  The first terrestrial gravitational wave interferometers have dramatically
underscored the scientific value of observing the Universe through an entirely
different window, and of folding this new channel of information with
traditional astronomical data for a multimessenger view. The Laser
Interferometer Space Antenna (LISA) will broaden the reach of gravitational
wave astronomy by conducting the first survey of the millihertz gravitational
wave sky, detecting tens of thousands of individual astrophysical sources
ranging from white-dwarf binaries in our own galaxy to mergers of massive black
holes at redshifts extending beyond the epoch of reionization. These
observations will inform - and transform - our understanding of the end state
of stellar evolution, massive black hole birth, and the co-evolution of
galaxies and black holes through cosmic time. LISA also has the potential to
detect gravitational wave emission from elusive astrophysical sources such as
intermediate-mass black holes as well as exotic cosmological sources such as
inflationary fields and cosmic string cusps.
