Early-time thermalization of cosmic components? A hint for solving cosmic tensions

Understand

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.