Constraining a matter-dominated cosmological model with bulk viscosity proportional to the Hubble parameter

TL;DR

This paper investigates a cosmological model with bulk viscosity proportional to the Hubble parameter as an explanation for accelerated expansion, testing its consistency with observational data and thermodynamic laws.

Contribution

It introduces and constrains a bulk viscosity cosmological model using multiple observational datasets and thermodynamic principles, highlighting its limitations and viability conditions.

Findings

Model agrees with the Second Law of Thermodynamics using SNe Ia data alone.

When combining SNe, CMB, and BAO data, the model violates thermodynamics.

The model predicts a low Hubble constant (~53 km/sec/Mpc) and fits data poorly with high chi-squared.

Abstract

We present and constrain a cosmological model where the only component is a pressureless fluid with bulk viscosity as an explanation for the present accelerated expansion of the universe. We study the particular model of a bulk viscosity coefficient proportional to the Hubble parameter. The model is constrained using the SNe Ia Gold 2006 sample, the Cosmic Microwave Background (CMB) shift parameter R, the Baryon Acoustic Oscillation (BAO) peak A and the Second Law of Thermodynamics (SLT). It was found that this model is in agreement with the SLT using only the SNe Ia test. However when the model is constrained using the three cosmological tests together (SNe+CMB+BAO) we found: 1.- The model violates the SLT, 2.- It predicts a value of H_0 \approx 53 km sec^{-1} Mpc^{-1} for the Hubble constant, and 3.- We obtain a bad fit to data with a \chi^2_{min} \approx 532. These results indicate that this model is viable just if the bulk viscosity is triggered in recent times.