Integrated Sachs Wolfe effect

The integrated Sachs Wolfe effect is something that happens to fluctuations in the temperature of the cosmic microwave background, according to the standard Big Bang model, unless the Universe is perfectly flat and the cosmological constant is zero.

It is due to the gravitational redshift induced by photons falling into and climbing out of very dense regions of space, called potential wells, in between the Earth and the surface of last scattering (close to the particle horizon). The naive sachs wolfe effect is also due to gravitational redshift, but is the effect only at the surface of last scattering itself.