Astro2020 Science White Paper:Cosmology with a Space-Based Gravitational Wave Observatory

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