The State of the Dark Energy Equation of State

TL;DR

The paper addresses constraining the dark energy equation of state $w_Q$ using a joint analysis of CMB, large-scale structure, SN-Ia, and HST measurements. It explores the possibility of $w_Q<-1$ via a tracking non-canonical scalar-field model with ${\cal L}=f(\phi)g(X)-V(\phi)$ and derives conditions for negative $w_Q$ and stability. The likelihood analysis yields $-1.38<w_Q<-0.82$ at 95% C.L. with best-fit $\\Omega_M\\approx 0.27$ and $w_Q\\approx -1.05$, compatible with a cosmological constant. The results illustrate degeneracies between $w_Q$ and $\\Omega_M$, show that allowing $w_Q<-1$ does not radically change conclusions, and highlight the potential of future surveys to tighten constraints.

Abstract

By combining data from seven cosmic microwave background experiments (including the latest WMAP results) with large scale structure data, the Hubble parameter measurement from the Hubble Space Telescope and luminosity measurements of Type Ia supernovae we demonstrate the bounds on the dark energy equation of state $w_Q$ to be $-1.38< w_Q <-0.82$ at the 95% confidence level. Although our limit on $w_Q$ is improved with respect to previous analyses, cosmological data does not rule out the possibility that the equation of state parameter $w_Q$ of the dark energy $Q$ is less than -1. We present a tracking model that ensures $w_Q \le -1$ at recent times and discuss the observational consequences.

Figures (4)

• Figure 1: The effect of varying $w_Q$ on the COBE-normalized CMB angular power spectrum and present CMB data. Since the shift of the power spectrum is proportional to ${\cal R} \ell$, the discrepancy is more important for higher values of $\ell$.
• Figure 2: Degenerate CMB power spectra The models are computed assuming flatness ($\Omega_{tot}=\Omega_M+\Omega_Q=1$). On large angular scales the Integrated Sachs Wolfe effect breaks the degeneracy for highly negative values of $w_Q$. In general, the degeneracy of the spectra can be broken with a strong prior on $h$ or on $\Omega_M$.
• Figure 3: Effect of including perturbations in the dark energy fluid with a constant equation of state in the CMB power spectrum. Since dark energy dominates at late redshifts, the effect is present only on large angular scales.
• Figure 4: Likelihood contours in the ($\Omega_M$, $w_Q$) plane for the joint CMB+HST+SN-Ia+2dF analysis described in the text. We take the best-fit values for the remaining parameters. The contours correspond to 0.32, 0.05 and 0.01 of the peak value of the likelihood, which are the 68%, 95% and 99% confidence levels respectively. Also plotted are the likelihood contours from type-Ia Supernovae alone.