Transcript: When clusters of galaxies are observed with microwaves something very interesting happens. The microwaves show a decrement or a hole where the cluster is. For awhile astronomers did not understand this effect, but it turns out to have a clean and clear theoretical explanation. What happens is that the hot, dense material at the center of clusters scatters the microwave photons of the background radiation from the big bang up to higher frequencies leaving a deficit of those microwave photons in the direction of the cluster. This is a very important effect in cosmology because it’s proof that the clusters are at cosmological distances since the microwave background photons emerge from the entire universe at a redshift of a thousand. It’s called the Sunyaev-Zeldovich effect after the two Russian theorists who first predicted it, and it’s now been observed in dozens of clusters. It’s also a powerful but indirect way to measure the mass of a cluster.