Early-Matter-Like Dark Energy and the Cosmic Microwave Background

TL;DR

This paper investigates early-matter-like dark energy within unified dark matter models, analyzing its impact on cosmic microwave background anisotropies and identifying conditions for compatibility with observations.

Contribution

It provides power series expansions for perturbations in models with early-matter-like dark energy and compares the resulting CMB anisotropy with observational data.

Findings

CMB anisotropy can match observations if the effective sound speed is near zero.

Variations in the effective sound speed affect the CMB temperature and polarization spectra.

The study supports the viability of UDM models with specific dark energy properties.

Abstract

Early-matter-like dark energy is defined as a dark energy component whose equation of state approaches that of cold dark matter (CDM) at early times. Such a component is an ingredient of unified dark matter (UDM) models, which unify the cold dark matter and the cosmological constant of the LambdaCDM concordance model into a single dark fluid. Power series expansions in conformal time of the perturbations of the various components for a model with early-matter-like dark energy are provided. They allow the calculation of the cosmic microwave background (CMB) anisotropy from the primordial initial values of the perturbations. For a phenomenological UDM model, which agrees with the observations of the local Universe, the CMB anisotropy is computed and compared with the CMB data. It is found that a match to the CMB observations is possible if the so-called effective velocity of sound c_eff of the early-matter-like dark energy component is very close to zero. The modifications on the CMB temperature and polarization power spectra caused by varying the effective velocity of sound are studied.