Phenomenological models for Unified Dark Matter with fast transition

TL;DR

This paper explores phenomenological models of unified dark matter with rapid transitions between matter-like and dark energy-like phases, analyzing their viability and observational distinguishability from standard cosmological models.

Contribution

It introduces and analyzes a simple phenomenological UDM model with a fast transition, assessing its background evolution and perturbation properties for observational consistency.

Findings

The model can be compatible with current observations.

The effective speed of sound constrains the model parameters.

The model predicts measurable differences from mbda CDM.

Abstract

A fast transition between a standard matter-like era and a late $\Lambda$ cold dark matter ($\Lambda$CDM)-like epoch (or more in general, a CDM+DE era), generated by a single Unified Dark Matter component, can provide a new interesting paradigm in the context of general relativity, alternative to $\Lambda$CDM itself or other forms of DE or modified gravity theories invoked to explain the observed acceleration of the Universe. UDM models with a fast transition have interesting features, leading to measurable predictions, thus they should be clearly distinguishable from $\Lambda$CDM (and alternatives) through observations. Here we look at different ways of prescribing phenomenological UDM models with fast transition, then focusing on a particularly simple model. We analyse the viability of this model by studying features of the background model and properties of the adiabatic UDM perturbations, which depend on the effective speed of sound and the functional form of the Jeans scale. As a result, theoretical constraints on the parameters of the models are found that allow for a behaviour compatible with observations.