4.2 Sunyaev Zel'dovich Effect and X-Ray Measurements
The inverse-Compton scattering of photons from the cosmic microwave background off of hot electrons in the X-ray gas of rich clusters results in a measurable decrement in the microwave background spectrum known as the Sunyaev-Zel'dovich (SZ) effect (Sunyaev & Zel'dovich 1969). Given a spatial distribution of the SZ effect and a high-resolution X-ray map, the density and temperature distributions of the hot gas can be obtained; the mean electron temperature can be obtained from an X-ray spectrum. The method makes use of the fact that the X-ray flux is distance-dependent, whereas the Sunyaev-Zel'dovich decrement in the temperature is not.
Once again, the advantages of this method are that it can be applied at large distances and, in principle, it has a straightforward physical basis. Some of the main uncertainties result from potential clumpiness of the gas (which would result in reducing H0), projection effects (if the clusters observed are prolate, H0 could be larger), the assumption of hydrostatic equilibrium, details of the models for the gas and electron densities, and potential contamination from point sources.
To date, a range of values of H0 have been published based on this method ranging from ~ 40-80 km/sec/Mpc (e.g., Birkinshaw 1998). The uncertainties are still large, but as more and more clusters are observed, higher-resolution (2D) maps of the decrement, and X-ray maps and spectra become available, the prospects for this method are improving enormously.
Carlstrom, (this meeting) presented exquisite new measurements of the Sunyaev-Zel'dovich decrement, measured in two dimensions with an interferometer, for a number of nearby clusters. He also highlighted the imminent advances on the horizon for X-ray imaging using NASA's soon-to-be launched Chandra X-ray Observatory (the satellite formerly known as AXAF). There is promise of a significant increase in accuracy for this method in the near future.