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 cosmic microwave background, baryon acoustic oscillations, and Type Ia supernova 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 similar to 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(-0.72)(+0.96) (68% confidence level). This lowers the sound horizon at last scattering, which increases the Hubble constant to 71.09 +/- 0.75 km s(-1) Mpc(-1) with a combination of local measurements as prior and to 69.22(-0.86)(+0.67) km s(-1) 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 kms(-1) 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.