Scalar-Fluid theories: cosmological perturbations and large-scale structure

TL;DR

This paper explores a new Lagrangian framework for scalar field and fluid interactions, analyzing their effects on cosmological structure formation and deriving perturbation equations with potential observational signatures.

Contribution

It introduces a unified formalism for scalar-fluid interactions, including pressures and derivative couplings, and analyzes their impact on large-scale structure formation.

Findings

Derived full perturbation equations for coupled scalar-fluid models.

Identified conditions for zero effective sound speed restricting Lagrangian forms.

Found new classes of models affecting background expansion and structure growth.

Abstract

Recently a new Lagrangian framework was introduced to describe interactions between scalar fields and relativistic perfect fluids. This allows two consistent generalizations of coupled quintessence models: non-vanishing pressures and a new type of derivative interaction. Here the implications of these to the formation of cosmological large-scale structure are uncovered at the linear order. The full perturbation equations in the two cases are derived in a unified formalism and their Newtonian, quasi-static limit is studied analytically. Requiring the absence of an effective sound speed for the coupled dark matter fluid restricts the Lagrangian to be a linear function of the matter number density. This still leaves new potentially viable classes of both algebraically and derivatively interacting models wherein the coupling may impact the background expansion dynamics and imprint signatures into the large-scale structure.