Why Gravitational Wave Science Needs Pulsar Timing Arrays And Why Pulsar   Timing Arrays Need Both Arecibo and the GBT: A Response to the NSF-AST   Portfolio Review from the NANOGrav Collaboration
  Gravitational waves (GWs) are ripples in space-time that are known to exist but have not yet been detected directly. Once they are, a key feature of any viable theory of gravity will be demonstrated and a new window on the Universe opened. GW astronomy was named as one of five key discovery areas in the New Worlds, New Horizons Decadal Report. Pulsar timing probes GW frequencies, and hence source classes, that are inaccessible to any other detection method and can uniquely constrain the nonlinear nature of General Relativity. Pulsar timing is therefore a critical capability with its own discovery space and potential. Fulfilling this capability requires the complementary enabling features of both the Green Bank Telescope (GBT) and the Arecibo Observatory. 
The NANOGrav Collaboration
astro-ph.IM astro-ph.SR gr-qc