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It is by now well established that the magnitude of the two-pointangular-correlation function of the cosmic microwave background temperatureanisotropies is anomalously low for angular separations greater than about 60degrees. Physics explanations of this anomaly typically focus on the propertiesof the Universe at the surface of last scattering, relying on the fact thatlarge-angle temperature fluctuations are dominated by the Sachs-Wolfe effect(SW). However, these fluctuations also receive important contributions from theintegrated Sachs-Wolfe effect (ISW) at both early (eISW) and late ($\ell$ISW)times. Here we study the correlations in those large-angle temperaturefluctuations and their relative contributions to $S_{1/2}$ -- the standardmeasure of the correlations on large angular scales. We find that in thebest-fitting $\Lambda$CDM cosmology, while the auto-correlation of the earlycontributions (SW plus eISW) dominates $S_{1/2}$, there are also significantcontributions originating from cross-terms between the early and latecontributions. In particular, realizations of $\Lambda$CDM with low $S_{1/2}$are typically produced from a combination of somewhat low pure-earlycorrelations and accidental cancellations among early-late correlations. Wealso find that if the pure $\ell$ISW auto-correlations were the onlycontribution to $S_{1/2}$ in $\Lambda$CDM, then the $p$-value of the observedcut-sky $S_{1/2}$ would be unremarkable. This suggests that physical mechanismsoperating only at or near the last scattering surface could explain theobserved lack of large-angle correlations, though this is not the typicalresolution within $\Lambda$CDM.

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