Showing papers on "Friedmann–Lemaître–Robertson–Walker metric published in 2008"
TL;DR: In this article, the future evolution of the dark energy universe in modified gravities including $F(R)$ gravity, string-inspired scalar Gauss-Bonnet and modified GaussBonnet ones, and ideal fluid with the inhomogeneous equation of state (EoS) was investigated.
Abstract: We investigate the future evolution of the dark energy universe in modified gravities including $F(R)$ gravity, string-inspired scalar-Gauss-Bonnet and modified Gauss-Bonnet ones, and ideal fluid with the inhomogeneous equation of state (EoS). Modified Friedmann-Robertson-Walker (FRW) dynamics for all these theories may be presented in universal form by using the effective ideal fluid with an inhomogeneous EoS without specifying its explicit form. We construct several examples of the modified gravity which produces accelerating cosmologies ending at the finite-time future singularity of all four known types by applying the reconstruction program. Some scenarios to resolve the finite-time future singularity are presented. Among these scenarios, the most natural one is related with additional modification of the gravitational action in the early universe. In addition, late-time cosmology in the non-minimal Maxwell-Einstein theory is considered. We investigate the forms of the non-minimal gravitational coupling which generates the finite-time future singularities and the general conditions for this coupling in order that the finite-time future singularities cannot emerge. Furthermore, it is shown that the non-minimal gravitational coupling can remove the finite-time future singularities or make the singularity stronger (or weaker) in modified gravity.
374 citations
TL;DR: In this article, it was shown that the cosmological event horizon of de Sitter space is also of the Hawking radiation with thermal spectrum, where the apparent horizon is assumed to have such a temperature without any proof.
Abstract: Hawking radiation is an important quantum phenomenon of black hole, which is closely related to the existence of event horizon of black hole. The cosmological event horizon of de Sitter space is also of the Hawking radiation with thermal spectrum. By use of the tunneling approach, we show that there is indeed a Hawking radiation with temperature, $T=1/2\pi \tilde r_A$, for locally defined apparent horizon of a Friedmann-Robertson-Walker universe with any spatial curvature, where $\tilde r_A$ is the apparent horizon radius. Thus we fill in the gap existing in the literature investigating the relation between the first law of thermodynamics and Friedmann equations, there the apparent horizon is assumed to have such a temperature without any proof. In addition, we stress the implication of the Hawking temperature associated with the apparent horizon.
253 citations
TL;DR: In this article, a modified Friedmann equation H-2 = 8 pi G/3 rho(1-rho/rho(crit) was derived for FRW universe with any spatial curvature.
Abstract: Applying Clausius relation, delta Q = TdS, to apparent horizon of a FRW universe with any spatial curvature, and assuming that the apparent horizon has temperature T = 1/(2 pi(r) over tildeA), and a quantum corrected entropy-area relation, S = A/4G + alpha lnA/4G, where (r) over tildeA and A are the apparent horizon radius and area, respectively, and alpha is a dimensionless constant, we derive modified Friedmann equations, which does not contain a bounce solution. On the other hand, loop quantum cosmology leads to a modified Friedmann equation H-2 = 8 pi G/3 rho(1-rho/rho(crit)). We obtain an entropy expression of apparent horizon of FRW universe described by the modified Friedmann equation. In the limit of large horizon area, resulting entropy expression gives the above corrected entropy-area relation, however, the prefactor alpha in the logarithmic term is positive, which seems not consistent with most of results in the literature that quantum geometry leads to a negative contribution to the area formula of black hole entropy.
194 citations
TL;DR: In this article, a modified Friedmann equation was proposed to obtain an entropy expression of the apparent horizon of a FRW universe with any spatial curvature, where the expected entropy expression is the same as the one obtained in this paper, however, the prefactor in the logarithmic term is positive, which seems not consistent with most of results in the literature that quantum geometry leads to a negative contribution to the area formula of black hole entropy.
Abstract: Applying Clausius relation, $\delta Q=TdS$, to apparent horizon of a FRW universe with any spatial curvature, and assuming that the apparent horizon has temperature $T=1/(2\pi \tilde {r}_A)$, and a quantum corrected entropy-area relation, $S=A/4G +\alpha \ln A/4G$, where $\tilde {r}_A$ and $A$ are the apparent horizon radius and area, respectively, and $\alpha$ is a dimensionless constant, we derive modified Friedmann equations, which does not contain a bounce solution. On the other hand, loop quantum cosmology leads to a modified Friedmann equation $H^2 =\frac{8\pi G}{3}\rho (1-\rho/\rho_{\rm crit})$. We obtain an entropy expression of apparent horizon of FRW universe described by the modified Friedmann equation. In the limit of large horizon area, resulting entropy expression gives the above corrected entropy-area relation, however, the prefactor $\alpha$ in the logarithmic term is positive, which seems not consistent with most of results in the literature that quantum geometry leads to a negative contribution to the area formula of black hole entropy.
173 citations
TL;DR: In this article, the authors investigate quintom cosmology in FRW universes using isomorphic models consisting of three coupled oscillators, one of which carries negative kinetic energy, and obtain their qualitative characteristics as well as their quantitative asymptotic behavior.
163 citations
TL;DR: In this paper, an exact Swiss-cheese model of the universe, where inhomogeneous LTB patches are embedded in a flat FLRW background, was studied to see how observations of distant sources are affected.
Abstract: We study an exact Swiss-cheese model of the universe, where inhomogeneous LTB patches are embedded in a flat FLRW background, in order to see how observations of distant sources are affected. We focus mainly on the redshift, both perturbatively and non-perturbatively: the net effect given by one patch is suppressed by (L/RH)3 (where L is the size of one patch and RH is the Hubble radius). We disentangle this effect from the Doppler term (which is much larger and has been used recently (Biswas et al 2007 J. Cosmol. Astropart. Phys. JCAP12(2007)017 [astro-ph/0606703]) to try to fit the SN curve without dark energy) by making contact with cosmological perturbation theory. Then, the correction to the angular distance is discussed analytically and estimated to be larger, , perturbatively and non-perturbatively (although it should go to zero after angular averaging).
160 citations
Book•
06 Mar 2008
TL;DR: In this paper, the concept of distance in cosmology was introduced and the first Lap in CMB Anisotropies Improved Fluid Description of Pre-Decoupling Physics Kinetic Hierarchies of Multipole Moments Early Initial Conditions Surfing on the Gauges Interacting Fluids Spectator Fields Appendices: The Concept of Distance in Cosmology Kinetic Description of Hot Plasmas Scalar Modes of the Geometry Metric Fluctuations: Gauge Independent Treatment
Abstract: Why CMB Physics Electromagnetic Emission of the Observable Universe From CMB to the Standard Cosmological Model Problems with the Standard Cosmological Model (SCM) Beyond the SCM Essentials of Inflationary Dynamics Inhomogeneities in FRW Models The First Lap in CMB Anisotropies Improved Fluid Description of Pre-Decoupling Physics Kinetic Hierarchies of Multipole Moments Early Initial Conditions Surfing on the Gauges Interacting Fluids Spectator Fields Appendices: The Concept of Distance in Cosmology Kinetic Description of Hot Plasmas Scalar Modes of the Geometry Metric Fluctuations: Gauge Independent Treatment
141 citations
TL;DR: In this paper, the phase space of Friedmann-Lemaitre-Robertson-Walker (FLRW) models derived from scalar-tensor gravity was studied.
Abstract: We study the phase space of Friedmann–Lemaitre–Robertson–Walker (FLRW) models derived from scalar–tensor gravity where the non-minimal coupling is F() = ξ2 and the effective potential is V() = λn. Our analysis allows one to unfold many features of the cosmology of this class of theories. For example, the evolution mechanism towards states indistinguishable from GR is recovered and proved to depend critically on the form of the potential V(). Also, transient almost-Friedmann phases evolving towards accelerated expansion and unstable inflationary phases evolving towards stable ones are found. Some of our results are shown to hold also for the string–dilaton action.
104 citations
TL;DR: In this paper, the existence of a Noether symmetry yields a general solution of the Wheeler-DeWitt equation where can be expressed as a superposition of states of the form e i S.
79 citations
TL;DR: In this article, a general class of cosmological models driven by a nonlocal scalar field inspired by string field theory is studied, and a special solution of the nonlocal Friedmann equations is found.
Abstract: A general class of cosmological models driven by a nonlocal scalar field inspired by string field theory is studied. Using the fact that the considering linear nonlocal model is equivalent to an infinite number of local models we have found an exact special solution of the nonlocal Friedmann equations. This solution describes a monotonically increasing universe with the phantom dark energy.
74 citations
TL;DR: In this paper, a generalized FRW model of space-time is studied, taking into consideration the anisotropic structure of fields which are depended on the position and the direction (velocity).
Abstract: A generalized FRW model of space-time is studied, taking into consideration the anisotropic structure of fields which are depended on the position and the direction (velocity). The Raychaudhouri and Friedman-like equations are investigated assuming the Finslerian character of space-time. A long range vector field of cosmological origin is considered in relation to a physical geometry where the Cartan connection has a fundamental role. The Friedman equations are produced including extra anisotropic terms. The variation of anisotropy z
t
is expressed in terms of the Cartan torsion tensor of the Finslerian manifold. A physical generalization of the Hubble and other cosmological parameters arises as a direct consequence of the equations of motion.
TL;DR: In this article, the authors studied Palatini f(R) cosmology using Noether symmetry approach for the matter-dominated universe and showed that this symmetry always exists for f (R)∼Rn and the Noether constant is a function of the Newton's gravitational constant and the current matter content of the universe.
TL;DR: In this article, the APS construction of the quantum Hamiltonian is analyzed under the assumption that the cosmological constant is a constant, and the essential self-adjointness of the operator whose square-root defines in [1] is proved.
Abstract: The flat Friedman?Robertson?Walker (FRW) model coupled to the massless scalar field according to the improved, background scale-independent version of Ashtekar, Paw?owski and Singh [1] is considered. The core of the theory is addressed directly: the APS construction of the quantum Hamiltonian is analyzed under the assumption that the cosmological constant ? ? 0. We prove the essential self-adjointness of the operator whose square-root defines in [1] the quantum Hamiltonian operator and therefore provide the explicit definition. If ? 0 being some constants) plus a trace class operator.
TL;DR: In this paper, it was shown that the preferred state is of Hadamard form, hence the backreaction on the metric is finite and the state can be used as a starting point for renormalization procedures.
Abstract: In a recent paper, we proved that a large class of spacetimes, not necessarily homogeneous or isotropous and relevant at a cosmological level, possesses a preferred codimension one submanifold, i.e., the past cosmological horizon, on which it is possible to encode the information of a scalar field theory living in the bulk. Such bulk-to-boundary reconstruction procedure entails the identification of a preferred quasifree algebraic state for the bulk theory, enjoying remarkable properties concerning invariance under isometries (if any) of the bulk and energy positivity, and reducing to well-known vacua in standard situations. In this paper, specialising to open FRW models, we extend previously obtained results and we prove that the preferred state is of Hadamard form, hence the backreaction on the metric is finite and the state can be used as a starting point for renormalisation procedures. That state could play a distinguished role in the discussion of the evolution of scalar fluctuations of the metric, an analysis often performed in the development of any model describing the dynamic of an early Universe which undergoes an inflationary phase of rapid expansion in the past.
TL;DR: In this paper, the dynamics of homogeneous isotropic FRW cosmologies with positive spatial curvature in $f(R)$-gravity were studied, paying special attention to the existence of Einstein static models.
Abstract: We study the dynamics of homogeneous isotropic FRW cosmologies with positive spatial curvature in $f(R)$-gravity, paying special attention to the existence of Einstein static models and only study forms of $f(R)=R^n$ for which these static models have been shown to exist. We construct a compact state space and identify past and future attractors of the system and recover a previously discovered future attractor corresponding to an expanding accelerating model. We also discuss the existence of universes which have both a past and future bounce, a phenomenon which is absent in General Relativity.
TL;DR: In this paper, the evolution of the scale factor in such a construction must be governed not by the standard Einstein equations for the FLRW metric, but by the modified Friedmann equations derived by Buchert (Gen Relat Gravit 32:105, 2000; 33:1381, 2001).
Abstract: The present matter density of the Universe, while highly inhomogeneous on small scales, displays approximate homogeneity on large scales. We propose that whereas it is justified to use the Friedmann–Lemaitre–Robertson–Walker (FLRW) line element (which describes an exactly homogeneous and isotropic universe) as a template to construct luminosity distances in order to compare observations with theory, the evolution of the scale factor in such a construction must be governed not by the standard Einstein equations for the FLRW metric, but by the modified Friedmann equations derived by Buchert (Gen Relat Gravit 32:105, 2000; 33:1381, 2001) in the context of spatial averaging in Cosmology. Furthermore, we argue that this scale factor, defined in the spatially averaged cosmology, will correspond to the effective FLRW metric provided the size of the averaging domain coincides with the scale at which cosmological homogeneity arises. This allows us, in principle, to compare predictions of a spatially averaged cosmology with observations, in the standard manner, for instance by computing the luminosity distance versus red-shift relation. The predictions of the spatially averaged cosmology would in general differ from standard FLRW cosmology, because the scale-factor now obeys the modified FLRW equations. This could help determine, by comparing with observations, whether or not cosmological inhomogeneities are an alternative explanation for the observed cosmic acceleration.
TL;DR: In this paper, a Hamilton-Jacobi method beyond semiclassical approximation in black hole physics was developed by Baerjee and Majhi, and the corrections to the Hawking-like temperature and entropy of apparent horizon for FRW universe were also obtained.
Abstract: Recently, a Hamilton-Jacobi method beyond semiclassical approximation in black hole physics was developed by \emph{Banerjee} and \emph{Majhi}\cite{beyond0}. In this paper, we generalize their analysis of black holes to the case of Friedmann-Robertson-Walker (FRW) universe. It is shown that all the higher order quantum corrections in the single particle action are proportional to the usual semiclassical contribution. The corrections to the Hawking-like temperature and entropy of apparent horizon for FRW universe are also obtained. In the corrected entropy, the area law involves logarithmic area correction together with the standard inverse power of area term.
TL;DR: In this paper, it was shown that the Friedmann equations of the FRW universe can be recast as a similar form of the first law ThdSh = dE+WdV of thermodynamics at the apparent horizon of a viscous fluid.
Abstract: It is shown that the differential form of Friedmann equations of Friedman?Robertson?Walker (FRW) universe can be recast as a similar form of the first law ThdSh = dE+WdV of thermodynamics at the apparent horizon of FRW universe filled with the viscous fluid. It is also shown that by employing the general expression of temperature associated with the apparent horizon of FRW universe and assumed that the temperature Tm of the energy inside the apparent horizon is proportional to the horizon temperature Tm = bTh, we are able to show that the generalized second law of thermodynamics holds in the Einstein gravity provided .
TL;DR: In this paper, the phenomenological decay of Λ has been investigated in the presence of perfect fluid for Bianchi type-I space-time, and the physical significance of the cosmological models has also been discussed.
Abstract: Einstein’s field equations with variable gravitational and cosmological “constant” are considered in presence of perfect fluid for Bianchi type-I space-time. Consequences of the four cases of the phenomenological decay of Λ have been discussed which are consistent with observations. The physical significance of the cosmological models have also been discussed.
TL;DR: In this paper, the authors investigated the dynamics of a flat FRW cosmological model with a non-minimally coupled scalar field with the coupling term R \xi R \psi^{2}$ in the scalar fields action.
Abstract: In this publication we investigate dynamics of a flat FRW cosmological model with a non-minimally coupled scalar field with the coupling term $\xi R \psi^{2}$ in the scalar field action. The quadratic potential function $V(\psi)\propto \psi^{2}$ is assumed. All the evolutional paths are visualized and classified in the phase plane, at which the parameter of non-minimal coupling $\xi$ plays the role of a control parameter. The fragility of global dynamics with respect to changes of the coupling constant is studied in details. We find that the future big rip singularity appearing in the phantom scalar field cosmological models can be avoided due to non-minimal coupling constant effects. We have shown the existence of a finite scale factor singular point (future or past) where the Hubble function as well as its first cosmological time derivative diverges.
TL;DR: In this article, a method for deriving Friedmann-Robertson-Walker (FRW) solutions developed in Chimento and Jakubi (1996 Int. J. Phys. Mod. D 571-84) is generalized to account for models with non-minimal coupling between the dark energy and the dark matter that are based on an action principle.
Abstract: A method for deriving Friedmann-Robertson-Walker (FRW) solutions developed in Chimento and Jakubi (1996 Int. J. Mod. Phys. D 5 71-84) is generalized to account for models with non-minimal coupling between the dark energy and the dark matter that are based on an action principle. New quintessence and phantom (flat) FRW solutions are found. Their physical significance is discussed. Additionally, the aforementioned method is modified so that, 'coincidence-free' solutions can be readily derived. Besides, we review some aspects of the phantom barrier crossing. In this regard we present a model which is free from the coincidence problem and, at the same time, does the crossing of the phantom barrier ω = -1 at late time. Finally, we give additional comments on the non-predictive properties of scalar-field cosmological models with or without energy transfer.
TL;DR: In this article, it was shown that the generalized second law of thermodynamics holds at the apparent horizon of FRW universe and preserves dominant energy condition and that the differential form of Friedmann equations can be recast as a similar form of the first law.
Abstract: It is shown that the differential form of Friedmann equations of FRW universe can be recast as a similar form of the first law, $T_{h}dS_{h} = dE + WdV$, of thermodynamics at the apparent horizon of FRW universe filled with the viscous fluid. It is also shown that the generalized second law of thermodynamics holds at the apparent horizon of FRW universe and preserves dominant energy condition.
TL;DR: In this article, the general relativistic cosmological Friedmann equations are expanded explicitly to include energy forms not usually seen and the evolution of the universe as predicted by the Friedmann equation when dominated by a single, isotropic, stable, static, perfect fluid energy form is discussed for different values of the gravitational pressure to density ratio.
Abstract: The general relativistic cosmological Friedmann equations, which describe how the scale factor of the universe evolves, are expanded explicitly to include energy forms not usually seen The evolution of the universe as predicted by the Friedmann equations when dominated by a single, isotropic, stable, static, perfect-fluid energy form is discussed for different values of the gravitational pressure to density ratio w These energy forms include phantom energy (w 1∕3) A brief history and possible futures of Friedmann universes dominated by a single energy form are discussed
TL;DR: In this paper, a Friedmann-Robertson-Walker quantum cosmological model in the presence of Chaplygin gas and perfect fluid for early and late time epochs is presented.
TL;DR: In this article, the authors show that the Lemaitre-Tolman-Bondi (LTB) solution is a gauge artefact for realistic sub-horizon inhomogeneities.
Abstract: Lemaitre-Tolman-Bondi (LTB) solutions are used frequently to describe the collapse or expansion of spherically symmetric inhomogeneous mass distributions in the Universe. These exact solutions are obtained in the synchronous gauge where non-linear dynamics (with respect to the Friedmann -Lemaitre-Roberston-Walker (FLRW) background) induce large deviations from the FLRW metric. In this paper we show explicitly that this is a gauge artefact (for realistic sub-horizon inhomogeneities). We write down the non-linear gauge transformation from synchronous to Newtonian gauge for a general LTB solution using the fact that the peculiar velocities are small. In the latter gauge we recover the solution in the form of a weakly perturbed FLRW metric that is assumed in standard cosmology. Furthermore we show how to obtain the LTB solutions directly in Newtonian gauge and illustrate how the Newtonian approximation remains valid in the non-linear regime where cosmological perturbation theory breaks down. Finally we discuss the implications of our results for the backreaction scenario.
TL;DR: In this paper, a detailed description for power-law scaling Friedmann-Robertson-Walker cosmological scenarios dominated by two interacting perfect fluid components during the expansion is provided, where neither component is conserved separately and the energy densities are proportional to 1 / t 2.
TL;DR: In this paper, the bending of light by galaxies or clusters of galaxies in the presence of the Λ term was analyzed using Friedmann-Robertson-Walker (FRW) coordinates, which are used for the description of actual observations.
Abstract: In this paper, we analyze the bending of light by galaxies or clusters of galaxies in the presence of the Λ term. Using Friedmann–Robertson–Walker (FRW) coordinates, which are used for the description of actual observations, we demonstrate that the cosmological constant does not practically influence the lensing effect.
TL;DR: In this paper, the authors show that the Lemaitre-Tolman-Bondi (LTB) solution is a gauge artefact for realistic sub-horizon inhomogeneities.
Abstract: Lemaitre-Tolman-Bondi (LTB) solutions are used frequently to describe the collapse or expansion of spherically symmetric inhomogeneous mass distributions in the Universe. These exact solutions are obtained in the synchronous gauge where nonlinear dynamics (with respect to the FLRW background) induce large deviations from the FLRW metric. In this paper we show explicitly that this is a gauge artefact (for realistic sub-horizon inhomogeneities). We write down the nonlinear gauge transformation from synchronous to Newtonian gauge for a general LTB solution using the fact that the peculiar velocities are small. In the latter gauge we recover the solution in the form of a weakly perturbed FLRW metric that is assumed in standard cosmology. Furthermore we show how to obtain the LTB solutions directly in Newtonian gauge and illustrate how the Newtonian approximation remains valid in the nonlinear regime where cosmological perturbation theory breaks down. Finally we discuss the implications of our results for the backreaction scenario.
TL;DR: In this paper, a general Lagrangian for a decaying vector field is proposed, which corresponds to a photon-like but massive particle of 4 degrees of dynamical freedom, and the model includes a continuous spectrum of plausible gravity theories, for example, quintessence, f(R), Einstein-Aether, MOND, TeVeS, BSTV, and V-Λ theories, as special cases.
Abstract: We propose that galactic dark matter can be described by a nonuniform dark energy fluid. The underlying field is a decaying vector field, which might corresponds to a photon-like but massive particle of 4 degrees of dynamical freedom. We propose a very general Lagrangian for this vector field. The model includes a continuous spectrum of plausible gravity theories, for example, quintessence, f(R), Einstein-Aether, MOND, TeVeS, BSTV, V-Λ theories, and the inflaton scalar field as special cases. We study in detail a special class of models with a fixed norm of the timelike vector field in the physical metric, which includes a nonlinear K4 term and a Ricci scalar term. We derive the Einstein equations in the perturbed form, which are needed for simulating structure growth in an FRW universe to test such theories. A special case of the model V-Λ shows promise of resembling the ΛCDM cosmology. We show that the vector field has the effect of a nonuniform dark fluid, which resembles dark matter in galaxies and dark energy in the late universe.
TL;DR: In this article, the authors studied the closed universe recollapse conjecture for positively curved FRW models with a perfect fluid matter source and a scalar field which arises in the conformal frame of the R + R 2 theory.
Abstract: We study the closed universe recollapse conjecture for positively curved FRW models with a perfect fluid matter source and a scalar field which arises in the conformal frame of the R + �R 2 theory. By including ordinary matter, we extend the analysis of a previous work. We analyze the structure of the resulted fourdimensional dynamical system with the methods of the center manifold theory and the normal form theory. It is shown that an initially expanding closed FRW universe, starting close to the Minkowski spacetime, cannot avoid recollapse. We discuss the posibility that potentials with a positive minimum may prevent the recollapse of closed universes.