A correlation of the cosmic microwave sky with large scale structure

We cross correlate the large-scale cosmic microwave background (CMB) sky measured by WMAP with two probes of large-scale structure at z ~ 1. The hard X-ray background, measured by the HEAO-1 satellite, is positively correlated with the WMAP data at the 2.5-3.0 sigma level. The number counts of radio galaxies in the NVSS survey are also correlated at a slightly weaker level (2.-2.5 sigma). These correlations appear to arise from both hemispheres on the sky and are resilient to changes in the levels of masking of the Galaxy and point sources, suggesting that foregrounds are not responsible for the signal. The implication is that some of the observed CMB fluctuations arise at low redshifts. The level of the correlations is consistent with that expected for the cosmological constant (Omega_Lambda = 0.72) concordance model resulting from the integrated Sachs-Wolfe effect. Thus, we may be observing dark energy's effect on the growth of structure.