# Fluids, Superfluids and Supersolids: Dynamics and Cosmology of Self Gravitating Media

## TL;DR

This paper develops a unified, symmetry-based framework to analyze cosmological perturbations in various self-gravitating media, including fluids, superfluids, solids, and supersolids, with implications for dark sector modeling.

## Contribution

It introduces a general effective field theory approach for self-gravitating media based on scalar fields and internal symmetries, encompassing multiple media types and their perturbation dynamics.

## Key findings

- Scalar perturbations include adiabatic and non-adiabatic modes.
- Tensor modes become massive in solids and supersolids.
- Special isentropic media with zero entropy perturbation are identified.

## Abstract

We compute cosmological perturbations for a generic self-gravitating media described by four derivatively- coupled scalar fields. Depending on the internal symmetries of the action for the scalar fields, one can describe perfect fluids, superfluids, solids and supersolids media. Symmetries dictate both dynamical and thermodynamical properties of the media. Generically, scalar perturbations include, besides the gravitational potential, an additional non-adiabatic mode associated with the entropy per particle {\sigma}. While perfect fluids and solids are adiabatic with {\sigma} constant in time, superfluids and supersolids feature a non-trivial dynamics for {\sigma}. Special classes of isentropic media with zero {\sigma} can also be found. Tensor modes become massive for solids and supersolids. Such an effective approach can be used to give a very general and symmetry driven modelling of the dark sector.