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.

A stable flat universe with variable cosmological constant in $f(R,T)$ gravity

Abstract: In this paper, a general FRW cosmological model has been constructed in f(R,T) gravity reconstruction with variable cosmological constant. A number of solutions to the field equations has been generated by utilizing a form for the Hubble parameter that leads to Berman’s law of constant deceleration parameter q = m − 1. The possible decelerating and accelerating solutions have been investigated. For (q > 0) we get a stable flat decelerating radiation-dominated universe at q = 1. For (q < 0) we get a stable accelerating solution describing a flat universe with positive energy density and negative cosmological constant. Nonconventional mechanisms that are expected to address the late-time acceleration with negative cosmological constant have been discussed.

Proper size of the visible Universe in FRW metrics with a constant spacetime curvature

Abstract: In this paper, we continue to examine the fundamental basis for the Friedmann–Robertson–Walker (FRW) metric and its application to cosmology, specifically addressing the question: What is the proper size of the visible universe? There are several ways of answering the question of size, though often with an incomplete understanding of how far light has actually traveled in reaching us today from the most remote sources. The difficulty usually arises from an inconsistent use of the coordinates or an over-interpretation of the physical meaning of quantities such as the so-called proper distance R(t) = a(t)r, written in terms of the (unchanging) co-moving radius r and the universal expansion factor a(t). In this paper, we prove for the five non-trivial FRW metrics with a constant spacetime curvature that, when the expansion began from an initial singularity, the visible universe today has a proper size equal to Rh(t0/2), i.e., the gravitational horizon at half its current age. The exceptions are de Sitter and Lanczos, whose contents had pre-existing positions away from the origin. In so doing, we confirm earlier results showing the same phenomenon in a broad range of cosmologies, including ΛCDM, based on the numerical integration of null geodesic equations through an FRW metric.

Novel null-test for the Λ cold dark matter model with growth-rate data

Abstract: Current and upcoming surveys will measure the cosmological parameters with an extremely high accuracy. The primary goal of these observations is to eliminate some of the currently viable cosmological models created to explain the late-time accelerated expansion (either real or only inferred). However, most of the statistical tests used in cosmology have a strong requirement: the use of a model to fit the data. Recently there has been an increased interest on finding tests that are model independent, i.e. to have a function that depends entirely on observed quantities and not on the model, see for instance [C. Clarkson, B. Bassett and T. H. C. Lu, Phys. Rev. Lett.101 (2008) 011301, arXiv:0712.3457 [astro-ph]]. In this paper, we present an alternative consistency check at the perturbative level for a homogeneous and isotropic Universe filled with a dark energy component. This test makes use of the growth of matter perturbations data and it is able to detect a deviation from the standard cosmological model, which could later be attributed to a clustering dark energy component, a tension in the data or a modification of gravity, within the framework of a Friedmann–Lemaitre–Robertson–Walker (FLRW) universe.

Cosmological string models from milne spaces and sl(2, ℤ) orbifold

Abstract: The (n + 1)-dimensional Milne universe with extra free directions is used to construct simple FRW cosmological string models in four dimensions, describing expansion in the presence of matter with p= κρ, κ = (4 - n)/3n. We then consider the n = 2 case and make SL(2, ℤ) orbifold identifications. The model is surprisingly related to the null orbifold with an extra reflection generator. The study of the string spectrum involves the theory of harmonic functions in the fundamental domain of SL(2, ℤ). In particular, from this theory one can deduce a bound for the energy gap and the fact that there are an infinite number of excitations with a finite degeneracy. We discuss the structure of wave functions and give examples of physical winding states becoming light near the singularity.