  A moderate investment of observing time with the International X-ray
Observatory to study high-redshift galaxy clusters detected in future
large-scale surveys, will provide cosmological measurements of fundamental
importance. IXO observations, combined with lensing follow-up, will measure the
perturbation growth factor from z=0-2 with an accuracy comparable to, or
possibly better than, that expected from observations of cosmic shear with
JDEM, and redshift-space distortions with EUCLID. The growth of structure data
derived from clusters will significantly improve our knowledge of the dark
energy equation of state and will aid in constraining non-GR models for cosmic
acceleration. IXO observations of the largest, dynamically relaxed clusters
will provide a powerful, independent measurement of the cosmological expansion
history using the apparent f_gas(z) trend. Systematic and statistical errors
from this technique are competitive with SNIa and BAO studies, making the test
extremely useful for improving the accuracy and reliability of the geometric
cosmological measurements planned for LSST and JDEM. Only by employing a range
of powerful, independent approaches, including those discussed here, can robust
answers to puzzles as profound as the origin of cosmic acceleration be
expected.
