Sachs-Wolfe effect

The Sachs-Wolfe effect (SW effect, SWE) is the temperature CMB anisotropies resulting from gravitational redshift (i.e., from passing near or through something massive), and is the favored primary explanation for cosmic microwave background (CMB) fluctuations over angular scales above ten degrees. The non-integrated Sachs-Wolfe effect is gravitational redshift at the time of the last scattering (recombination) due to density variations at that time.

The integrated Sachs-Wolfe effect (or ISW) is an effect on the radiation after recombination, and reflects the subsequent matter and energy distribution. The effect occurs when photons enter and exit a region of different density such as a galaxy cluster or a void. A blueshift results from entering a high-density region and a redshift on leaving. Also, the length of the photon's travel is affected by the curvature, such that photons of slightly different ages reach us from that part of the celestial sphere. These effects are small (largely canceling) and may be undetectable except that dark energy causes the high-density region (gravitational potential well) to evolve during the photon's trip, reducing the canceling-effect sufficiently that clusters within a giga-light-year of us should produce a detectable ISW. Voids have the opposite effect, i.e., they shift the apparent temperature in the opposite direction. Due to dark energy's significant effects, descriptions of the ISW have significantly changed since around 1999 when dark energy's existence was uncovered. Efforts are made to associate observed anisotropy with the effect through correlations and by attempting to pin the apparent effects to known features, i.e., galaxy clusters and voids.