Detecting the integrated Sachs-Wolfe effect with stacked voids

TL;DR

This study revisits the detection of the integrated Sachs-Wolfe effect using stacked cosmic voids, confirming previous signals but highlighting complexities in interpretation and the influence of selection effects and void properties.

Contribution

The paper introduces a robust protocol for analyzing stacked voids and extends the analysis to larger catalogs, addressing foreground contamination and statistical significance.

Findings

Confirmed the detection of the iSW signal with stacked voids.

Identified the need for rescaling voids to compare different catalogs.

Highlighted the influence of selection effects and void overlap on the results.

Abstract

The stacking of cosmic microwave background (CMB) patches has been recently used to detect the integrated Sachs-Wolfe effect (iSW). When focusing on the locations of superstructures identified in the Sloan Digital Sky Survey (SDSS), Granett et al. (2008a, Gr08) found a signal with strong significance and an amplitude reportedly higher than expected within the LambdaCDM paradigm. We revisit the analysis using our own robust protocol, and extend the study to the two most recent and largest catalogues of voids publicly available. We quantify and subtract the level of foreground contamination in the stacked images and determine the contribution on the largest angular scales from the first multipoles of the CMB. We obtain the radial temperature and photometry profiles from the stacked images. Using a Monte Carlo approach, we computed the statistical significance of the profiles for each catalogue and identified the angular scale at which the signal-to-noise ratio (S/N) is maximum. We essentially confirm the signal detection reported by Gr08, but for the other two catalogues, a rescaling of the voids to the same size on the stacked image is needed to find any significant signal (with a maximum at ~2.4 sigmas). This procedure reveals that the photometry peaks at unexpectedly large angles in the case of the Gr08 voids, in contrast to voids from other catalogues. Conversely, the photometry profiles derived from the stacked voids of these other catalogues contain small central hot spots of uncertain origin. We also stress the importance of a posteriori selection effects that might arise when intending to increase the S/N, and we discuss the possible impact of void overlap and alignment effects. We argue that the interpretation in terms of an iSW effect of any detected signal via the stacking method is far from obvious.