Early-time thermalization of cosmic components? A hint for solving
  cosmic tensions
  We study an expanding two-fluid model of non-relativistic dark matter and
radiation which are allowed to interact during a certain time span and to
establish an approximate thermal equilibrium. Such interaction which generates
an effective bulk viscous pressure at background level is expected to be
relevant for times around the transition from radiation to matter dominance. We
quantify the magnitude of this pressure for dark matter particles masses within
the range $1 {\rm eV} \lesssim m_{\chi} \lesssim 10 {\rm eV}$ around the
matter-radiation equality epoch (i.e., redshift $z_{\rm eq}\sim 3400$) and
demonstrate that the existence of a transient bulk viscosity has consequences
which may be relevant for addressing current tensions of the standard
cosmological model: i) the additional (negative) pressure contribution modifies
the expansion rate around $z_{\rm eq}$, yielding a larger $H_0$ value and ii)
large-scale structure formation is impacted by suppressing the amplitude of
matter overdensity growth via a new viscous friction-term contribution to the
M\'esz\'aros effect. As a result, both the $H_0$ and the $S_8$ tensions of the
current standard cosmological model are significantly alleviated.
