Perturbations in a holographic universe and in other stiff fluid cosmologies

Abstract

We examine the generation and evolution of perturbations in a universe dominated by a fluid with stiff equation of state p = ρ. The recently proposed Holographic Universe is an example of such a model. We compute the spectrum of scalar and tensor perturbations, without relying on a microphysical description of the p = ρ fluid. The spectrum is scale invariant deep inside the Hubble horizon. In contrast, infrared perturbations that enter the Hubble horizon during the stiff fluid dominated (holographic) phase yield oscillatory and logarithmic terms in the power spectrum. We show that vector perturbations grow during the stiff fluid dominated epoch and may result in a turbulent and anisotropic Universe at the end of the holographic phase. Therefore, the required period of inflation following the holographic phase cannot be much shorter than that required in standard inflationary models.PACS numbers: 98.80.Cq.

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“…Recently a similar observation was also put forward in Ref. [102] in the framework of the so-called holographic proposal which implies the presence of a barotropic fluid with stiff equation of state during the post-inflationary evolution [103,104].…”

Stochastic backgrounds of relic gravitons: a theoretical appraisal

“…(6.29)-(6.30) and discussion therein). The possibility of having a post-inflationary phase stiffer than radiation has been also investigated in different contexts such as in [146,147,148].…”

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“…Thorough the years various completions of the standard paradigm have been proposed but none of them provides a strong gravitational wave background either at small or at large frequencies. An exception is represented by the presence of and early stiff phase [20,37] which can arise also in braneworld scenarios [38] as well as in other contexts [39]. In Fig.…”