The extreme limit of the generalized Chaplygin gas

TL;DR

This paper examines the generalized Chaplygin gas as a unified dark matter and dark energy model, finding that observational effects like the ISW effect strongly constrain its parameters and impact the universe's expansion history.

Contribution

It provides new constraints on the generalized Chaplygin gas model using the ISW effect and supernova data, exploring its viability as a unified dark sector candidate.

Findings

ISW effect excludes models with $10^{-3} < b5 < 350$

Large b5 values lead to a universe transitioning from matter-dominated to de Sitter phase

Supernova data places the transition redshift at $z_{tr} = 0.22

Abstract

Unified Dark Matter models describe Dark Matter and Dark Energy as a single entity which is, in the simplest case, embodied in a perfect barotropic fluid. It is a well-established fact that small adiabatic perturbations of Unified Dark Matter have an evolution characterised by oscillations and decay which provide predictions on the Cosmic Background Radiation anisotropies which are in poor agreement with observation. In this paper we investigate the generalised Chaplygin gas and we find that the Integrated Sachs-Wolfe effect excludes the model for $10^{-3} < \alpha < 350$. We discuss the implications on the background evolution of the Universe if large values of $\alpha$ are considered. In this case, the Universe expansion mimics a matter-dominated phase abruptly followed by a de Sitter one at the transition redshift $z_tr$. Thanks to an analysis of the type Ia supernovae Constitution set we are able to place $z_tr = 0.22$.