Cosmological perturbations in elastic dark energy models

Abstract

We discuss the general framework for a perfect continuum medium in cosmology and show that an interesting generalization of the fluids normally used is for the medium to have rigidity and, hence, be analogous to an elastic solid. Such models can provide perfect, adiabatic fluids which are stable even when the pressure is negative, if the rigidity is sufficiently large, making them natural candidates to describe the dark energy. In fact, if the medium is adiabatic and isotropic, they provide the most general description of linearized perturbations. We derive the equations of motion and wave propagation speeds in the isotropic case. We point out that anisotropic models can also be incorporated within the same formalism and that they are classified by the standard Bravais Lattices. We identify the adiabatic and isocurvature modes allowed in both the scalar and vector sectors and discuss the predictions they make for CMB and matter power spectra. We comment on the relationship between these models and other fluid-based approaches to dark energy, and discuss a possible microphysical manifestation of this class of models as a continuum description of defect-dominated scenarios.

Quantization of perturbations in an inflating elastic solid

“…This model has previously been considered as a potential model of dark energy [1,2] and recently similarities between a relativistic elastic solid and massive gravity have been noted [3]. In this work, we discuss in detail how an elastic solid can drive an inflationary epoch in the early Universe.…”

“…[7] for a continuous relativistic medium and the findings of Ref. [2] for the linear perturbations in an isotropic relativistic elastic solid.…”

“…[2], specifying the pressure and elasticity tensors is sufficient for describing the behaviour of linear perturbations in a relativistic elastic solid. By relating derivatives in F and M, the spacetime pressure and elasticity tensors for an isotropic elastic solid are…”

“…1 Linear perturbations in an elastic solid satisfy the equations of motion found in Ref. [2], which uses the framework for describing a macroscopic relativistic medium developed in Ref. [7].…”

Viable Palatini-f(R)cosmologies with generalized dark matter

“…This model allows an analytical treatment and its 2 Though our present approach is phenomenological and not aimed at explaining the origin of dark matter stress Π, we might note that cosmic media satisfying effectively w = 0 but Π = 0 has indeed been considered [43]. (See also [44] for a recent discussion anisotropically stressed cosmological fluids and [36] for an extensive review of structure formation with generalized dark matter).…”