A phenomenon in which irregularities in the distribution of mass in the early Universe give rise to localized temperature variations in the cosmic microwave background. It occurs because photons are gravitationally redshifted by regions where the density of matter is higher than average. The ripples detected by the Cosmic Background Explorer (COBE) satellite in 1992 are thought to be a manifestation of this effect. It is named after the American astrophysicists Rainer Kurt Sachs (1932– ) and Arthur Michael Wolfe (1939– ).
Subjects: Astronomy and Astrophysics.