Addressing astrophysical and cosmological problems with secretly asymmetric dark matter

Abstract

We present a simple model of dark matter that can address astrophysical and cosmological puzzles across a wide range of scales. The model is an application of the secretly asymmetric dark matter mechanism, where several flavors of dark matter are fully asymmetric despite an exact dark matter number symmetry $U(1{)}_{\ensuremath{\chi}}$. The dark matter relic abundance arises from these asymmetries, generated in the early Universe through right-handed neutrino decays. The $U(1{)}_{\ensuremath{\chi}}$ is gauged by a massless dark photon, and asymmetries with opposite signs in the different DM flavors result in the formation of bound states. Dark acoustic oscillations in the early Universe lead to a suppression in the matter power spectrum for addressing the ${\ensuremath{\sigma}}_{8}$ problem. The dark photon as a relativistic degree of freedom contributes to $\mathrm{\ensuremath{\Delta}}{N}_{\mathrm{eff}}$, easing the tension between the cosmic microwave background and low redshift ${H}_{0}$ measurements. Finally, scattering between the bound states after structure formation leads to a flattening of the dark matter distribution at the cores of haloes.