Viscous dark matter and 21 cm cosmology
  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.
