New measurements of the expansion rate of the Universe have plunged the standard model of cosmology into a severe crisis. In this paper, 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 subdominant 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 $H_0=71.4\pm 1.0\mathrm{km}{\mathrm{s}}^{-1}{\mathrm{Mpc}}^{-1}$ (68% C.L.).

• Received 14 November 2019
• Revised 4 July 2020
• Accepted 22 January 2021

DOI:https://doi.org/10.1103/PhysRevD.103.L041303