Hot new early dark energy bridging cosmic gaps: Supercooled phase transition reconciles stepped dark radiation solutions to the Hubble tension with BBN
Mathias Garny, Florian Niedermann, Henrique Rubira, Martin S. Sloth
Abstract
We propose a simple model that can alleviate the ${H}_{0}$ tension while remaining consistent with big bang nucleosynthesis (BBN). It is based on a dark sector described by a standard Lagrangian featuring a $SU(N)$ gauge symmetry with $N\ensuremath{\ge}3$ and a massive scalar field with a quartic coupling. The scalar acts as a dark Higgs leading to spontaneous symmetry breaking $SU(N)\ensuremath{\rightarrow}SU(N\ensuremath{-}1)$ via a first-order phase transition \`a la Coleman-Weinberg. This setup naturally realizes previously proposed scenarios featuring strongly interacting dark radiation (SIDR) with a mass threshold within hot new early dark energy. For a wide range of reasonable model parameters, the phase transition occurs between the BBN and recombination epochs and releases a sufficient amount of latent heat such that the model easily respects bounds on extra radiation during BBN while featuring a sufficient SIDR density around recombination for increasing the value of ${H}_{0}$ inferred from the cosmic microwave background. Our model can be summarized as a natural mechanism providing two successive increases in the effective number of relativistic degrees of freedom after BBN but before recombination $\mathrm{\ensuremath{\Delta}}{N}_{\mathrm{BBN}}\ensuremath{\rightarrow}\mathrm{\ensuremath{\Delta}}{N}_{\mathrm{NEDE}}\ensuremath{\rightarrow}\mathrm{\ensuremath{\Delta}}{N}_{\mathrm{IR}}$ alleviating the Hubble tension. The first step is related to the phase transition, and the second is related to the dark Higgs becoming nonrelativistic. This setup predicts further signatures, including a stochastic gravitational wave background and features in the matter power spectrum that can be searched for with future pulsar timing and Lyman-$\ensuremath{\alpha}$ forest measurements.