Probing the dark side: Constraints on the dark energy equation of state from CMB, large scale structure and Type Ia supernovae

TL;DR

This paper relaxes the common prior $w_Q>-1$ and analyzes a constant $w_Q$ dark-energy model using CMB, LSS, and Type Ia SN data to derive unbiased constraints. A joint likelihood with $C_l$ and $P(k)$ from CMB+2dF, plus SN distance moduli, yields a 95.4% CL bound of $-2.68 < w_Q < -0.78$; imposing $w_Q>-1$ tightens to $-1 \le w_Q < -0.71$, while a cosmological constant remains a good fit. The analysis shows relaxing the prior does not substantially bias the region above $-1$, though some datasets prefer $w_Q<-1$, and highlights the non-trivial lower bound that emerges from combining probes. Forecasts for Planck and SNAP/SNfactory data suggest that $w_Q$ could be constrained at the ~5% level, contingent on controlling systematic uncertainties such as gravitational lensing.

Abstract

We have reanalysed constraints on the equation of state parameter, w_Q = P/rho, of the dark energy, using several cosmological data sets and relaxing the usual constraint w_Q > -1. We find that combining Cosmic Microwave Background, large scale structure and Type Ia supernova data yields a non-trivial lower bound on w_Q. At 95.4% confidence we find, assuming a flat geometry of the universe, a bound of -2.68 < w_Q < -0.78 if w_Q is taken to be a completely free parameter. Reassuringly we also find that the constraint w_Q > -1 does not significantly bias the overall allowed region for w_Q. When this constraint is imposed the 95.4% confidence bound is -1 < w_Q < -0.71. Also, a pure cosmological constant (w = -1) is an excellent fit to all available data. Based on simulations of future data from the Planck CMB experiment and the SNAP and SNfactory supernova experiments we estimate that it will be possible to constrain w_Q at the 5% level in the future.

Figures (7)

• Figure 1: The 68.3 % (dark shaded) and 95.4 % (light shaded) confidence allowed regions for $\Omega_m$ and $w_Q$ using CMB, HST, BBN and LSS data.
• Figure 2: Different CMB power spectra for different values of $w_Q$. In all cases the model parameters are those of the fiducial $\Lambda$CDM model, $\Omega_m=0.3$, $\Omega_\Lambda=0.7$, $\Omega_b h^2 = 0.020$, $H_0 = 70 \,\, {\rm km} \, {\rm s}^{-1} \, {\rm Mpc}^{-1}$. The full line is for $w_Q=-1$, the dashed for $w_Q=-2$, and the dotted for $w_Q=-4$ (in order of decreasing $C_l$ at low $l$).
• Figure 3: The 68.3 % (dark shaded) and 95.4 % (light shaded) confidence allowed regions for $\Omega_m$ and $w_Q$ using the 54 type Ia SNe from the Supernova Cosmology Project.
• Figure 4: The 68.3 % (dark shaded) and 95.4 % (light shaded) confidence allowed regions for $\Omega_m$ and $w_Q$ using all available data.
• Figure 5: The 68.3 % (dark shaded) and 95.4 % (light shaded) confidence allowed regions for $\Omega_m$ and $w_Q$ using all available data and imposing the bound $w_Q \geq -1$.