[\sigma _8\] Discrepancy and its solutions

In the recent past, measurements of \[\sigma _8\] from large scale structure observations have shown some discordance with its value obtained from Planck CMB within the \[\Lambda \]CDM frame. This discordance naturally leads to a mismatch in the value of \[H_0\] also. Under the presumption that these discordances are not due to systematics, several attempts have been made to ameliorate the tensions. In this article, we describe the methods of determination of \[\sigma _8\] from large scale as well as CMB observations. We discuss that these discrepancies vanish if we consider the energy momentum tensor for an imperfect fluid which could arise due to self-interaction of dark matter or in an effective description of large scale structure. We demonstrate how the presence of viscosities in cold dark fluid on large scales ameliorate the problem elegantly than other solutions. We also estimate the neutrino mass in the viscous cosmological setup.