Confronting the Chaplygin gas with data: background and perturbed cosmic dynamics

Abstract

In this paper, we undertake a unified study of background dynamics and cosmological perturbations in the presence of the Chaplygin gas. This is done by first constraining the background cosmological parameters of different Chaplygin gas models with SNIa data, and then feeding these observationally constrained parameters in the analysis of cosmological perturbations. Based on the statistical criteria we followed, none of the models has a substantial observational support but we show that the so-called 'original' and 'generalized' Chaplygin gas models have some observational support and less observational support, respectively, whereas the 'modified' and 'modified generalized' Chaplygin gas models miss out on the less observational support category but cannot be ruled out. The so-called 'generalized cosmic Chaplygin gas' model, on the other hand, falls under the no observational support category of the statistical criterion and can be ruled out. We follow the 1 + 3 covariant formalism of perturbation theory and derive the evolution equations of the fluctuations in the matter density contrast of the matter-Chaplygin gas system for the models with some or less statistical support. The solutions to these coupled systems of equations are then computed in both short-wavelength and long-wavelength modes.

Perturbations in Bianchi-V Spacetimes with Varying Λ, G and Viscous Fluids

“…Moreover, our results suggest that the longer the wavelength (i.e., the smaller the value of the wavenumber k and hence the dimensionless parameter γ), the larger the perturbation amplitudes, ceteris paribus. We have demonstrated this finding in Figures 8-13, and it is in line with other findings [45][46][47][48] in the literature. In the short-wavelength regime, we noticed the perturbation amplitudes reaching maximum values at about the same redshift that the fractional background matter density peaked; see Figures 2, 7, 8, 10 and 12 for a comparison.…”

Perturbations in the interacting vacuum

“…From the plots, we see that h(z) and q(z) mimic the ΛCDM model. All results are consistent with what is expected for the expansion history of the universe [36,37].…”

Observational constraints of diffusive dark-fluid cosmology

“…In the following we will provide the result for observational constraints for the diffused dark fluid models we have introduced in this work. We have used the distance modulus equation which can be obtained by combining the different cosmological distance definitions to fit against the supernovae data in our MCMC simulation, which is presented in the work of [32].…”

“…In Table I we provide some statistical result which allows us to determine the best diffusive model case in comparisons to the ΛCDM model. The statistical analysis test that we have used is the Akaike information criterion (AIC) and Bayesian/Schwarz information criterion (BIC) selections which were used in a similar work in [32]. These information criteria evaluate the plausibility of an alternative model explaining the data compared to an "accepted/true model".…”

“…In our case the ΛCDM model will be considered as the "true model". Following the suggestion made in [32] as the calculated values for the AIC and BIC can by very random, we will also use the difference in AIC (i.e., ∆AIC) and BIC observational and less observational support, respectively. Case I is seen to have a value just missing out on the substantial observational support category, but is still with a closer value to the boundary for less observational support.…”