An exploration of an early gravity transition in light of cosmological
  tensions
  We study a step-like transition in the value of the effective Planck mass (or
effective gravitational constant) on cosmological scales prior to
recombination. We employ CMB, BAO, and SNIa data and find they are sufficient
to strongly constrain our implementation of the Effective Field Theory of Dark
Energy and Modified Gravity, used to model the transition, to a limited
parameter space. The data prefer a $\sim 5\%$ shift in the value of the
effective Planck mass ($<10 \%$ at $2 \sigma$) prior to recombination. This
Transitional Planck Mass (TPM) model is free to undergo its transition at any
point over multiple decades of scale factor prior to recombination,
$\log_{10}(a) = -5.32^{+0.96}_{-0.72}$ (68\% CL). This lowers the sound horizon
at last scattering, which increases the Hubble constant to $71.09 \pm 0.75$ km
$\textrm{s}^{-1}\textrm{Mpc}^{-1}$ with a combination of local measurements as
prior and to $69.22^{+0.67}_{-0.86}$ km $\textrm{s}^{-1}\textrm{Mpc}^{-1}$ when
the prior is excluded. The TPM model improves $\chi^2$ with respect to
$\Lambda$CDM by $\Delta \chi^2 = -23.72$ with the $H_0$ prior and $\Delta
\chi^2 = -4.8$ without the prior. The model allows for both $H_0 > 70$
km$\textrm{s}^{-1}\textrm{Mpc}^{-1}$ and $S_8 < 0.80$ simultaneously with lower
values of $S_8$ due to a reduction in the matter density $\Omega_m$ to offset
the increase in $H_0$ relative to $\Lambda$CDM. While this is a particular
modified gravity model, studying other variants of modified gravity may be a
productive path for potentially resolving cosmological tensions, while avoiding
the need for a cosmological constant.
