Observational constraints on dark energy and cosmic curvature

Current observational bounds on dark energy depend on our assumptions about the curvature of the universe. We present a simple and efficient method for incorporating constraints from cosmic microwave background (CMB) anisotropy data and use it to derive constraints on cosmic curvature and dark energy density as a free function of cosmic time using current CMB, Type Ia supernova (SN Ia), and baryon acoustic oscillation data. We show that there are two CMB shift parameters, R≡Ω_mH₀²r(z_CMB) (the scaled distance to recombination) and l_a≡πr(z_CMB)/r_s(z_CMB) (the angular scale of the sound horizon at recombination), with measured values that are nearly uncorrelated with each other. Allowing nonzero cosmic curvature, the three-year WMAP (Wilkinson Microwave Anisotropy Probe) data give R=1.71±0.03, l_a=302.5±1.2, and Ω_bh²=0.02173±0.00082, independent of the dark energy model. The corresponding bounds for a flat universe are R=1.70±0.03, l_a=302.2±1.2, and Ω_bh²=0.022±0.00082. We give the covariance matrix of (R,l_a,Ω_bh²) from the three-year WMAP data. We find that (R,l_a,Ω_bh²) provide an efficient and intuitive summary of CMB data as far as dark energy constraints are concerned. Assuming the Hubble Space Telescope (HST) prior of H₀=72±8(km/s)Mpc^-1, using 182 SNe Ia (from the HST/GOODS program, the first year Supernova Legacy Survey, and nearby SN Ia surveys), (R,l_a,Ω_bh²) from WMAP three-year data, and SDSS (Sloan Digital Sky Survey) measurement of the baryon acoustic oscillation scale, we find that dark energy density is consistent with a constant in cosmic time, with marginal deviations from a cosmological constant that may reflect current systematic uncertainties or true evolution in dark energy. A flat universe is allowed by current data: Ω_k=-0.006_-0.012-0.025^+0.013+0.025 for assuming that the dark energy equation of state w_X(z) is constant, and Ω_k=-0.002_-0.018-0.032^+0.018+0.041 for w_X(z)=w₀+w_a(1-a) (68% and 95% confidence levels). The bounds on cosmic curvature are less stringent if dark energy density is allowed to be a free function of cosmic time, and are also dependent on the assumption about the early time property of dark energy. We demonstrate this by studying two examples. Significant improvement in dark energy and cosmic curvature constraints is expected as a result of future dark energy and CMB experiments.