Accelerating Universes with Scaling Dark Matter

TL;DR

This paper analyzes cosmological models with dark energy having an equation of state less than -1/3, identifying critical points and discussing observational challenges in distinguishing variable from constant dark energy properties.

Contribution

It provides a comprehensive analysis of critical points in universes with dark energy and discusses the observational difficulties in detecting variations in the dark energy equation of state.

Findings

Critical points are identified for constant equations of state.

Large variations in w_X at low redshift are hard to detect with current methods.

Precise measurements at higher redshifts are necessary to distinguish variable dark energy.

Abstract

Friedmann-Robertson-Walker universes with a presently large fraction of the energy density stored in an $X$-component with $w_X<-1/3$, are considered. We find all the critical points of the system for constant equations of state in that range. We consider further several background quantities that can distinguish the models with different $w_X$ values. Using a simple toy model with a varying equation of state, we show that even a large variation of $w_X$ at small redshifts is very difficult to observe with $d_L(z)$ measurements up to $z\sim 1$. Therefore, it will require accurate measurements in the range $1<z<2$ and independent accurate knowledge of $\Omega_{m,0}$ (and/or $\Omega_{X,0}$) in order to resolve a variable $w_X$ from a constant $w_X$.