Early Dark Energy and Dark Photon Dark Matter from Waterfall Symmetry Breaking

We investigate a cosmological model wherein a waterfall symmetry breaking occurs during the radiation-dominated era. The model comprises a complex waterfall field, an axion field, and the gauge field (dark photon) generated through a tachyonic instability due to the Chern-Simons interaction. Prior to symmetry breaking, the total energy density incorporates a vacuum energy from the waterfall field, establishing a novel scenario for Early Dark Energy (EDE). Subsequent to the symmetry breaking, the dark photon dynamically acquires mass via the Higgs mechanism, potentially contributing to the dark matter abundance. Hence, our model can simultaneously address the $H_0$ tension and the origin of dark matter.