The ISW effect and the lack of large-angle CMB temperature correlations

It is by now well established that the magnitude of the two-point angular-correlation function of the cosmic microwave background temperature anisotropies is anomalously low for angular separations greater than about 60 degrees. Physics explanations of this anomaly typically focus on the properties of the Universe at the surface of last scattering, relying on the fact that large-angle temperature fluctuations are dominated by the Sachs-Wolfe effect (SW). However, these fluctuations also receive important contributions from the integrated Sachs-Wolfe effect (ISW) at both early (eISW) and late (lISW) times. Here, we study the correlations in those large-angle temperature fluctuations and their relative contributions to S-1/2 - the standard measure of the correlations on large angular scales. We find that in the best-fitting lambda cold dark matter (Lambda CDM) cosmology, while the autocorrelation of the early contributions (SW plus eISW) dominates S-1/2, there are also significant contributions originating from cross-terms between the early and late contributions. In particular, realizations of Lambda CDM with low S-1/2 are typically produced from a combination of somewhat lowpure-early correlations and accidental cancellations among early-late correlations. We also find that if the pure lISW autocorrelations were the only contribution to S-1/2 in Lambda CDM, then the p-value of the observed cut-sky S-1/2 would be unremarkable. This suggests that the physical mechanisms operating only at or near the last scattering surface could explain the observed lack of large-angle correlations, though this is not the typical resolution within Lambda CDM.