A varying dark energy effective speed of sound parameter in the phantom Universe

Abstract

We analyse the phenomenological effects of a varying Dark Energy (DE) effective speed of sound parameter, $$c^{2}_{\text {sd}}$$ c sd 2 , 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, $$f\sigma _{8}$$ f σ 8 growth rate and forecast the current matter power spectrum. We vary the $$c^{2}_{\text {sd}}$$ c sd 2 parameter in the interval [0, 1] and compute the relative deviation with respect $$c^{2}_{\text {sd}}=1$$ c sd 2 = 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.