Phenomenology of Modified Gravity at Recombination

TL;DR

This paper explores how modifications to gravity during recombination affect cosmological observations like the CMB and BAO, potentially helping to resolve tensions in measurements of the Hubble constant.

Contribution

It provides a phenomenological framework for understanding the imprints of modified gravity on early universe observables, expanding the tools for testing gravity with cosmological data.

Findings

Modifications to gravity influence acoustic oscillations in the CMB.

Altered gravity can help reconcile CMB and local Hubble constant measurements.

The study broadens the observational constraints on gravity theories during recombination.

Abstract

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.