MG15 - Talk detail

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Abstract

The recent observation of the neutron star merger event GW170817 and of its electromagnetic counterpart GRB170817A has established that the speed of gravitational waves deviates from the speed of light 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. 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 mimic both dark energy and dark matter with a non-vanishing speed of sound. In this way, we are able to reproduce the ΛCDM cosmological model without introducing Cold Dark Matter. We also discuss the presence of gradient instabilities of the model.

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