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The Question

(Submitted March 26, 1997)

I've read that the Sunyaev-Zeldovich Effect (SZE) can be used as an independent means to calculate the Hubble Constant. My question is: What is the SZE and what observing programs are either ongoing or planned to measure it and the Hubble Constant? Also, I'd appreciate any references you can point me to for further reading.

The Answer

The Sunyaev-Zeldovich effect refers to the scattering of cosmic microwave background (CMB) photons off of hot gas. It can be used as a cosmological measure because clusters of galaxies have hot gas associated with them, at temperatures of 10^7 K or more. When the CMB photons pass through the hot gas, the photons scatter off of the hot gas and gain energy (this is called inverse-Compton scattering). This distorts the spectrum of the CMB in the directions of clusters. The amount of distortion depends on the size, temperature, and density of the hot cluster gas. We measure the temperature and density of the gas with X-ray observations. Using the X-ray temp., density and the distortion of the CMB we can estimate the size of the gas-emitting region. This size estimate along with the brightness gives an estimate of the distance to the cluster. This is the key ingredient to measure the Hubble constant. This discussion is a little simplified but it gives the basic idea.

Using radio and X-ray observations, the SZE has been used to estimate Hubble's constant. Measurements reported from 1990 to 1995 give values ranging from 32 to 82 km/s/Mpc. Relevant X-ray measurements are continuing to be done using the ASCA and ROSAT satellites.

For a recent review, written at a level for professional astronomers, see "Comptonization of the Cosmic Microwave Background: The Sunyaev- Zeldovich Effect" by Y. Rephaeli in Annual Review of Astronomy and Astrophysics, Vol 33 (1995), p. 541.

We're not aware of any discussion in a popular level magazine, like Scientific American or Sky and Telescope, but you might check their indices.

Jim Lochner & Andy Ptak
for Imagine the Universe!

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