[Paper Review] On the detection of the integrated Sachs-Wolfe effect with stacked voids

S. Ilić, M. Langer+1 more|arXiv (Cornell University)|Jan 24, 2013

Cosmology and Gravitation Theories4 references6 citations

TL;DR

This study re-evaluates the detection of the integrated Sachs-Wolfe (iSW) effect using stacked cosmic voids from the Sloan Digital Sky Survey, applying a robust protocol to quantify foreground contamination and large-scale CMB multipoles. It confirms the iSW signal reported by Granett et al. (2008a) but finds that only after rescaling voids to a uniform size on the sky does a significant signal emerge (peaking at ~2.4σ) in two newer void catalogues, raising concerns about selection effects and the physical interpretation of the signal.

ABSTRACT

The stacking of Cosmic Microwave Background (CMB) patches has been recently used to detect the integrated Sachs-Wolfe effect (iSW). Focusing on the locations of superstructures they 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 LCDM 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 (Pan et al. 2012, Sutter et al. 2012). We quantify and subtract the level of foreground contamination in the stacked images and we 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 compute for each catalogue the statistical significance of the profiles and identify the angular scale at which the signal-to-noise ratio (SNR) is maximal. 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 with the Sutter et al. catalogue. Conversely, the photometry profiles derived from the stacked voids of Sutter et al. 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 SNR, 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.

Motivation & Objective

• To re-express the detection of the integrated Sachs-Wolfe (iSW) effect using stacked voids with a more robust analysis protocol than previous studies.
• To assess the significance of the iSW signal using the two most recent and largest publicly available void catalogues (Pan et al. 2012; Sutter et al. 2012).
• To quantify and subtract foreground contamination in stacked CMB images and account for contributions from the first CMB multipoles on large angular scales.
• To investigate whether the observed temperature and photometry profiles from stacked voids are consistent with iSW expectations or influenced by systematics.
• To evaluate the impact of a posteriori selection effects, void overlap, and alignment on the interpretation of the iSW signal.

Proposed method

• Stacked cosmic microwave background (CMB) patches are constructed by aligning CMB maps centered on the locations of voids identified in the Sloan Digital Sky Survey (SDSS).
• Foreground contamination in the stacked CMB images is quantified and subtracted using dedicated component separation techniques.
• The contribution of the first few CMB multipoles (large-scale modes) is isolated and removed to focus on the signal at intermediate angular scales.
• Radial temperature and photometry profiles are extracted from the cleaned stacked images to analyze the spatial structure of the signal.
• A Monte Carlo simulation framework is employed to compute the statistical significance of the observed profiles and determine the angular scale with maximum signal-to-noise ratio (SNR).
• The voids from different catalogues are rescaled to a common angular size on the sky to enable consistent comparison across datasets.

Experimental results

Research questions

• RQ1 Is the iSW effect signal detected in stacked voids statistically significant when using a robust, standardized analysis protocol?
• RQ2 How do the results from the two most recent and largest void catalogues (Pan et al. 2012; Sutter et al. 2012) compare to the original detection by Granett et al. (2008a)?
• RQ3 To what extent does foreground contamination and large-scale CMB power affect the interpretation of the stacked signal?
• RQ4 Does the observed photometry profile from stacked voids exhibit features inconsistent with iSW predictions, such as unexpected peaks at large angles or central hot spots?
• RQ5 How do a posteriori selection effects, void overlap, and alignment influence the reliability of the iSW signal detection via stacking?

Key findings

• The study essentially confirms the iSW signal detection reported by Granett et al. (2008a), but only after applying a consistent rescaling of voids to a common angular size on the sky.
• For the Pan et al. (2012) and Sutter et al. (2012) catalogues, a significant signal is only recovered after rescaling, with a maximum signal-to-noise ratio of approximately 2.4σ.
• The photometry profile from the Gr08 voids shows an unexpected peak at large angular scales, which is inconsistent with standard iSW expectations.
• In contrast, the photometry profiles from the Sutter et al. catalogue exhibit small, unexplained central hot spots, suggesting potential systematics or non-physical features.
• The study highlights that the interpretation of the stacked signal as a genuine iSW effect is complicated by a posteriori selection effects and potential void overlap or alignment artifacts.
• The results suggest that caution is required when interpreting stacked void signals as evidence for the iSW effect, as systematics may significantly influence the observed profiles and significance levels.