New Early Dark Energy

TL;DR

This paper proposes a first order phase transition in a dark sector as a solution to the cosmological crisis caused by new measurements of the Universe's expansion rate, improving the fit to observational data.

Contribution

It introduces a novel NEDE model involving a phase transition in the dark sector that better explains cosmological observations compared to the standard model.

Findings

Significant improvement in fit to CMB, BAO, and SNe data.

Hubble constant estimate of 71.4 km/s/Mpc with 1 km/s/Mpc uncertainty.

Supports early dark energy as a resolution to the Hubble tension.

Abstract

New measurements of the expansion rate of the Universe have plunged the standard model of cosmology into a severe crisis. In this letter, we propose a simple resolution to the problem that relies on a first order phase transition in a dark sector in the early Universe, before recombination. This will lead to a short phase of a New Early Dark Energy (NEDE) component and can explain the observations. We model the false vacuum decay of the NEDE scalar field as a sudden transition from a cosmological constant source to a decaying fluid with constant equation of state. The corresponding fluid perturbations are covariantly matched to the adiabatic fluctuations of a sub-dominant scalar field that triggers the phase transition. Fitting our model to measurements of the cosmic microwave background (CMB), baryonic acoustic oscillations (BAO, and supernovae (SNe) yields a significant improvement of the best-fit compared with the standard cosmological model without NEDE. We find the mean value of the present Hubble parameter in the NEDE model to be $H_0=71.4 \pm 1.0 ~\textrm{km}\, \textrm{s}^{-1}\, \textrm{Mpc}^{-1}$ ($68\, \%$ C.L.).