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

¹ Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029 Blindern 0315, Oslo, Norway
² Instituto de Astronomía Teórica y Experimental, CONICET-UNC, Laprida 854, X5000BGR, Córdoba, Argentina
³ Observatorio Astronómico de Córdoba, UNC, Laprida 854, X5000BGR, Córdoba, Argentina

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 28 May 2025
Accepted: 27 May 2026

Abstract

We find a significant cosmic microwave background (CMB) temperature excess in the direction of local underdensities within z < 0.03. By contrast, fewer 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 the 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 of 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 what is found in simulations.