A varying Dark Energy effective speed of sound parameter in the phantom Universe

TL;DR

This paper investigates how a varying dark energy effective speed of sound affects cosmological perturbations and structure growth in models predicting future cosmic rip events, highlighting potential observational signatures.

Contribution

It introduces a phenomenological analysis of a varying dark energy sound speed in phantom models with future rip scenarios, exploring its impact on perturbations and large-scale structure.

Findings

Varying $c^{2}_{sd}$ influences the evolution of cosmological perturbations.

The gravitational potential sign flip occurs at large scales due to the varying sound speed.

Deviations in matter power spectrum depend on the $c^{2}_{sd}$ parameter variation.

Abstract

We analyse the phenomenological effects of a varying Dark Energy (DE) effective speed of sound parameter, $c^{2}_{\textrm{sd}}$, on the cosmological perturbations of three phantom DE models. Each of these models induce a particular abrupt future event known as Big Rip (BR), Little Rip (LR), and Little Sibling of the Big Rip (LSBR). In this class of abrupt events, all the bound structures in the Universe would be ripped apart at a finite cosmic time. We compute the evolution of the perturbations, $f\sigma_{8}$ growth rate and forecast the current matter power spectrum. We vary the $c^{2}_{\textrm{sd}}$ parameter in the interval $[0,1]$ and compute the relative deviation with respect $c^{2}_{\textrm{sd}}=1$. In addition, we analyse the effect of gravitational potential sign flip that occurs at very large scale factors as compared with the current one.