Friedmann–Lemaître–Robertson–Walker metric

About: Friedmann–Lemaître–Robertson–Walker metric is a research topic. Over the lifetime, 4113 publications have been published within this topic receiving 87752 citations. The topic is also known as: FLRW metric.

Nonlinear electrodynamics and FRW cosmology

Abstract: Maxwell electrodynamics, considered as a source of the classical Einstein field equations, leads to the singular isotropic Friedmann solutions. We show that this singular behavior does not occur for a class of nonlinear generalizations of the electromagnetic theory. A mathematical toy model is proposed for which the analytical nonsingular extension of FRW solutions is obtained.

Evolution of universes in quadratic theories of gravity

Abstract: We use a dynamical systems approach to investigate Bianchi type I and II universes in quadratic theories of gravity. Because of the complicated nature of the equations of motion we focus on the stability of exact solutions and find that there exists an isotropic Friedmann-Robertson-Walker (FRW) universe acting as a past attractor. This may indicate that there is an isotropization mechanism at early times for these kind of theories. We also discuss the Kasner universes, elucidate the associated center manifold structure, and show that there exists a set of nonzero measure which has the Kasner solutions as a past attractor. Regarding the late-time behavior, the stability shows a dependence of the parameters of the theory. We give the conditions under which the de Sitter solution is stable and also show that for certain values of the parameters there is a possible late-time behavior with phantomlike behavior. New types of anisotropic inflationary behavior are found which do not have counterparts in general relativity.

Generalized second law of thermodynamics in f(T)-gravity

Abstract: We investigate the validity of the generalized second law (GSL) of gravitational thermodynamics in the framework of f(T) modified teleparallel gravity. We consider a spatially flat FRW universe containing only the pressureless matter. The boundary of the universe is assumed to be enclosed by the Hubble horizon. For two viable f(T) models containing f(T) = T+μ1{(−T)}n and f(T) = T−μ2T(1−eβT0/T), we first calculate the effective equation of state and deceleration parameters. Then, {we investigate the null and strong energy conditions and conclude that a sudden future singularity appears in both models. Furthermore, using a cosmographic analysis we check the viability of two models. Finally, we examine the validity of the GSL and find that for both models it} is satisfied from the early times to the present epoch. But in the future, the GSL is violated for the special ranges of the torsion scalar T.

Conditions for the cosmological viability of the most general scalar-tensor theories and their applications to extended Galileon dark energy models

Abstract: In the Horndeski's most general scalar-tensor theories with second-order field equations, we derive the conditions for the avoidance of ghosts and Laplacian instabilities associated with scalar, tensor, and vector perturbations in the presence of two perfect fluids on the flat Friedmann-Lemaitre-Robertson-Walker (FLRW) background. Our general results are useful for the construction of theoretically consistent models of dark energy. We apply our formulas to extended Galileon models in which a tracker solution with an equation of state smaller than -1 is present. We clarify the allowed parameter space in which the ghosts and Laplacian instabilities are absent and we numerically confirm that such models are indeed cosmologically viable.

An Example of a New Type of Cosmological Solutions of Einstein's Field Equations of Gravitation

Abstract: Kurt Gödel became interested in general relativity theory while he and Einstein were both on staff of the Institute for Advanced Studies in Princeton, and saw a lot of each other (Ref. 25, p.7, Ref. 19, p.157). Gödel’s resultant two technical papers were highly original, and had a major impact. Indeed (see Ref. 38, p.111-112, Refs. 29,8) the beginning of the modern studies of singularities in general relativity in many ways had its seeds in the presentation by Gödel in 1949 [10] of an exact solution of Einstein’s equations for pressure-free matter, which could be thought of as a singularity-free rotating but non-expanding cosmological model. This was one of the papers presented in a special issue of Reviews of Modern Physics dedicated to Einstein on his 70th birthday. Gödel used this space-time as an example helping to clarify the nature of time in general relativity, for it is an exact solution of the Einstein equations in which there are closed timelike lines. He shortly thereafter published a further paper [12] discussing a family of exact solutions of Einstein’s equations representing rotating and