Going Further 15.1: The Sunyaev-Zel’dovich Effect
The Sunyaev-Zel’dovich effect is caused by interactions of photons with charged particles. We have already discussed on several occasions how photons can be scattered by charged particles, typically electrons. Sometimes this scattering merely changes the direction of the photons without changing their energy. That is what happens to sunlight as it passes through Earth’s atmosphere, and the preferential scattering of short wavelengths over longer causes the sky to appear blue on a clear day.
However, it is also possible for the scattering of light to transfer energy to or from the photons. If the population of scattering electrons has on average more energy than the population of photons, then energy is transferred to the photons, which appear hotter as a result. On the other hand, if the typical energy of the electrons is lower than the energy of the photons, then the light will transfer energy to the electrons. The light cools at it looses energy to the electrons.
The microwave background began as light with a typical temperature around 3000K. Since the Universe has grown by a factor of about a thousand since the CMB was created, the CMB light now is characterized by a temperature of around 3K, and of course, it was cooling from 3000K to 3K over its entire journey. However, some of the microwave photons have encountered galaxy clusters as they traveled through the cosmos. These photons have had their temperature modified by the scattering described previously.
In addition to containing galaxies, galaxy clusters also contain immense amounts of hot gas. The gas, which actually dominates the total baryonic mass of the clusters, has a temperature around 10 million kelvin, much higher than that of the CMB. This is because it sits deep in the gravitational well of the cluster. When CMB photons encounter the hot gas they gain energy from its particles because the CMB is so much cooler than the typical electron in the cluster gas. As a result, the photons passing through a cluster have a slightly increased temperature than CMB photons that don’t pass through a cluster. The spectrum is distorted and shifted to higher energy. The CMB photons mark the positions of the clusters on the sky because they cause the clusters to stand out as small spots in the microwave background. At higher wavelengths than the peak of the CMB, these spots are hotter, and at longer wavelengths (lower energies), these spots are colder, because the photons that would normally be present have been shifted to a different part of the spectrum. An example of an SZ cluster observation is shown in Figure B.15.1.
Figure B.15.1: SZ effect
The SZ effect is important at high-l values, or small angular scales, because galaxy clusters are not very large on the sky. The small SZ temperature fluctuations are not caused by any primordial cosmological parameters, they are modifications of the original CMB spectrum by intervening mass concentrations as its photons travel to us. These fluctuations must be accounted for in the cosmological analysis of the CMB since they introduce a spurious signal (or noise, if you prefer) on the true cosmological perturbations.
However, the SZ effect is interesting in its own right because it provides a way to understand how galaxy clusters have formed and evolved over the history of the Universe. Because the CMB comes to us from essentially the beginning of the Universe, it probes as far as can be probed by photons. What’s more, the SZ effect depends only on the angular size of the clusters and the temperature of their gas, it is not affected by the 1/r2 dimming law that affects many astronomical measurements. Thus SZ provides a powerful means to study structures anywhere along the line of sight. One of the many telescopes studying the SZ effect is the South Pole Telescope. Animated Figure B.15.2 shows this telescope being built.