Alleviating $H_0$ tension in Horndeski gravity

TL;DR

This paper demonstrates that specific subclasses of Horndeski gravity can reduce the H_0 tension by increasing the late-time Hubble constant without conflicting with early universe observations.

Contribution

It introduces two models within Horndeski gravity where the H_0 tension is alleviated by controlling the kinetic energy dependence, maintaining consistency with CMB constraints.

Findings

H_0 can be increased to ~74 km/s/Mpc in the models.

Models satisfy stability conditions for scalar perturbations.

Confronted models with SNIa data showing compatibility.

Abstract

We show that the $H_0$ tension can be alleviated in the framework of Horndeski/generalized galileon gravity. In particular, since the terms depending on $G_5$ control the friction in the Friedmann equation, we construct specific sub-classes in which it depends only on the field's kinetic energy. Since the latter is small at high redshifts, namely at redshifts which affected the CMB structure, the deviations from $\Lambda$CDM cosmology are negligible, however as time passes it increases and thus at low redshifts the Hubble function acquires increased values in a controlled way. We consider two Models, one with quadratic and one with quartic dependence on the field's kinetic energy. In both cases we show the alleviation of the tension, resulting to $H_0 \approx 74$ km/s/Mpc for particular parameter choices. Finally, we examine the behavior of scalar metric perturbations, showing that the conditions for absence of ghost and Laplacian instabilities are fulfilled throughout the evolution, and we confront the models with SNIa data.