**3.2. Sachs-Wolfe Effect**

The simplest contribution to the CMB anisotropy from density
fluctuations is just
a gravitational redshift, known as the *Sachs-Wolfe effect*
[18].
A photon coming from a region which is slightly overdense will have a
slightly larger
redshift due to the deeper gravitational well at the surface of last
scattering.
Conversely, a photon coming from an underdense region will have a
slightly smaller
redshift. Thus we can calculate the CMB temperature anisotropy due to
the slightly varying Newtonian potential
from density
fluctuations at the surface of last scattering:

(58) |

where the factor 1/3 is a general relativistic correction. Fluctuations on large angular scales (low multipoles) are actually larger than the horizon at the time of last scattering, so that this essentially kinematic contribution to the CMB anisotropy is dominant on large angular scales.