On (non-)dynamical dark energy

TL;DR

This paper develops a detailed perturbative analysis of a cosmological model with baryons, dark matter, and a scalar field, showing that deviations from a cosmological constant hinder structure formation, thus supporting the standard model.

Contribution

It introduces a new set of equations for scalar perturbations in a non-interacting multi-fluid cosmology with a scalar field, constraining dark energy models.

Findings

Tiny deviations from $w_S=-1$ impair structure formation.

Results strongly favor the standard cosmological model.

Many dynamical dark energy models are ruled out.

Abstract

The current Universe is composed by a mixture of relativistic species, baryonic matter, dark matter and dark energy which evolve in a non-trivial way at perturbative level. An advanced description of the cosmological dynamics should include non-standard features beyond the simplistic approach idealized by the standard cosmology in which cosmic components do not interact, are adiabatic and dissipationless. We promote a full perturbative analysis of linear scalar perturbations of a non-interacting cosmological model containing baryons, dark matter (both pressureless) and a scalar field allowing for the presence of relative entropic perturbations between the three fluids. Assuming an effective scalar-field sound speed equal to one and neglecting anisotropic stresses we establish a new set of equations for the scalar cosmological perturbations. As a consequence of this new approach, we show that tiny departures from a constant scalar field equation of state $w_S=-1$ damage structure formation in a non-acceptable manner. Hence, by strongly constraining $w_S$ our results provide compelling evidence in favor of the standard cosmological model and rule out a large class of dynamical dark energy models