Early dark energy from massive neutrinos -- a natural resolution of the
  Hubble tension
  The Hubble tension can be significantly eased if there is an early component
of dark energy that becomes active around the time of matter-radiation
equality. Early dark energy models suffer from a coincidence problem -- the
physics of matter-radiation equality and early dark energy are completely
disconnected, so some degree of fine-tuning is needed in order for them to
occur nearly simultaneously. In this paper we propose a natural explanation for
this coincidence. If the early dark energy scalar couples to neutrinos then it
receives a large injection of energy around the time that neutrinos become
non-relativistic. This is precisely when their temperature is of order their
mass, which, coincidentally, occurs around the time of matter-radiation
equality. Neutrino decoupling therefore provides a natural trigger for early
dark energy by displacing the field from its minimum just before
matter-radiation equality. We discuss various theoretical aspects of this
proposal, potential observational signatures, and future directions for its
study.
