Evidence for a sign change of the ISW effect in the very recent universe? hot voids and cold overdensities at $z<0.03$

Abstract

We find a significant CMB temperature excess in the direction of local underdensities within $z<0.03$. By contrast, less than $0.2\%$ of simulated CMB maps show a similarly significant temperature excess in nearby voids. Combined with earlier findings showing a $>5σ$ cooling of CMB photons in galactic filaments in the same redshift range, we now may have possible evidence for a negative Integrated Sachs-Wolfe (ISW) effect in the very recent Universe. In addition to having opposite sign, the observed amplitude is an order of magnitude larger than the predicted Rees-Sciama and ISW effects, pointing to an unknown physical process, possibly related with a non-standard model of dark energy. We discuss the results in light of the latest Data Release 2 results of the Dark Energy Spectroscopic Instrument (DESI) showing hints for dynamical dark energy. Removing the quadrupole, we find the CMB temperatures measured in nearby voids to a large degree uncorrelated with the temperature measured around nearby galaxies and the observed mean difference between these temperatures is almost $6.5σ$ larger than found in simulations.