Lightening the Dark Matter from its Viscosity and Explanation of EDGES Anomaly

Abstract

We study the visible photon production from the viscous dissipation of the dark matter (DM) fluid. The visible photon production depends on the magnitude of the dark matter viscosity and becomes important at the late times. We argue that for sufficiently large dark matter viscosity, the number of the resonantly converted visible photons becomes large which populates the Rayleigh-Jeans (RJ) tails of the Cosmic Microwave Background (CMB) radiation. Consequently, these excess visible photons possibly can explain the reported EDGES anomaly in the 21 cm signal. Further, we explore the parameter space for which the 21 cm signal can provide the region to probe the dark radiation and the DM viscosity.