Viscosity in cosmic fluids
  The effective theory of large-scale structure formation based on $\Lambda$CDM
paradigm predicts finite dissipative effects in the resulting fluid equations.
In this work, we study how viscous effect that could arise if one includes
self-interaction among the dark-matter particles combines with the effective
theory. It is shown that these two possible sources of dissipation can operate
together in a cosmic fluid and the interplay between them can play an important
role in determining dynamics of the cosmic fluid. In particular, we demonstrate
that the viscosity coefficient due to self-interaction is added inversely with
the viscosity calculated using effective theory of $\Lambda$CDM model. Thus the
larger viscosity has less significant contribution in the effective viscosity.
Using the known bounds on $\,\sigma/m$ for self-interacting dark-matter, where
$\,\sigma\,$ and $m$ are the cross-section and mass of the dark-matter
particles respectively, we discuss role of the effective viscosity in various
cosmological scenarios.
