  There are two big questions cosmologists would like to answer -- How does the
Universe work, and what are its origin and destiny? A long wavelength
gravitational wave detector -- with million km interferometer arms, achievable
only from space -- gives a unique opportunity to address both of these
questions. A sensitive, mHz frequency observatory could use the inspiral and
merger of massive black hole binaries as standard sirens, extending our ability
to characterize the expansion history of the Universe from the onset of dark
energy-domination out to a redshift z ~ 10. A low-frequency detector,
furthermore, offers the best chance for discovery of exotic gravitational wave
sources, including a primordial stochastic background, that could reveal clues
to the origin of our Universe.
