# Testing dissipative dark matter in causal thermodynamics

## TL;DR

This paper investigates a cosmological model with dissipative dark matter using causal thermodynamics, testing its consistency with observational data and exploring its implications for universe acceleration without a cosmological constant.

## Contribution

It applies the Israel-Stewart theory to a specific analytic dark matter model and tests its observational viability, revealing both its potential and limitations.

## Key findings

- Supports accelerated expansion without cosmological constant
- Identifies large non-adiabatic sound speed issues
- Finds some inconsistencies with fluid description of dark matter

## Abstract

In this paper we study the consistency of a cosmological model representing a universe filled with a one-component dissipative dark matter fluid, in the framework of the causal Israel-Stewart theory, where a general expression arising from perturbation analysis for the relaxation time $\tau$ is used. This model is described by an exact analytic solution recently found in [N. Cruz, E. Gonz\'alez and G. Palma, Gen. Rel. Grav. \textbf{52}, 62 (2020), which depends on several model parameters as well as integration constants, allowing the use of Type Ia Supernovae and Observational Hubble data to perform an astringent observational test. The constraint regions found for the parameters of the solution allow the existence of an accelerated expansion of the universe at late times, after the domination era of the viscous pressure, which holds without the need of including a cosmological constant. Nevertheless, the fitted parameter values lead to drawbacks as a very large non-adiabatic contribution to the speed of sound, and some inconsistencies, not totally conclusive, with the description of the dissipative dark matter as a fluid, which is nevertheless a common feature of these kind of models.