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, ${\Omega }_{\mathrm{DE}}$, equation of state $w$ and speed of sound $c_s^2$, we find $0.3\le {\Omega }_{\mathrm{DE}}\le 0.8$ at 95% C.L., independently of $c_s^2$ and $w$. If dark energy is assumed to be a cosmological constant ($w=-1$), the bound on density shrinks to $0.41\le {\Omega }_{\mathrm{DE}}\le 0.79$. Models without dark energy are excluded at more than $4\sigma$. The bounds on $w$ depend rather strongly on the assumed value of $c_s^2$. We find that models with more negative equation of state (such as phantom models) are a worse fit to the data in the case $c_s^2=1$ than in the case $c_s^2=0$.