Early dark energy by a dark Higgs field and axion-induced nonthermal trapping

Abstract

We propose a new scenario of early dark energy (EDE) with a dark Higgs field trapped at the origin. To keep this dark Higgs trapped until around the matter-radiation equality, we use dark photons produced nonthermally by coherent oscillations of axions, which have a much stronger trapping effect than thermal mass. When the trapping ends, the dark Higgs quickly decays into dark photons, which are then redshifted as radiation. In fact, dark photons are self-interacting dark radiation, and the dark Higgs field can also be in thermal equilibrium if its self-coupling is small enough. In some cases, these particles can act as the Wess-Zumino dark radiation. The dark Higgs EDE scenario works well for an ordinary Mexican-hat potential, and the dark Higgs naturally sits at the origin from the beginning, since it is the symmetry-enhanced point. Thus, unlike the axion EDE, there is no need for elaborate potentials or fine-tuning with respect to the initial condition. Interestingly, the axion not only produces dark photons, but also explains dark matter. We find the viable parameter region of the axion decay constant and the axion mass where dark matter and the ${H}_{0}$ tension can be simultaneously explained. We also discuss the detectability of the axion in the presence of axion-photon coupling, and show that the axion can be the QCD axion.