Searching for Low-Frequency Gravitational Waves with Pulsar Timing
I am associate member of the North American Nanohertz Observatory for Gravitational Waves, a US/Canadian research collaboration working to detect nHz-frequency gravitational radiation using a pulsar timing array. Pulsars are rapidly rotating neutron stars, compact and degenerate cores of massive stars that have undergone death by supernova and whose radio emission falls within the line-of-sight of earth at periodic intervals. The fastest and most stable pulsars have spin-periods on the order of milliseconds and variablility of less than a second every 10\(^{20}\) seconds, rivaling atomic clocks in their accuracy. This level of stability makes millisecond pulsars ideal for detecting low-frequency gravitational waves, ripples in the fabric of space-time generated by inspiraling super-massive black hole binaries. The superposition of gravitational waves from the super-massive black hole binary mergers that have taken place throughout the history of the universe forms a stochastic background of gravitational waves.
As they propagate through space, gravitational waves change the path-length between the pulsar and earth, causing a detectable delay in the pulse arrival time as seen by radio telescopes. Correlating delays in the pulse arrival times of many MSPs throughout the galaxy is equivalent to creating a galactic-scale gravitational wave telescope, known as a pulsar timing array. NANOGrav currently searches for inter-pulsar time of arrival correlations in over 70 millisecond pulsars using the Arecibo Radio Telescope, Green Bank Radio Telescope, and Very Large Array.