# Viscous dark matter and 21 cm cosmology

## TL;DR

This paper explores how viscous dissipation in dark matter can influence the 21 cm cosmological signal, potentially altering constraints on dark matter properties and providing new bounds on its viscosity.

## Contribution

It introduces the impact of dissipative effects in dark matter on 21 cm cosmology, modifying previous constraints and proposing viscosity bounds from EDGES data.

## Key findings

- Viscous dark matter significantly affects baryon-dark matter energy transfer.
- Inclusion of viscosity alters constraints on dark matter mass and interaction cross-section.
- EDGES data can set bounds on dark matter viscosity much larger than structure formation limits.

## Abstract

The EDGES experiment has detected the global absorption signal of 21 cm line at $z\sim17$ in cosmic dawn era and reported its amplitude larger than the standard cosmological prediction. One of the possible explanation requires that the baryons were much cooler than the standard scenario. This requires an interaction between the dark and baryonic sectors with some appropriate cross-section, $ \hat{\sigma} $. In this work, we examine the role that dissipative effects of cosmic fluid might play in influencing the 21 cm signal. We show that the presence of viscous dissipation of dark matter can significantly affect the energy transfer between the baryonic and dark matter fluids. It is demonstrated that the inclusion of the dissipative mechanism in the dark sector, strongly modify the earlier constraints on dark matter mass and $ \hat{\sigma} $ obtain from EDGES observation. Further, we argue that EDGES absorption signal can put an independent bound on dark matter viscosity which is many order of magnitude larger than the maximum viscosity allowed by the structure formation.