Extra Radiation Cosmologies: Implications of the Hubble Tension for eV-scale Neutrinos

Abstract: We present a new analysis on sterile neutrino cosmologies using the Dark Energy Spectroscopic Instrument (DESI) second data release (DR2) baryon acoustic oscillation (BAO) measurements in combination with cosmic microwave background (CMB), CMB lensing, and supernova data. We show that BAO observables are intrinsically less sensitive to the combined effects of relativistic energy density, $N_{\rm eff}$, and the sum of neutrino masses, $\Sigma m_\nu$, which are both augmented in sterile neutrino cosmologies. With SH0ES local expansion rate, $H_0$, data, we find $N_{\rm eff} = 3.43 \pm 0.13$, reducing the Hubble tension to $2.4\sigma$. For a 0.1~eV sterile neutrino, we find $N_{\rm eff}=3.50$ as the best fit. For this representative $N_{\rm eff}$, we find an upper limit of $m_s < 0.17$ eV (95\% CL), greater than a factor of four weaker than standard constraints on $\Sigma m_\nu$. When SH0ES is included, light sterile neutrinos with masses $m_s\simeq0.1$--$0.2$ eV are favored at $\gtrsim 3\sigma$, whereas eV-scale sterile masses remain strongly excluded by the data in the cosmologies we study. Our findings confirm our previous results that partially thermalized sub-eV sterile neutrinos are preferred by the SH0ES $H_0$ data. The preferred $m_s$ mass scale overlaps with, but is not identical to, that favored in neutrino oscillation solutions to short-baseline anomalies.