  Using a directional spherical wavelet analysis we detect the integrated
Sachs-Wolfe (ISW) effect, indicated by a positive correlation between the
Wilkinson Microwave Anisotropy Probe (WMAP) and NRAO VLA Sky Survey (NVSS)
data, at the 3.9 sigma level. In a flat universe the detection of the ISW
effect provides direct and independent evidence for dark energy. Moreover, we
use our detection to constrain the dark energy density Omega_Lambda. We obtain
estimates for Omega_Lambda consistent with other analysis techniques and data
sets and rule out a zero cosmological constant at greater than 99%
significance.
