Non-adiabatic Chaplygin gas

TL;DR

This paper investigates the non-adiabatic perturbations in a split generalized Chaplygin gas model, showing that the effective sound speed remains small, avoiding instabilities and oscillations seen in earlier adiabatic models.

Contribution

It demonstrates that non-adiabatic pressure perturbations lead to a small effective sound speed, resolving issues of oscillations and instabilities in Chaplygin gas models.

Findings

Effective sound speed c_s^2 is proportional to dark energy to dark matter perturbation ratio.

Dark-energy perturbations are negligible compared to dark-matter perturbations on relevant scales.

No oscillations or instabilities occur in the non-adiabatic model.

Abstract

The split of a generalised Chaplygin gas with an equation of state p = -A/\rho^{\alpha} into an interacting mixture of pressureless matter and a dark-energy component with equation of state p_{\Lambda} = - \rho_{\Lambda} implies the existence of non-adiabatic pressure perturbations. We demonstrate that the square of the effective (non-adiabatic) sound speed c_s of the medium is proportional to the ratio of the perturbations of the dark energy to those of the dark matter. Since, as demonstrated explicitly for the particular case \alpha = -1/2, dark-energy perturbations are negligible compared with dark-matter perturbations on scales that are relevant for structure formation, we find |c_s^2| << 1. Consequently, there are no oscillations or instabilities which have plagued previous adiabatic Chaplygin-gas models.