Mimicking dark matter and dark energy in a mimetic model compatible with GW170817

TL;DR

This paper develops a modified mimetic gravity model with higher-order terms that complies with gravitational wave speed constraints and can mimic both dark matter and dark energy, reproducing the CDM model without cold dark matter.

Contribution

It introduces a higher-order mimetic gravity model that satisfies GW170817 constraints and mimics dark energy and dark matter simultaneously.

Findings

Model satisfies gravitational wave speed constraints

Reproduces CDM cosmology without cold dark matter

Features non-vanishing scalar perturbations with a non-zero sound speed

Abstract

The recent observation of the the gravitational wave event GW170817 and of its electromagnetic counterpart GRB170817A, from a binary neutron star merger, has established that the speed of gravitational waves deviates from the speed of light by less than one part in $10^{15}$. As a consequence, many extensions of General Relativity are inevitably ruled out. Among these we find the most relevant sectors of Horndeski gravity. In its original formulation, mimetic gravity is able to mimic cosmological dark matter, has tensorial perturbations that travel exactly at the speed of light but has vanishing scalar perturbations and this fact persists if we combine mimetic with Horndeski gravity. In this work, we show that implementing the mimetic gravity action with higher-order terms that break the Horndeski structure yields a cosmological model that satisfies the constraint on the speed of gravitational waves and mimics both dark energy and dark matter with a non-vanishing speed of sound. In this way, we are able to reproduce the $\Lambda$CDM cosmological model without introducing particle cold dark matter.