Neutrino-Assisted Early Dark Energy: Theory and Cosmology
  The tension between measurements of the Hubble constant obtained at different
redshifts may provide a hint of new physics active in the relatively early
universe, around the epoch of matter-radiation equality. A leading paradigm to
resolve the tension is a period of early dark energy, in which a scalar field
contributes a subdominant part of the energy budget of the universe at this
time. This scenario faces significant fine-tuning problems which can be
ameliorated by a non-trivial coupling of the scalar to the standard model
neutrinos. These become non-relativistic close to the time of matter-radiation
equality, resulting in an energy injection into the scalar that kick-starts the
early dark energy phase, explaining its coincidence with this seemingly
unrelated epoch. We present a minimal version of this neutrino-assisted early
dark energy model, and perform a detailed analysis of its predictions and
theoretical constraints. We consider both particle physics constraints -- that
the model constitute a well-behaved effective field theory for which the
quantum corrections are under control, so that the relevant predictions are
within its regime of validity -- and the constraints provided by requiring a
consistent cosmological evolution from early through to late times. Our work
paves the way for testing this scenario using cosmological data sets.
