Unified Dark Matter scalar field models with fast transition

TL;DR

This paper explores scalar field models for unified dark matter that can rapidly transition from matter-like behavior to dark energy-like behavior, addressing fine-tuning issues and analyzing perturbation properties.

Contribution

It introduces a new approach to constructing UDM scalar field models with fast transitions, avoiding fine-tuning and analyzing perturbations and effective sound speed.

Findings

Models can mimic early matter-dominated and late dark energy phases

Perturbations are inherently non-adiabatic in scalar field models

Conditions for model viability are established based on sound speed and Jeans length

Abstract

We investigate the general properties of Unified Dark Matter (UDM) scalar field models with Lagrangians with a non-canonical kinetic term, looking specifically for models that can produce a fast transition between an early Einstein-de Sitter CDM-like era and a later Dark Energy like phase, similarly to the barotropic fluid UDM models in JCAP1001(2010)014. However, while the background evolution can be very similar in the two cases, the perturbations are naturally adiabatic in fluid models, while in the scalar field case they are necessarily non-adiabatic. The new approach to building UDM Lagrangians proposed here allows to escape the common problem of the fine-tuning of the parameters which plague many UDM models. We analyse the properties of perturbations in our model, focusing on the the evolution of the effective speed of sound and that of the Jeans length. With this insight, we can set theoretical constraints on the parameters of the model, predicting sufficient conditions for the model to be viable. An interesting feature of our models is that what can be interpreted as w_{DE} can be <-1 without violating the null energy conditions.