Light dark photon and fermionic dark radiation for the Hubble constant and the structure formation

Abstract

Motivated by the tensions in the Hubble constant \(H_0\) and the structure growth \(\sigma_8\) between \(Planck\) results and other low redshift measurements, we discuss some cosmological effects of a dark sector model in which dark matter (DM) interacts with fermionic dark radiation (DR) through a light gauge boson (dark photon). Such kind of models are very generic in particle physics with a dark sector with dark gauge symmetries. The effective number of neutrinos is increased by \(\delta N_{eff} \sim 0.5\) due to light dark photon and fermionic DR, thereby resolving the conflicts in \(H_0\). The elastic scattering between DM and DR induces suppression for DM's density perturbation, but without acoustic oscillations. For weakly-interacting DM around \(100\)GeV, the new gauge coupling should be \(\sim 10^{-4}\) to have sizable effect on matter power spectrum in order to relax the tension in \(\sigma_8\).