The Sachs-Wolfe effect is a property of the cosmic background radiation (CBR), in which gravitational bodies redshift the CBR, causing it to appear uneven.
Variations in the cosmic background radiation as observed today are "fossils" of interactions between energy and matter which took place much earlier in the universe's history. As the universe expanded and cooled, it did not do so isotropically (the same in all directions). As a consequence, some regions of space contained greater concentrations of matter than others.
Photons, as the particles which mediate the electromagnetic force, are subject to the influence of gravity. When they pass through a region which contains a relatively greater concentration of matter, they undergo a redshift. Conversely, they undergo a blueshift relative to the average background as a consequence of passing through a region of low density.
See also: Integrated Sachs Wolfe effect
Reference
- Sachs, Rainer Kurt, and Arthur Michael Wolfe, "Perturbations of a cosmological model and angular variations of the cosmic microwave background", Astrophysical Journal, 1967, 147, 73.
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