Integrated Sachs-Wolfe effect from the cross correlation of WMAP 3 year and the NRAO VLA sky survey data: New results and constraints on dark energy

We cross correlate the new 3 year Wilkinson Microwave Anistropy Probe (WMAP) cosmic microwave background data with the NRAO VLA Sky Survey radio galaxy data and find further evidence of late integrated Sachs-Wolfe (ISW) effect taking place at late times in cosmic history. Our detection makes use of a novel statistical method (P. Baldi, G. Kerkyacharian, D. Marinucci, and D. Picard, math.ST/0606154 and P. Baldi, G. Kerkyacharian, D. Marinucci, D. Picard, math.ST/0606599) based on a new construction of spherical wavelets, called needlets. The null hypothesis (no ISW) is excluded at more than 99.7% confidence. When we compare the measured cross correlation with the theoretical predictions of standard, flat cosmological models with a generalized dark energy component parameterized by its density, ΩDE, equation of state w and speed of sound cs2, we find 0.3≤ΩDE≤0.8 at 95% C.L., independently of cs2 and w. If dark energy is assumed to be a cosmological constant (w=-1), the bound on density shrinks to 0.41≤ΩDE≤0.79. Models without dark energy are excluded at more than 4σ. The bounds on w depend rather strongly on the assumed value of cs2. We find that models with more negative equation of state (such as phantom models) are a worse fit to the data in the case cs2=1 than in the case cs2=0