Exploring the Dark Sector: Interacting Radiation in Light of Modern Cosmological Probes

Abstract

[abridged] We constrain a phenomenological dark radiation (DR) framework consisting of free-streaming and fluid-like components, providing a model-independent extension of the standard radiation sector. Using Planck CMB data, DESI DR2 BAO measurements, and Pantheon+ and DES Y5 (Dovekie) supernova samples, we derive constraints on additional relativistic degrees of freedom and assess their impact on cosmological tensions. We obtain $N_{\rm fld}<0.66$ (95% C.L.) from CMB data alone, while the combination with BAO yields $N_{\rm fs}=2.93\pm0.23$ and $N_{\rm fld}=0.36^{+0.16}_{-0.21}$ (68% C.L.), consistent with Standard Model expectations for free-streaming radiation. The DR framework significantly alleviates the Hubble tension through an enhanced early-time expansion rate, which reduces the sound horizon scale. The tension with SH0ES is reduced from highly significant in $Λ$CDM to statistically non-significant for CMB+BAO data according to the $\mathcal{T}$-statistic. Bayesian model comparison shows no decisive preference for DR over $Λ$CDM when SH0ES is excluded, with results in the regime of weakly disfavoured. However, including SH0ES data leads to decisive Bayesian evidence in favour of the DR scenario. Overall, DR provides a compelling framework for resolving the Hubble tension. When CMB, BAO, Pantheon$+$ and SH0ES data are considered, we find an increased effective radiation content, $N_{\rm tot}=3.63^{+0.13}_{-0.15}$, with a fraction of free-streaming radiation, $f_{\rm fs}=0.392\pm 0.026$, a reduced sound horizon scale, $r_d = 141.8^{+1.3}_{-1.2}\,\mathrm{Mpc}$, and a higher primordial helium fraction, $Y_{\rm He}=0.2530 \pm 0.0017$, which lies at the level of approximately $\sim 2$-$2.5σ$ above direct determinations from metal-poor H II regions, while remaining broadly consistent with other abundance measurements within current uncertainties.