Analysis of the Sunyaev-Zel'dovich effect (SZE) and X-ray data provides a
method of directly determining distances to galaxy clusters at any
redshift. Clusters of galaxies contain hot (kB
Te
10 keV)
gas, known as the intracluster medium (ICM), trapped in their
potential wells. Cosmic microwave background (CMB) photons passing
through a massive cluster interact with the energetic ICM electrons
with a probability of 
0.01. This
inverse-Compton
scattering preferentially boosts the energy of a scattered CMB photon,
causing a small (
1 mK) distortion
in the CMB spectrum, known as the Sunyaev-Zel'dovich effect
(Sunyaev &
Zel'dovich, 1972;
Sunyaev &
Zel'dovich, 1970).
The SZE is proportional to the pressure integrated along the line of
sight, 
T
ne Te
d
. X-ray emission
from the ICM has a different dependence on the density
SX
ne2
eH
d, where
eH is
the X-ray cooling
function. Taking advantage of the different density dependences and
with some assumptions about the geometry of the cluster, the distance
to the cluster may be determined. SZE and X-ray determined distances
are independent of the extragalactic distance ladder and provide
distances to high-redshift galaxy clusters. This method does not rely
on clusters being standard candles or rulers and relies only on
relatively simple properties of highly ionized plasma.
The promise of direct distances has been one of the primary motivations for SZE observations. Efforts over the first two decades after the SZE was first proposed in 1970 (Sunyaev & Zel'dovich, 1972; Sunyaev & Zel'dovich, 1970) yielded few reliable detections. Over the last decade, new detectors and observing techniques have allowed high-quality detections and images of the effect for more than 60 clusters with redshifts as high as one. SZE observations are routine enough to build up samples of clusters and place constraints on cosmological parameters.
The SZE offers a unique and powerful observational tool for
cosmology. Distances to galaxy clusters yield a measurement of the
Hubble constant, H0. With a sample of high-redshift
clusters, SZE
and X-ray distances can be used to determine the geometry of the
Universe. In addition, the SZE has been used to measure gas fractions
in galaxy clusters (e.g.,
Myers et al., 1997;
Grego et al., 2001),
which can be used to measure the matter density of the Universe,
m,
assuming the composition of clusters represents a fair sample of the
universal composition. Upcoming deep, large-scale SZE surveys will
measure the evolution of the number density of galaxy clusters, which
is critically dependent on the underlying cosmology. In principle, the
equation of state of the "dark energy" may be determined from the
evolution of the number density of clusters.
In this review, we first outline the properties of the SZE in Section 2 and provide a brief overview of the current state of the observations in Section 3. SZE/X-ray determined distances are discussed in Section 4, briefly discussing the current state of SZE/X-ray distances, sources of systematics, and future potential. SZE surveys are briefly discussed in Section 5 and a summary is given in Section 6. The physics of the SZE is covered in previous reviews (Sunyaev & Zel'dovich, 1980; Birkinshaw, 1999; Rephaeli, 1995), with Birkinshaw (1999) and Carlstrom et al. (2000) providing reviews of the observations. Carlstrom et al. (2002) provide a review with focus on cosmology from the SZE, with special attention to SZE surveys.