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

Abstract

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 $1\mathrm{eV}\lesssim m_{\chi }\lesssim 10\mathrm{eV}$ around the matter-radiation equality epoch (i.e., redshift $z_{\mathrm{eq}}\sim 3400$) 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 $z_{\mathrm{eq}}$, 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 Mészáros effect. As a result, both the $H_0$ and the $S_8$ tensions of the current standard cosmological model are significantly alleviated.

arXiv comment8 pages/5 figures. V.2: version accepted for publication in PRD

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• Challenges for $\Lambda$CDM: An update
• Cosmology Intertwined: A Review of the Particle Physics, Astrophysics, and Cosmology Associated with the Cosmological Tensions and Anomalies
• Can the Hubble tension be resolved by bulk viscosity?
• Limit on the dark matter mass from its interaction with photons
• What Prevents Resolving the Hubble Tension through Late-Time Expansion Modifications?
• Equality scale-based and sound horizon-based analysis of the Hubble tension
• Bulk viscosity from early-time thermalization of cosmic fluids in light of DESI DR2 data