Viscous dark matter and 21 cm cosmology VISCOUS DARK MATTER and 21 CM COSMOLOGY ... BHATT, MISHRA, and NAYAK

The EDGES experiment has detected the global absorption signal of the 21 cm line at z∼17 in cosmic dawn era and reported its amplitude larger than the standard cosmological prediction. One 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, σ^. 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 σ^ obtain from EDGES observation. Further, we argue that the EDGES absorption signal can put an independent bound on dark matter viscosity, which is an order of magnitude larger than the maximum viscosity allowed from the structure formation. Also to preserve the requirement of the coldness of dark matter, we estimate another independent bound on the dark matter viscosity, which is smaller in comparison with the upper limit on the viscosity obtained from the structure formation. To best of our knowledge, the constraint on the dark matter viscosity obtained from the requirement of the dark matter coldness paradigm is a stringent bound on the dark matter viscosity.