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.

Equation of state for the Universe from similarity symmetries

Abstract: In this paper we proposed to use the group of analysis of symmetries of the dynamical system to describe the evolution of the Universe. This method is used in searching for the unknown equation of state. It is shown that group of symmetries enforce the form of the equation of state for noninteracting scaling multifluids. We showed that symmetries give rise to the equation of state in the form p =-Λ + w 1ρ(a) + w 2 a β + 0 and energy density ρ = Λ+ρ01 a -3(1+w) +ρ02 a α +ρ03 a -3, which is commonly used in cosmology. The FRW model filled with scaling fluid (called homological) is confronted with the observations of distant type Ia supernovae. We found the class of model parameters admissible by the statistical analysis of SNIa data.We showed that the model with scaling fluid fits well to supernovae data. We found that Ωm,0 ≃ 0.4 and n ≃ -1 (β = -3n), which can correspond to (hyper) phantom fluid, and to a high density universe. However if we assume prior that Ωm,0 = 0.3 then the favoured model is close to concordance ΛCDM model. Our results predict that in the considered model with scaling fluids distant type Ia supernovae should be brighter than in the ΛCDM model, while intermediate distant SNIa should be fainter than in the ΛCDM model. We also investigate whether the model with scaling fluid is actually preferred by data over ΛCDM model. As a result we find from the Akaike model selection criterion: it prefers the model with noninteracting scaling fluid.

Does the cosmological term influence gravitational lensing

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.

Bianchi I model: an alternative way to model the present-day Universe

Abstract: Although the new era of high-precision cosmology of the cosmic microwave background (CMB) radiation improves our knowledge to understand the infant as well as the present-day Universe, it also leads us to question the main assumption of the exact isotropy of the CMB. There are two pieces of observational evidence that hint towards there being no exact isotropy. These are: first, the existence of small anisotropy deviations from isotropy of the CMB radiation and secondly, the presence of large angle anomalies, although the existence of these anomalies is currently a huge matter of debate. These hints are particularly important since isotropy is one of the two main postulates of the Copernican principle on which the Friedmann Robertson Walker (FRW) models are built. This almost-isotropic CMB radiation implies that the universe is almost an FRW universe, as is proved by previous studies. Assuming that the matter component forms the deviations from isotropy in the CMB density fluctuations when matter and radiation decouples, we here attempt to find possible constraints on the FRW-type scale and Hubble parameter by using the Bianchi type I (BI) anisotropic model which is asymptotically equivalent to the standard FRW. To obtain constraints on such an anisotropic model, we derive average and late-time shear values that come from the anisotropy upper limits of the recent Planck data based on a model independent shear parameter of Maartens, Ellis & Stoeger and from the theoretical consistency relation. These constraints lead us to obtain a BI model which becomes an almost-FRW model in time, and which is consistent with the latest observational data of the CMB.

FRW cosmology with the extended Chaplygin gas

Abstract: In this paper, 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 about the resulting effective equation of state parameter. Using density perturbations we investigate the stability of the theory.

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=k \rho $, $k=(4-n)/3n$. 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.