The integrated Sachs-Wolfe effect

Dark energy influences cosmic microwave background (CMB) photons, in particular via the integrated Sachs-Wolfe effect (ISW). CMB photons zipping across the universe gain energy by falling into gravitational potential wells, and lose energy when they climb out again (top trajectory). For the shallow potentials on large scales, which correspond to over- and under- dense regions extending across hundreds of megaparsecs, the overall loss and gain of energy cancel. But this is only true in a universe in which the full critical energy density comes from atoms and dark matter. In the presence of dark energy, however, the ISW effect comes in to play: the expansion of the universe is fast enough to stretch the potentials and make them shallower, which means that a photon falling into an overdense region gains more energy than it loses when climbing out (bottom trajectory). Regions of space in which matter has clustered should therefore correspond to hotter CMB photons, whereas underdense regions should lead to colder CMB photons. By comparing the CMB and the large-scale structure of the universe at different wavelengths, four independent groups of cosmologists have found signs of the ISW effect, providing a new line of evidence that is consistent with cosmic acceleration driven by dark energy.

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