Dark Radiation candidates after Planck

TL;DR

The paper analyzes Planck-era constraints on extra relativistic species and applies them to three beyond-Standard-Model scenarios: light sterile neutrinos in $\text{(3+1)}$ models, thermal hadronic axions, and extended dark sectors. It translates $N_{\textrm{eff}}$ measurements into bounds on sterile-neutrino mixing, axion mass, and dark-sector degrees of freedom, using the Blennow framework for dark sectors. The results show only mild evidence for additional dark radiation, excluding large sterile mixings for $m_s\sim0.1-0.3$ eV and disfavouring hadronic axions with $m_a\gtrsim0.4$ eV under some data combinations, while broader data allows compatibility at smaller masses; dark-sector scenarios with heavy-DOF heating are strongly constrained. The findings guide model-building of light relics and highlight the potential of future Planck polarization data to further tighten limits on dark radiation.

Abstract

Recent Cosmic Microwave Background (CMB) results from the Planck satellite, combined with previous CMB data and Hubble constant measurements from the Hubble Space Telescope, provide a constraint on the effective number of relativistic degrees of freedom of Neff=3.62^{+0.50}_{-0.48} at 95% CL. These new measurements provide a unique opportunity to place limits on models containing relativistic species at the decoupling epoch. Here we review the bounds or the allowed parameter regions in sterile neutrino models, hadronic axion models as well as on extended dark sectors with additional light species based on the latest Planck CMB observations.

Figures (3)

• Figure 1: The left (right) panel illustrates the $95\%$ CL allowed regions in the ($|U_{e4}|$, $|U_{\mu 4}|$) plane arising from the Planck measurements of the effective number of relativistic degrees of freedom $N_{\textrm{eff}}=3.62^{+0.50}_{-0.48}$ ($N_{\textrm{eff}}=3.83\pm 0.54$) for two values of the sterile neutrino mass.
• Figure 2: $\Delta N_{\textrm{eff}}$ as a function of the thermal axion mass (in eV). The left (right) panel illustrates the constraint $N_{\textrm{eff}}=3.62^{+0.50}_{-0.48}$ ($N_{\textrm{eff}}=3.83\pm 0.54$).
• Figure 3: The left (right) panel shows the $2\sigma$ required ranges for the number of heavy degrees of freedom heating the dark sector $g_h$ using $N_{\textrm{eff}}=3.62^{+0.50}_{-0.48}$ ($N_{\textrm{eff}}=3.83\pm 0.54$) for several values of $g_\ell$, the light degrees of freedom of the dark sector.