Testing Horndeski gravity as dark matter with $\texttt{hi_class}$

TL;DR

This paper tests a Horndeski gravity-based dark matter model using hi_class, finding it fails at the perturbative level despite matching background evolution, thus confirming its non-viability.

Contribution

The study applies hi_class to evaluate a Horndeski gravity dark matter model, providing new independent evidence of its failure at the perturbative level.

Findings

Model reproduces background evolution well.

Fails to match matter power spectrum and CMB spectrum.

Confirmed non-viability aligns with gravitational wave speed constraints.

Abstract

We consider a model of dark matter fluid based on a sector of Horndeski gravity. The model is very successful, at the background level, in reproducing the evolution of the Universe from early times to today. However, at the perturbative level the model fails. To show this, we use the code $\texttt{hi_class}$ and we compute the matter power spectrum and the cosmic microwave background spectrum. Our results confirm, in a new and independent way, that this sector of Horndeski gravity is not viable, in agreement with the recent constraints coming from the measurement of the speed of gravitational waves obtained from the observation of the neutron star merger event GW170817.