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New early dark energy (NEDE) is a component of vacuum energy at the electron volt scale, which decays in a first-order phase transition shortly before recombination [F. Niedermann and M. S. Sloth, New early dark energy]. The NEDE component has the potential to resolve the tension between recent local measurements of the expansion rate of the Universe using supernovae (SN) data and the expansion rate inferred from the early Universe through measurements of the cosmic microwave background (CMB) when assuming ΛCDM. We discuss in depth the two-scalar field model of the NEDE phase transition including the process of bubble percolation, collision, and coalescence. We also estimate the gravitational wave signal produced during the collision phase and argue that it can be searched for using pulsar timing arrays. In a second step, we construct an effective cosmological model, which describes the phase transition as an instantaneous process, and derive the covariant equations that match perturbations across the transition surface. Fitting the cosmological model to CMB, baryonic acoustic oscillations, and SN data, we report H 0 ¼ 69.6 þ1.0 −1.3 km s −1 Mpc −1 (68% C.L.) without the local measurement of the Hubble parameter, bringing the tension down to 2.5σ. Including the local input, we find H 0 ¼ 71.4 AE 1.0 km s −1 Mpc −1 (68% C.L.) and strong evidence for a nonvanishing NEDE component with a ≃4σ significance.

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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 ¼ 71.4 AE 1.0 km s −1 Mpc −1 (68% C.L.).