Pulsars as Cosmic Clocks

Pulsars were first discovered in 1967 by Ph.D. student Jocelyn Bell Burnell at the Mullard Radio Astronomy Observatory in Cambridge, England. Dr. Bell Burnell discovered repeating pulses separated by 1.3 seconds that originated from the same patch of sky every time they were observed. The pulse was unexplained at the time and it became colloquially known as LGM for “little green men,” but we now know it as pulsar B1919+21. Bell Burnell went on to discover the first four pulsars, but now over 3,000 have been discovered.

As pulsars rotate, they act as cosmic lighthouses.  Each time the beam of a pulsar points towards the Earth, we see a pulse of radio waves. These pulses can be “timed” just like the tick of a clock.  Millisecond pulsars are a subclass of pulsars which spin hundreds of times a second and approach the stability of the best atomic clocks on Earth. NANOGrav is a pulsar timing experiment. This involves observing millisecond pulsars over very long periods of time and developing timing models which predict exactly when future pulses will arrive. These models must account for many different astrophysical effects ranging from the intrinsic pulsar rotation to its motion around a binary companion to its travel through the interstellar medium. These timing models are so precise that NANOGrav can search for very small deviations from expected pulse arrival times due to very small astrophysical effects, such as those from gravitational waves.

Gravitational waves from merging supermassive black holes stretch and squeeze spacetime by only one part in one quadrillion. In order to detect these extremely subtle changes, NANOGrav uses a technique called pulsar timing to predict arrival times of the regular, clock-like pulses of rapidly rotating millisecond pulsars. Ripples from gravitational waves passing through our Galaxy cause pulses to arrive too early or too late in a predictable and distinctive pattern. By timing dozens of pulsars scattered throughout the galaxy as part of its pulsar timing array, NANOGrav uses a Galaxy-sized gravitational wave detector to study low-frequency gravitational waves.

Pulsar timing array experiments are similar to global positioning systems (GPS): in the same way that your phone will communicate with satellites in orbit around Earth to accurately determine your position, NANOGrav uses millisecond pulsars to look for small changes in the position of Earth due to the stretching and shrinking of spacetime induced by gravitational waves. The GPS satellites spread across the sky are akin to NANOGrav’s array of pulsars. Because the deviations NANOGrav looks for are so minuscule, only the most clock-like, “well-behaved” pulsars are suitable for our pulsar timing array.

NANOGrav collects timing data for dozens of millisecond pulsars on a regular basis using large radio telescopes. Not only is this data set state-of-the-art for detecting low-frequency gravitational waves, but it is also a powerful tool for probing unrelated astrophysical processes. For example, NANOGrav’s frequent pulsar timing observations have helped advance our understanding of how the cold matter between stars moves, changes, and affects the signals we study. NANOGrav also contributes to our understanding of how the extremely dense matter in neutron star interiors behaves, as our observations have led to extremely precise measurements of high-mass neutron stars.