Pulsar Timing Arrays: A New Window on the Gravitational Wave Universe

Millisecond pulsars are rapidly rotating neutron stars with phenomenal rotational stability. Pulsar timing arrays world-wide monitor over 100 of these cosmic clocks in order to search for perturbations due to gravitational waves at nanohertz frequencies. The tell-tale sign of a stochastic background of gravitational waves in pulsar timing data is the presence of quadrupolar spatial correlations. Recently, and for the first time ever, pulsar timing array collaborations have found evidence of these spatial correlations in multiple independent pulsar datasets.  The signal is consistent with that expected from an ensemble of supermassive black hole binaries, but could also be attributable to more exotic sources, such as cosmic strings or early universe inflation. I will describe these experiments and the most recent results, concentrating on those from the NANOGrav collaboration, and will discuss the increases in sensitivity expected from the combination of data observed with new and existing telescopes across the globe.