  Observations of the gravitational radiation from well-localized, inspiraling
compact object binaries can measure absolute source distances with high
accuracy. When coupled with an independent determination of redshift through an
electromagnetic counterpart, these standard sirens can provide an excellent
probe of the expansion history of the Universe and the dark energy. Short
gamma-ray bursts, if produced by merging neutron star binaries, would be
standard sirens with known redshifts detectable by ground-based GW networks
such as LIGO-II, Virgo, and AIGO. Depending upon the collimation of these GRBs,
a single year of observation of their gravitational waves can measure the
Hubble constant to about 2%. When combined with measurement of the absolute
distance to the last scattering surface of the cosmic microwave background,
this determines the dark energy equation of state parameter w to 9%. Similarly,
supermassive binary black hole inspirals will be standard sirens detectable by
LISA. Depending upon the precise redshift distribution, 100 sources could
measure w at the 4% level.
