The ISW puzzle.

The integrated Sachs-Wolfe (ISW) effect from stacking cosmic microwave background (CMB) images of superclusters and voids persists as a challenge to the concordance [Formula: see text]CDM paradigm. The signal is 4-5 times the expectation. The CMB Cold Spot (CS), the most significant CMB anomaly, resulted in the discovery of the Eridanus supervoid, one of the most enormous known structures. Bayesian statistics and a later Dark Energy Survey (DES) analysis suggest it is responsible for the CS, consistent with the observed fourfold enhancement over concordance predictions. These results motivate the average expansion rate approximation (AvERA) model, tracking coarse-grained inhomogeneities in an -body simulation. The AvERA expansion history provides a 'late solution' to the Hubble-constant tension with emerging curvature taking the role of Dark Energy, is consistent with all principal CMB and large-scale structure measurements, and solves the ISW puzzle. In addition, it predicts a sign reversal of the ISW effect that has been recently confirmed, albeit at a moderate significance, with eBOSS quasars. Deep and wide galaxy surveys, such as Euclid, will soon confirm or refute the 'late complexity' indicated by the ISW sign reversal, increase the overall statistical significance of the findings and settle whether the ISW puzzle necessitates any significant modification to the concordance [Formula: see text]CDM paradigm.This article is part of the discussion meeting issue 'Challenging the standard cosmological model'.