Detecting the integrated Sachs-Wolfe effect with stacked voids
  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.
