# Evolution of perturbations in a universe with exotic solid-like matter

## TL;DR

This paper investigates cosmological perturbations in a universe filled with a solid-like matter component, revealing unique behaviors of perturbations and potential challenges for observational comparisons in such models.

## Contribution

It introduces a model of a universe with solid-like matter fields and analyzes the evolution of perturbations, highlighting issues with superluminal sound speeds and mode behaviors.

## Key findings

- Superhorizon perturbations lack constant modes.
- Scalar, vector, tensor perturbations decay or grow at different rates.
- Perturbations can propagate superluminally for certain w values.

## Abstract

We study cosmological perturbations in a universe with only one matter component described by a triplet of fields. Configuration of these fields is the same as for body coordinates of a solid, and they enter the matter Lagrangian only through the kinetic term. We restrict ourselves only to cases with constant pressure to energy density ratio $w$. Superhorizon perturbations have no constant modes with scalar vector and tensor perturbations decaying or growing at different rates, and in cases with pressure to energy density ratio $w>(19-8\sqrt{7})/3\dot{=}-0.722$ perturbations propagate with superluminal sound speed. Regarding our universe, these results illustrate possible challenges with comparing the observational data to models similar to solid inflation, if the inflation is followed by a period during which the studied model is a sufficiently good approximation.