Sachs–Wolfe effect

The Sachs - Wolfe effect explains fluctuations in the redshift of the photons of the cosmic background radiation. He is next to the Silk damping and oscillations of the plasma in the early universe one of three effects, with which it is possible to astrophysics to calculate conditions in the early universe. It is named after Rainer Kurt Sachs and Arthur Michael Wolfe, who discovered it in 1967.

It allows to read from the fluctuations in the redshift of the cosmic background radiation, such as the structure of matter in the universe must be about 400,000 years after the Big Bang, that is, at the time of recombination, have been. In particular, can thus also determine the curvature parameter k of the space-time.

At this time, the distribution of matter was already anisotropic in some places in the universe, and so there was here and there small condensations whose gravitational potentials exceeded the isotropic environment. Clearly, one can imagine a potential as an indentation in a flat surface, with the size of the potential with increasing depth increases. If it enters a photon in such a gravitational potential, it receives energy due to the occurrence. While the photon moves into the potential, but the universe is expanding at a little further, so that the potential becomes flatter by the stretching of space-time in the vivid imagination. Therefore, the photon energy must give back less than it had previously obtained at the outlet. In effect, this means that the photon gains energy - this energy was previously in the potential. This is just the Sachs - Wolfe effect, through which the cosmic background radiation was anisotropic. While the photons on their way across the universe to us, they go through many more gravitational potentials, however, have only a negligible effect on the photons and therefore no longer belong to the Sachs - Wolfe effect.

With WMAP has managed in 2001 to get through this effect is strong evidence for the existence of the hypothetical dark energy. This in its nature yet unknown energy is responsible for the movement of expansion of the universe; it makes up about 70 % of the energy of the universe. The curvature parameter k of the space-time resulted from the measurements at k = 0, which means that the universe is a flat manifold. However, since a perfect measurement is impossible, however, it may also be that the universe is just very slightly curved and this is within the range of measurement error.

In May 2009, ESA's Planck telescope has been launched and will provide a ten times more resolution for the background radiation and allow better investigations.