ becomes negative during the matter era for late-time tracking solutions, so the two Galileon theories can be clearly distinguished at the level of linear cosmological perturbations.
TL;DR: In this paper, a model of an emergent universe is formulated using the mechanism of particle creation, where the universe is considered as a non-equilibrium thermodynamical system with dissipation due to particle creation mechanism.
TL;DR: In this paper, the cosmological perturbations around FLRW solutions to non-linear massive gravity with a new effective coupling to matter were investigated, and it was shown that all five degrees of freedom in the gravity sector propagate on generic self-accelerating FLRW backgrounds.
Abstract: We investigate the cosmological perturbations around FLRW solutions to non- linear massive gravity with a new effective coupling to matter proposed recently. Unlike the case with minimal matter coupling, all five degrees of freedom in the gravity sector propagate on generic self-accelerating FLRW backgrounds. We study the stability of the cosmological solutions and put constraints on the parameters of the theory by demanding the correct sign for the kinetic terms for scalar, vector and tensor perturbations.
TL;DR: In this article, the authors considered the matter bounce scenario in F(T) gravity and Loop Quantum Cosmology (LQC) for phenomenological potentials that at early times provide a nearly matter dominated Universe in the contracting phase, having a reheating mechanism in the expanding or contracting phase.
Abstract: We consider the matter bounce scenario in F(T) gravity and Loop Quantum Cosmology (LQC) for phenomenological potentials that at early times provide a nearly matter dominated Universe in the contracting phase, having a reheating mechanism in the expanding or contracting phase, i.e., being able to release the energy of the scalar field creating particles that thermalize in order to match with the hot Friedmann Universe, and finally at late times leading to the current cosmic acceleration. For these potentials, numerically solving the dynamical perturbation equations we have seen that, for the particular F(T) model that we will name teleparallel version of LQC, and whose modified Friedmann equation coincides with the corresponding one in holonomy corrected LQC when one deals with the flat Friedmann-Lemaitre-Robertson-Walker (FLRW) geometry, the corresponding equations obtained from the well-know perturbed equations in F(T) gravity lead to theoretical results that fit well with current observational data. More precisely, in this teleparallel version of LQC there is a set of solutions which leads to theoretical results that match correctly with last BICEP2 data, and there is another set whose theoretical results fit well with Planck's experimental data. On the other hand, in the standard holonomy corrected LQC, using the perturbed equations obtained replacing the Ashtekar connection by a suitable sinus function and inserting some counter-terms in order to preserve the algebra of constrains, the theoretical value of the tensor/scalar ratio is smaller than in the teleparallel version, which means that there is always a set of solutions that matches with Planck's data, but for some potentials BICEP2 experimental results disfavours holonomy corrected LQC.
TL;DR: In this paper, the mechanism of particle creation in the framework of irreversible thermodynamics has been discussed and the second order nonequilibrium thermodynamic prescription of Israel and Stewart has been presented with particle creation rate, treated as the dissipative effect.
Abstract: The paper deals with the mechanism of particle creation in the framework of irreversible thermodynamics. The second order nonequilibrium thermodynamical prescription of Israel and Stewart has been presented with particle creation rate, treated as the dissipative effect. In the background of a flat Friedmann-Robertson-Walker (FRW) model, we assume the nonequilibrium thermodynamical process to be isentropic so that the entropy per particle does not change and consequently the dissipative pressure can be expressed linearly in terms of the particle creation rate. Here the dissipative pressure behaves as a dynamical variable having a nonlinear inhomogeneous evolution equation and the entropy flow vector satisfies the second law of thermodynamics. Further, using the Friedmann equations and by proper choice of the particle creation rate as a function of the Hubble parameter, it is possible to show (separately) a transition from the inflationary phase to the radiation era and also from the matter dominated era to late time acceleration. Also, in analogy to analytic continuation, it is possible to show a continuous cosmic evolution from inflation to late time acceleration by adjusting the parameters. It is found that in the de Sitter phase, the comoving entropy increases exponentially with time, keeping entropy per particle unchanged. Subsequently, the above cosmological scenarios have been described from a field theoretic point of view by introducing a scalar field having self-interacting potential. Finally, we make an attempt to show the cosmological phenomenon of particle creation as Hawking radiation, particularly during the inflationary era.
TL;DR: In this article, the authors present exact Friedmann Lemaitre Robertson Walkers (FLRW) solutions in generalized massive gravity where the mass parameters are naturally promoted to Lorentz-invariant functions of the Stuckelberg fields.
Abstract: We present exact Friedmann Lemaitre Robertson Walkers (FLRW) solutions in generalized massive gravity where the mass parameters are naturally promoted to Lorentz-invariant functions of the Stuckelberg fields. This new dependence relaxes the constraint that would otherwise prevent massive gravity from possessing exact FLRW solutions. It does so without the need to introduce additional degrees of freedom. We find self-accelerating cosmological solutions and show that, with a mild restriction on the region of phase space, these cosmological solutions exhibit full stability, i.e. absence of ghosts and gradient instabilities for all the tensor, vector and scalar modes, for all cosmic time. We perform the full decoupling limit analysis, including vector degrees of freedom, which can be used to confirm the existence of an active Vainshtein mechanism about these solutions.
TL;DR: In this paper, the authors present an analysis and translation of the cosmological problem of the general theory of relativity in the light of the first evidence for an expanding universe and consider his views of issues such as the curvature of space, the cosMological constant, the singularity and the timespan of the expansion.
Abstract: We present an analysis and translation of Einstein’s 1931 paper “Zum kosmologischen Problem der allgemeinen Relativitatstheorie” or “On the cosmological problem of the general theory of relativity”. In this little-known paper, Einstein proposes a cosmic model in which the universe undergoes an expansion followed by a contraction, quite different to the monotonically expanding Einstein-de Sitter model of 1932. The paper offers many insights into Einstein’s cosmology in the light of the first evidence for an expanding universe and we consider his views of issues such as the curvature of space, the cosmological constant, the singularity and the timespan of the expansion. A number of original findings concerning Einstein’s 1931 model are presented. We find that Einstein’s calculations of the present radius and matter density of the universe contain some anomalies: we suggest that his estimate of the age of the universe is based on a questionable calculation of Friedmann’s: we find that the model is not periodic, contrary to what is often stated. A first English translation of Einstein’s paper is included as an Appendix.
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TL;DR: In this article, the authors considered the reconstruction scenario of new agegraphic dark energy (NADE) model and the theory of gravity with $G$ representing the Gauss-Bonnet invariant in the flat FRW spacetime.
Abstract: In this work, we consider the reconstruction scenario of new agegraphic dark energy (NADE) model and $f(G)$ theory of gravity with $G$ representing the Gauss-Bonnet invariant in the flat FRW spacetime. In this context, we assume a solution of the scale factor in power-law form and study the correspondence scenario. A new agegraphic $f(G)$ model is constructed and discussed graphically for the evolution of the universe. Using this model, we investigate the different eras of the expanding universe and stability with the help of the equation of state (EoS) parameter $\omega_{eff}$ and squared speed of sound $v_s^2$, respectively. It is mentioned here that the reconstructed model represents the quintessence era of the accelerated expansion of the universe with instability. Moreover, the statefinder trajectories are studied and we find out that the model is not capable of reaching the $\Lambda$CDM phase of the universe.
TL;DR: In this paper, a modified cosmic Chaplygin cosmology with non-zero cosmological constant in non-flat universe was studied and the authors obtained time-dependent dark energy density by numerical analysis of non-linear differential equation and fitting curves.
Abstract: In this paper we study modified cosmic Chaplygin cosmology with non-zero cosmological constant in non-flat Universe. By using well-known forms of scale factor we obtain time-dependent dark energy density by numerical analysis of non-linear differential equation and fitting curves. We use observational data to fix solution and discuss about stability of our system. First of all we consider cosmological constant as a constant in Einstein equation, and then study possibility of variable cosmological constant.
TL;DR: The emergent gravity paradigm as discussed by the authors provides an alternative perspective on cosmology and interprets the expansion of the universe itself as a quest towards holographic equipartition, where the field equations of gravity are invariant under the addition of a constant to the matter Lagrangian.
Abstract: Observations indicate that our universe is characterized by a late-time accelerating phase, possibly driven by a cosmological constant $\Lambda$, with the dimensionless parameter $\Lambda L_P^2 \simeq 10^{-122}$, where $L_P = (G \hbar /c^3)^{1/2}$ is the Planck length. In this review, we describe how the emergent gravity paradigm provides a new insight and a possible solution to the cosmological constant problem. After reviewing the necessary background material, we identify the necessary and sufficient conditions for solving the cosmological constant problem. We show that these conditions are naturally satisfied in the emergent gravity paradigm in which (i) the field equations of gravity are invariant under the addition of a constant to the matter Lagrangian and (ii) the cosmological constant appears as an integration constant in the solution. The numerical value of this integration constant can be related to another dimensionless number (called CosMIn) that counts the number of modes inside a Hubble volume that cross the Hubble radius during the radiation and the matter dominated epochs of the universe. The emergent gravity paradigm suggests that CosMIn has the numerical value $4 \pi$, which, in turn, leads to the correct, observed value of the cosmological constant. Further, the emergent gravity paradigm provides an alternative perspective on cosmology and interprets the expansion of the universe itself as a quest towards holographic equipartition. We discuss the implications of this novel and alternate description of cosmology.
TL;DR: In this article, the authors investigate the evolution of the universe from a static to a dynamic model, and show that the primary reason for the switch was not observational evidence, but the realisation that his static model was unstable.
Abstract: In 1917 Einstein initiated modern cosmology by postulating, based on general relativity, a homogenous, static, spatially curved universe. To counteract gravitational contraction he introduced the cosmological constant. In 1922 Alexander Friedman showed that Albert Einstein’s fundamental equations also allow dynamical worlds, and in 1927 Georges Lemaitre, backed by observational evidence, concluded that our universe was expanding. Einstein impetuously rejected Friedman’s as well as Lemaitre’s findings. However, in 1931 he retracted his former static model in favour of a dynamic solution. This investigation follows Einstein on his hesitating path from a static to the expanding universe. Contrary to an often advocated belief the primary motive for his switch was not observational evidence, but the realisation that his static model was unstable.
TL;DR: In this article, a general dynamical equation of FRW universe filled with a perfect fluid was derived by considering a generic form of the entropy as a function of area. And the modified Raychaudhuri equation due to GUP in emergence of cosmic space framework and investigate it using fixed point method.
TL;DR: In this paper, the authors consider a FLRW space-time, filled with a perfect fluid described by a barotropic equation of state (p = γ ρ ), and give the equivalent mini-superspace description and use the reparametrization invariance of the resulting Lagrangian to work in the equivalent constant potential description.
TL;DR: A cosmological model with an inhomogeneous viscous dark fluid coupled with dark matter in a flat Friedmann-Robertson-Walker (FRW) universe is investigated in this article.
Abstract: A cosmological model with an inhomogeneous viscous dark fluid coupled with dark matter in a flat Friedmann–Robertson–Walker (FRW) universe is investigated. The influence of dark matter on the behavior of an inhomogeneous viscous fluid of this kind, responsible for cosmic acceleration and for the appearance of different types of singularities, is analyzed in detail. In particular, the critical points corresponding to the solutions of the background equations in a useful approximation are obtained explicitly.
TL;DR: In this paper, the authors analyzed the stability of the model proposed by Maggiore and Mancarella, where gravity is modified by a term to produce the late-time acceleration of the expansion of the universe.
Abstract: We analyse the classical stability of the model proposed by Maggiore and Mancarella, where gravity is modified by a term $\sim m^2 R \Box^{-2} R$ to produce the late-time acceleration of the expansion of the universe. Our study takes into account all excitations of the metric that can potentially drive an instability. There are some subtleties in identifying these modes, as a non-local field theory contains dynamical fields which yet do not correspond to degrees of freedom. Since some of them are ghost-like, we clarify the impact of such modes on the stability of the solutions of interest that are the flat space-time and cosmological solutions. We then find that flat space-time is unstable under scalar perturbations, but the instability manifests itself only at cosmological scales, i.e. out of the region of validity of this solution. It is therefore the stability of the FLRW solution which is relevant there, in which case the scalar perturbations are known to be well-behaved by numerical studies. By finding the analytic solution for the late-time behaviour of the scale factor, which leads to a big rip singularity, we argue that the linear perturbations are bounded in the future because of the domination of Hubble friction. In particular, this effect damps the scalar ghost perturbations which were responsible for destabilizing Minkowski space-time. Thus, the model remains phenomenologically viable.
TL;DR: In this article, the cosmological FRW flat solutions generated in general massive gravity theories were studied and the upshot is that even though dark energy can be simply accounted by massive gravity modifications, its equation of state w{sub eff} has to deviate from -1.
Abstract: We study the cosmological FRW flat solutions generated in general massive gravity theories. Such a model are obtained adding to the Einstein General Relativity action a peculiar non derivative potentials, function of the metric components, that induce the propagation of five gravitational degrees of freedom. This large class of theories includes both the case with a residual Lorentz invariance as well as the case with rotational invariance only. It turns out that the Lorentz-breaking case is selected as the only possibility. Moreover it turns out that that perturbations around strict Minkowski or dS space are strongly coupled. The upshot is that even though dark energy can be simply accounted by massive gravity modifications, its equation of state w{sub eff} has to deviate from -1. Indeed, there is an explicit relation between the strong coupling scale of perturbations and the deviation of w{sub eff} from -1. Taking into account current limits on w{sub eff} and submillimiter tests of the Newton's law as a limit on the possible strong coupling scale, we find that it is still possible to have a weakly coupled theory in a quasi dS background. Future experimental improvements on short distance tests of the Newton's law may be more » used to tighten the deviation of w{sub eff} form -1 in a weakly coupled massive gravity theory. « less
TL;DR: In this article, the authors used the Union2 dataset and 59 high-redshift gamma-ray bursts (C:411Es) to give constraints on the anisotropy of the universe.
Abstract: Recently, an anisotropic cosmological model was proposed. An arbitrary one-form, which picks out a privileged axis in the universe, was added to the Friedmann Robertson Walker (FRW) line element. The distance-redshift relation was modified such that it is direction-dependent. In this paper, we use the Union2 dataset and 59 high-redshift gamma-ray bursts (C.:411Es) to give constraints on the anisotropy of the universe. The results show that the magnitude of anisotropy is about D = 0.044 0.018, and the privileged axis points toward the direction (to, ho) = (306.1" 18.7", 18.2" 11.2') in the galactic coordinate system. The anisotropy is small and the isotropic cosmological model is an excellent approximation.
TL;DR: In this article, the authors consider inhomogeneous viscous fluids in flat Friedmann-Robertson-Walker universe and investigate the possibility to reproduce the current cosmic acceleration providing a different future evolution with respect to the Cosmological Constant case.
Abstract: We consider inhomogeneous viscous fluids in flat Friedmann-Robertson-Walker universe. We analyze different kinds of such fluids and investigate the possibility to reproduce the current cosmic acceleration providing a different future evolution with respect to the Cosmological Constant case. In particular, we study the presence of finite-future time singularities. We also discuss a general class of “integrable” viscous fluid models whose bulk viscosities obey to a common differential equation.