2.6.3. Zeldovich-Sunyaev Effect
The observation of the Zeldovich-Sunyaev (ZS) effect for clusters tells us about the cluster depth (times electron density), which, when combined with angular diameter (times electron density square) from X ray observations, gives us the distance to the cluster provided that cluster is spherical (Birkinshaw et al. 1991, Myers et al. 1997). This is often taken as a physical method to measureH0. I give little weight to this method in these lectures, since it is difficult to estimate the systematic errors. The currently available results wildly vary from a cluster to a cluster. The most importnat is a bias towards elongation. None of the known clusters are quite spherical, and selection effects bias towards clusters elongated along the line of sight because of higher surface brightness. This may happen even if one uses a large sample. Additional systematics arise from the sensitivity of the ZS effect to the cluster envelopes; one must resort to a model to correct for this effect.