The Integrated Sachs-Wolfe Effect
A supercluster is a huge region of the Universe with relatively many galaxies; a supervoid is a huge region with relatively few. When light travels into a supercluster, it heats up, gaining energy as though it were rolling into a valley. When the light leaves the supercluster, it must give that energy back. But dark energy stretches and flattens the valley in the meantime, and lets the light keep some of the warmth it had in the supercluster. Similarly, light leaving supervoids gets a slight chill as a souvenir.
The movie at left shows how a supercluster leaves a slight warm imprint on a screen of light that passes through it. The screen starts out all the same color (i.e. temperature). Green is lukewarm; red is hot, or energetic; and blue is cold, or lacking in energy.
When the light in the middle of the screen goes into the cluster, it grows more energetic, i.e. hotter, since it is falling “down” into the cluster. When it climbs back out, it cools back down.
However, you might notice that the depression in the rubber sheet is getting shallower with time; that’s because dark energy is stretching it out. Thus the light doesn’t have as far to “climb” on its way out as on its way in, and it retains a yellow, warm tint.
Photons are affected by many processes, where does ISW fit in? A well known effect is gravitational lensing: photons passing near massive structures get distorted as if they were passing through a glass lens. The phenomenon warps galaxies into arcs like a fun house mirror. But, to produce such a big effect requires a strong gravitational field, and on the million-light-year scales we are considering, the distortion is negligible.
Clusters of galaxies contain hot ionized gas, and photons traveling through are scattered by the free electrons. The photons will get a kick in energy and, depending on the frequency, the cluster will show up as either a hot or cold spot imprinted on the CMB. The phenomenon is known as the Sunyaev-Zel’dovich effect (SZ for short). A photon measured in the frequency range covered by the WMAP satellite will lose energy when passing through a cluster. Thus clusters imprint a cold spot on the WMAP CMB due to this effect. However, this too happens at very small scales relative to the ISW effect and so it does not influence our measurement.