New Early Dark Energy
  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.).
