## Detalles de publicación

PP 010008

## Absence of significant cross-correlation between WMAP and SDSS

1. IAC; 2 ULL; 3 Centro Studi e Ricerche Enrico Fermi, Via Panisperna 89 A, Compendio del Viminale, 00184 Rome, Italy; 4 Istituto dei Sistemi Complessi CNR, Via dei Taurini 19, 00185 Rome, Italy

AIMS. Several authors have claimed to detect a significant cross-correlation between microwave WMAP anisotropies and the SDSS galaxy distribution. We repeat these analyses to determine the different cross-correlation uncertainties caused by re-sampling errors and field-to-field fluctuations. The first type of error concerns overlapping sky regions, while the second type concerns non-overlapping sky regions.

METHODS. To measure the re-sampling errors, we use bootstrap and jack-knife techniques. For the field-to-field fluctuations, we use three methods: 1) evaluation of the dispersion in the cross-correlation when correlating separated regions of WMAP with the original region of SDSS; 2) use of mock Monte Carlo WMAP maps; 3) a new method (developed in this article), which measures the error as a function of the integral of the product of the self-correlations for each map.

RESULTS. The average cross-correlation for b>30 deg is significantly stronger than the re-sampling errors---both the jack-knife and bootstrap techniques provide similar results---but it is of the order of the field-to-field fluctuations. This is confirmed by the cross-correlation between anisotropies and galaxies in more than the half of the sample being null within re-sampling errors.

CONCLUSIONS. Re-sampling methods underestimate the errors. Field-to-field fluctuations dominate the detected signals. The ratio of signal to re-sampling errors is larger than unity in a way that strongly depends on the selected sky region. We therefore conclude that there is no evidence yet of a significant detection of the integrated Sachs-Wolfe (ISW) effect. Hence, the value of Omega _\Lambda ~ 0.8 obtained by the authors who assumed they were observing the ISW effect would appear to have originated from noise analysis.

METHODS. To measure the re-sampling errors, we use bootstrap and jack-knife techniques. For the field-to-field fluctuations, we use three methods: 1) evaluation of the dispersion in the cross-correlation when correlating separated regions of WMAP with the original region of SDSS; 2) use of mock Monte Carlo WMAP maps; 3) a new method (developed in this article), which measures the error as a function of the integral of the product of the self-correlations for each map.

RESULTS. The average cross-correlation for b>30 deg is significantly stronger than the re-sampling errors---both the jack-knife and bootstrap techniques provide similar results---but it is of the order of the field-to-field fluctuations. This is confirmed by the cross-correlation between anisotropies and galaxies in more than the half of the sample being null within re-sampling errors.

CONCLUSIONS. Re-sampling methods underestimate the errors. Field-to-field fluctuations dominate the detected signals. The ratio of signal to re-sampling errors is larger than unity in a way that strongly depends on the selected sky region. We therefore conclude that there is no evidence yet of a significant detection of the integrated Sachs-Wolfe (ISW) effect. Hence, the value of Omega _\Lambda ~ 0.8 obtained by the authors who assumed they were observing the ISW effect would appear to have originated from noise analysis.