[1001.4000] Absence of significant cross-correlation between WMAP and SDSS

Authors:  M. Lopez-Corredoira, F. Sylos Labini, J. Betancort-Rijo
Abstract:  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
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Syksy Rasanen
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Joined: March 02 2005
Affiliation: University of Helsinki

[1001.4000] Absence of significant cross-correlation betwee

Post by Syksy Rasanen » February 24 2011

The authors claim that the cross-correlation between the CMB and large-scale structure is not statistically significant. Such a detection is usually interpreted as being due to the ISW effect, which is a prediction of dark energy models. (More generally, any cosmological models other than matter-dominated spatially flat FRW predict an ISW effect, though the quantitative details of course vary.)

The authors consider WMAP5 and SDSS DR7 data. They argue that the intrinsic field-to-field fluctuations are larger than the signal and that thus there is o significant detection. I don't quite follow the analysis, and would be grateful if more knowleadgle people would share their views.

Maciej Bilicki
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Joined: May 12 2010
Affiliation: University of Cape Town
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[1001.4000] Absence of significant cross-correlation betwee

Post by Maciej Bilicki » February 25 2011

I'd like to draw your attention to yet another paper where no significant detection of the Sachs-Wolfe effect is claimed, Francis & Peacock (2010) 0909.2494. They cross-correlated CMB with 2MASS. They say even that it might be that the S-W effect could remain undetected in a LambdaCDM universe, even if present!

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