Minimally Modified Gravity fitting Planck data better than $\Lambda$CDM

TL;DR

This paper explores a class of minimally modified gravity theories, demonstrating that a specific model within this class fits Planck cosmic microwave background data better than the standard Lambda Cold Dark Matter model by altering gravity at intermediate redshifts.

Contribution

The authors introduce and analyze $f( ext{H})$ theories, a new class of modified gravity, and show that a specific three-parameter model provides a better fit to cosmological data than $ ext{Lambda}$CDM.

Findings

The $f( ext{H})$ theory can modify gravity at intermediate redshifts.

The specific three-parameter model fits Planck data better than $ ext{Lambda}$CDM.

The model remains consistent with other late-time cosmological probes.

Abstract

We study the phenomenology of a class of minimally modified gravity theories called $f(\mathcal{H})$ theories, in which the usual general relativistic Hamiltonian constraint is replaced by a free function of it. After reviewing the construction of the theory and a consistent matter coupling, we analyze the dynamics of cosmology at the levels of both background and perturbations, and present a concrete example of the theory with a $3$-parameter family of the function $f$. Finally, we compare this example model to Planck data as well as some later-time probes, showing that such a realization of $f(\mathcal{H})$ theories fits the data significantly better than the standard $\Lambda$CDM model, in particular by modifying gravity at intermediate redshifts, $z\simeq743$.