  The North American Nanohertz Observatory for Gravitational Waves (NANOGrav)
is a consortium of astronomers whose goal is the creation of a galactic scale
gravitational wave observatory sensitive to gravitational waves in the
nHz-microHz band. It is just one component of an international collaboration
involving similar organizations of European and Australian astronomers who
share the same goal. Gravitational waves, a prediction of Einstein's general
theory of relativity, are a phenomenon of dynamical space-time generated by the
bulk motion of matter, and the dynamics of space-time itself. They are
detectable by the small disturbance they cause in the light travel time between
some light source and an observer. NANOGrav exploits radio pulsars as both the
light (radio) source and the clock against which the light travel time is
measured. In an array of radio pulsars gravitational waves manifest themselves
as correlated disturbances in the pulse arrival times. The timing precision of
today's best measured pulsars is less than 100 ns. With improved
instrumentation and signal-to-noise it is widely believed that the next decade
could see a pulsar timing network of 100 pulsars each with better than 100 ns
timing precision. Such a pulsar timing array (PTA), observed with a regular
cadence of days to weeks, would be capable of observing supermassive black hole
binaries following galactic mergers, relic radiation from early universe
phenomena such as cosmic strings, cosmic superstrings, or inflation, and more
generally providing a vantage on the universe whose revolutionary potential has
not been seen in the 400 years since Galileo first turned a telescope to the
heavens.
