Phenomenology of Modified Gravity at Recombination
  We discuss the phenomenological imprints of modifications to gravity in the
early universe with a specific focus on the time of recombination. We derive
several interesting results regarding the effect that such modifications have
on cosmological observables, especially on the driving and phasing of acoustic
oscillations, observed in the CMB and BAO, as well as the weak gravitational
lensing of the CMB and of galaxy shapes. This widens the pool of measurements
that can be used to test gravity with present and future surveys, in particular
realizing the full constraining power of the structure of the primary peaks of
the CMB spectrum. We investigate whether such a phenomenology can relax
tensions between cosmological measurements and find that a modification of the
gravitational constant at recombination would help in reconciling measurements
of the CMB with local measurements of the Hubble constant.
