Volume 513, April 2010
|Number of page(s)||5|
|Section||Cosmology (including clusters of galaxies)|
|Published online||09 April 2010|
Absence of significant cross-correlation between WMAP and SDSS
Instituto de Astrofísica de Canarias,
38200 La Laguna, Tenerife, Spain e-mail: firstname.lastname@example.org
2 Departamento de Astrofísica, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain
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
Accepted: 17 January 2010
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 Ω ≈ 0.8 obtained by the authors who assumed they were observing the ISW effect would appear to have originated from noise analysis.
Key words: cosmic microwave background / large-scale structure of Universe
© ESO, 2010
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