Dark matter-radiation interaction as a source of bulk viscosity: constraints from DESI DR2
ORAL
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 eV - 10 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 zeq∼3400) and at late times (leading to a higher inferred value of the Hubble constant H0); cf. H. Velten et al. [2104.05352].
We use the recent DESI DR2 BAO data to place an upper bound on the free parameter of the model τeq 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 τeq < 1.84×10-10 s (2σ), with the corresponding dimensionless bulk coefficient ξ·H0/Heq < 5.94×10-4 (2σ). In practice, this rules out any possible interaction between radiation and dark matter prior to the recombination epoch.
For further information, cf. H. Velten and W. Iania [2509.05148].
We use the recent DESI DR2 BAO data to place an upper bound on the free parameter of the model τeq 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 τeq < 1.84×10-10 s (2σ), with the corresponding dimensionless bulk coefficient ξ·H0/Heq < 5.94×10-4 (2σ). In practice, this rules out any possible interaction between radiation and dark matter prior to the recombination epoch.
For further information, cf. H. Velten and W. Iania [2509.05148].
*The authors thank FAPEMIG, FAPES, CAPES and CNPq for financial support. The authors would like to acknowledge the use of the computational resources provided by the Sci-Com Lab of the Department of Physics at UFES, which was funded by FAPES, CAPES and CNPq. W. Iania gratefully acknowledges the Sociedade Brasileira de Física (SBF) for enabling participation in the APS meeting through the reciprocal society membership program.
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Publication: Submitted manuscript (Physical Review D): H. Velten and W.Iania, "Bulk viscosity from early-time thermalization of cosmic fluids in light of DESI DR2 data", arXiv:2509.05148 [astro-ph.CO].
Presenters
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William Iania
- Federal University of Espirito Santo