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Exploring the Self Interacting Dark Matter Properties From Low Redshift Observations

The small scale observations indicate that the dark matter may be self-interacting. In this paper, we calculate the shear ($η$) and bulk viscosity ($ζ$) of Self Interacting Dark Matter (SIDM) fluid, in kinetic theory formalism. Further, using the KSS bound on $η/\mathfrak{s}$, we derive a new upper limit on the ratio of dark matter self interaction cross section to its mass, $ σ/m $. Then, using the $ σ/m $ constraint, we show that KSS bound allows only sub-GeV mass of SIDM particle. Further, with the assumption of a power-law form of $η$ and $ζ$, we study its evolution in the light of low redshift observations. We find that at the large redshift, the SIDM viscosity is small but at the small redshift it becomes sufficiently large and contributes significantly in cosmic dissipation. As a consequence, viscous SIDM can explain the low redshift observations and also consistent with the standard cosmological prediction.