It is due to the gravitational redshift induced by photons falling into and climbing out of regions of space with different density, called potential wells, in between the Earth and the surface of last scattering (close to the particle horizon). The non-integrated Sachs-Wolfe effect is also due to gravitational redshift, but is the effect only at the surface of last scattering itself.

There are two main contributions to the integrated effect. The first is shortly after photons leave the last scattering surface, and due to the evolution of the potential wells as the universe changes from being radition to matter dominated. The second, sometimes called the 'Late-time integrated Sachs Wolfe effect', arises much later as the evolution starts to feel the effect of the cosmological constant (or, more generally, dark energy), or curvature of the Universe if it is not flat. The latter effect has an observational signature in the amplitude of the large scale perturbations of cosmic microwave background and their correlation with large scale structures in the universe.

See also: Sachs-Wolfe effect

Related: Big Bang, Cosmic microwave background radiation, List of astronomical topics, Timeline of cosmic microwave background astronomy

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