Large-scale structure in mimetic Horndeski gravity

TL;DR

This paper investigates mimetic Horndeski gravity as a unified model for dark matter and dark energy, showing it can mimic LCDM on linear scales and discussing its distinguishability from other models through power spectrum analysis.

Contribution

It demonstrates that mimetic Horndeski models can replicate LCDM power spectra on linear scales and explores their potential to be distinguished from other dark energy models.

Findings

Mimetic Horndeski behaves like a perfect fluid with zero sound speed.

In the LCDM background, the power spectrum matches LCDM predictions.

Distinguishability depends on the equation of state and sound speed.

Abstract

In this paper, we propose to use the mimetic Horndeski model as a model for the dark universe. Both cold dark matter (CDM) and dark energy (DE) phenomena are described by a single component, the mimetic field. In linear theory, we show that this component effectively behaves like a perfect fluid with zero sound speed and clusters on all scales. For the simpler mimetic cubic Horndeski model, if the background expansion history is chosen to be identical to a perfect fluid DE (PFDE) then the mimetic model predicts the same power spectrum of the Newtonian potential as the PFDE model with zero sound speed. In particular, if the background is chosen to be the same as that of LCDM, then also in this case the power spectrum of the Newtonian potential in the mimetic model becomes indistinguishable from the power spectrum in LCDM on linear scales. A different conclusion may be found in the case of non-adiabatic perturbations. We also discuss the distinguishability, using power spectrum measurements from LCDM N-body simulations as a proxy for future observations, between these mimetic models and other popular models of DE. For instance, we find that if the background has an equation of state equal to -0.95 then we will be able to distinguish the mimetic model from the PFDE model with unity sound speed. On the other hand, it will be hard to do this distinction with respect to the LCDM model.