  Constellation-X (Con-X) will carry out two powerful and independent sets of
tests of dark energy based on X-ray observations of galaxy clusters, providing
comparable accuracy to other leading dark energy probes. The first group of
tests will measure the absolute distances to clusters, primarily using
measurements of the X-ray gas mass fraction in the largest, dynamically relaxed
clusters, but with additional constraining power provided by follow-up
observations of the Sunyaev-Zel'dovich (SZ) effect. As with supernovae studies,
such data determine the transformation between redshift and true distance,
d(z), allowing cosmic acceleration to be measured directly. The second,
independent group of tests will use the exquisite spectroscopic capabilities of
Con-X to determine scaling relations between X-ray observables and mass.
Together with forthcoming X-ray and SZ cluster surveys, these data will help to
constrain the growth of structure, which is also a strong function of
cosmological parameters.
