📝 SummarXiv

Abstract

If nonrelativistic dark matter and radiation are allowed to interact, reaching an approximate thermal equilibrium, this interaction induces a bulk viscous pressure changing the effective one-fluid description of the universe dynamics. By modeling such components as perfect fluids, a cosmologically relevant bulk viscous pressure emerges for dark matter particle masses in the range of $1\,\text{eV} - 10\,\text{eV}$ keeping thermal equilibrium with the radiation. Such a transient bulk viscosity introduces significant effects in the expansion rate near the matter-radiation equality (redshift $z_{\text{eq}} \sim 3400$) and at late times (leading to a higher inferred value of the Hubble constant $H_0$). We use the recent DESI DR2 BAO data to place an upper bound on the free parameter of the model $\tau_\text{eq}$ which represents the time scale in which each component follows its own internal perfect fluid dynamics until thermalization occurs. Our main result is encoded in the bound $\tau_\text{eq} < 1.84 \times 10^{-10} $ s (2$\sigma$), with the corresponding dimensionless bulk coefficient $\tilde{\xi} H_0/H_{eq}<5.94\times10^{-4}$ (2$\sigma$). In practice, this rules out any possible interaction between radiation and dark matter prior to the recombination epoch.