Weakening Gravity on Redshift-Survey Scales with Kinetic Matter Mixing

TL;DR

This paper investigates scalar-tensor models with kinetic matter mixing that weaken gravity on small scales, potentially resolving discrepancies between CMB data and low-redshift observations.

Contribution

It introduces a general framework for kinetic matter mixing in scalar-tensor theories, deriving stability conditions and analyzing its effects on cosmological observables.

Findings

Kinetic Matter Mixing weakens gravity on short scales.

Lowered $\sigma_8$ could address CMB-low redshift tension.

Implemented in Einstein-Boltzmann solver for cosmological predictions.

Abstract

We explore general scalar-tensor models in the presence of a kinetic mixing between matter and the scalar field, which we call Kinetic Matter Mixing. In the frame where gravity is de-mixed from the scalar this is due to disformal couplings of matter species to the gravitational sector, with disformal coefficients that depend on the gradient of the scalar field. In the frame where matter is minimally coupled, it originates from the so-called beyond Horndeski quadratic Lagrangian. We extend the Effective Theory of Interacting Dark Energy by allowing disformal coupling coefficients to depend on the gradient of the scalar field as well. In this very general approach, we derive the conditions to avoid ghost and gradient instabilities and we define Kinetic Matter Mixing independently of the frame metric used to described the action. We study its phenomenological consequences for a $\Lambda$CDM background evolution, first analytically on small scales. Then, we compute the matter power spectrum and the angular spectra of the CMB anisotropies and the CMB lensing potential, on all scales. We employ the public version of COOP, a numerical Einstein-Boltzmann solver that implements very general scalar-tensor modifications of gravity. Rather uniquely, Kinetic Matter Mixing weakens gravity on short scales, predicting a lower $\sigma_8$ with respect to the $\Lambda$CDM case. We propose this as a possible solution to the tension between the CMB best-fit model and low-redshift observables.