Velocity-dependent interacting dark energy and dark matter with a Lagrangian description of perfect fluids

TL;DR

This paper explores a velocity-dependent interaction between dark matter and dark energy modeled as perfect fluids, affecting perturbations and potentially alleviating cosmological tensions like  and H0.

Contribution

It introduces a novel velocity-dependent coupling in the dark sector using a Lagrangian formulation and analyzes its effects on cosmological perturbations and tensions.

Findings

Interaction suppresses dark matter clustering at late times.

Model can alleviate  and H0 tensions.

Dark energy behaves as radiation at high redshift and as a cosmological constant later.

Abstract

We consider a cosmological scenario where the dark sector is described by two perfect fluids that interact through a velocity-dependent coupling. This coupling gives rise to an interaction in the dark sector driven by the relative velocity of the components, thus making the background evolution oblivious to the interaction and only the perturbed Euler equations are affected at first order. We obtain the equations governing this system with the Schutz-Sorkin Lagrangian formulation for perfect fluids and derive the corresponding stability conditions to avoid ghosts and Laplacian instabilities. As a particular example, we study a model where dark energy behaves as a radiation fluid at high redshift while it effectively becomes a cosmological constant in the late Universe. Within this scenario, we show that the interaction of both dark components leads to a suppression of the dark matter clustering at late times. We also argue the possibility that this suppression of clustering together with the additional dark radiation at early times can simultaneously alleviate the $\sigma_8$ and $H_0$ tensions.