https://doi.org/10.1140/epjc/s10052-020-8341-8
Regular Article - Theoretical Physics
Viscosity in cosmic fluids
1
Theoretical Physics Division,Physical Research Laboratory, Ahmedabad, 380 009, Gujarat, India
2
Department of Physics, Indian Institute of Technology Gandhinagar, Palaj,Gandhinagar, 382 355, Gujartat, India
3
Department of Physics and Astrophysics, University of Delhi, Delhi, 110 007, India
* e-mail: pvn.sps@gmail.com
** e-mail: pravin@prl.res.in
Received:
19
February
2020
Accepted:
10
August
2020
Published online:
24
August
2020
The effective theory of large-scale structure formation based on 
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 CDM model. Thus the larger viscosity has less significant contribution in the effective viscosity. Using the known bounds on 
for self-interacting darkmatter, where 
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.
© The Author(s), 2020
