Abstract
We analyse the phenomenological effects of a varying Dark Energy (DE) effective speed of sound parameter, c^{2}_{text {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, fsigma _{8} growth rate and forecast the current matter power spectrum. We vary the c^{2}_{text {sd}} parameter in the interval [0, 1] and compute the relative deviation with respect c^{2}_{text {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.
Highlights
During the last two decades Cosmology has experienced a great improvement in the theoretical and observational scopes
As we have already mentioned in the introduction section, we focus on three genuine phantom models where each of them induce a particular future doomsday known as Big Rip (BR), Little Rip (LR), and Little Sibling of the Big Rip (LSBR)
We have analysed the cosmological perturbations of three genuine phantom Dark Energy (DE) models with a varying effective speed of sound parameter
Summary
During the last two decades Cosmology has experienced a great improvement in the theoretical and observational scopes. In the recent work [50], the authors use machine learning computation methods to reconstruct the relevant perturbation parameters including those involved with the anisotropic effects, pointing out a way to detect imprints of anisotropies on a wide range of DE models As it is shown in [46,50], when considering such anisotropies the DE sound speed could be negative without inducing instabilities at the perturbation level as long as the effective speed of sound parameter stands positive. A model that has gained some attention recently is the so called Early Dark Energy (EDE) model, which has been shown to be slightly favoured by observational data This model consists on considering a small but not negligible DE presence at early stages of the Universe (for example, before the matter-radiation decoupling time) which could induce significant footprints on the structure formation [51,52]. In the “Appendix 1” we show in detail the pressure decomposition into its adiabatic and non-adiabatic contributions
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