We cross-correlate the new 3 year Wilkinson Microwave Anistropy Probe ( WMAP ) cosmic microwave background ( CMB ) data with the NRAO VLA Sky Survey ( NVSS ) 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 ( ( 49 ) ; ( 50 ) ) 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 _ { DE } , equation of state w and speed of sound c _ { s } ^ { 2 } , we find 0.3 \leq \Omega _ { DE } \leq 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 \leq \Omega _ { DE } \leq 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 .