Cross-correlation of Wilkinson Microwave Anisotropy Probe third-year data and the Sloan Digital Sky Survey DR4 galaxy survey: new evidence for dark energy

We cross-correlate the third-year Wilkinson Microwave Anisotropy Probe (WMAP) data with galaxy samples extracted from the SDSS DR4 (SDSS4) covering 13 per cent of the sky, increasing the volume sampled in previous analysis by a factor of 3.7. The new measurements confirm a positive cross-correlation with higher significance (total signal-to-noise ratio of about 4.7). The correlation as a function of angular scale is well fitted by the integrated Sachs-Wolfe (ISW) effect for Lambda cold dark matter (Lambda CDM) flat Friedmann-Robertson-Walker models with a cosmological constant. The combined analysis of different samples gives Omega(Lambda) = 0.80-0.85 (68 per cent confidence level, CL) or 0.77-0.86 (95 per cent CL). We find similar best-fitting values for Omega(Lambda) for different galaxy samples with median redshifts of z similar or equal to 0.3 and z similar or equal to 0.5, indicating that the data scale with redshift as predicted by the LCDM cosmology (with equation of state parameter w = -1). This agreement is not trivial, but cannot yet be used to break the degeneracy constraints in the w versus Omega(Lambda) plane using only the ISW data.