# Challenging bulk viscous unified scenarios with cosmological observations

## TL;DR

This paper explores a generalized bulk viscous unified cosmological model, constraining it with observational data, and finds that while non-zero viscosity is favored, the standard Lambda-CDM model remains preferred statistically.

## Contribution

It introduces a general power-law bulk viscous model and tests it against cosmological observations, providing new insights into viscous effects in cosmic evolution.

## Key findings

- Non-zero bulk viscosity is favored by data.
- Some models reduce the H_0 tension.
- Lambda-CDM remains statistically preferred.

## Abstract

In a spatially flat Friedmann-Lema\^{i}tre-Robertson-Walker universe, we investigate a unified cosmic fluid scenario endowed with bulk viscosity in which the coefficient of the bulk viscosity has a power law evolution. The power law in the bulk viscous coefficient is a general case in this study which naturally includes several choices as special cases. Considering such a general bulk viscous scenario, in the present work we have extracted the observational constraints using the latest cosmological datasets and examine their behaviour at the level of both background and perturbations. From the observational analyses, we find that a non-zero bulk viscous coefficient is always favored and some of the models in this series are able to weaken the current tension on $H_0$ for some dataset. However, from the Bayesian evidence analysis, $\Lambda$CDM is favored over the bulk viscous model.