Abstract
Pulsars are rapidly rotating neutron stars that act like cosmic lighthouses, emitting beams of radio waves that sweep past Earth with extraordinary regularity. Some of these, the millisecond pulsars, rival atomic clocks in their timing precision. By carefully tracking the arrival times of pulses from many such objects distributed across the sky, astronomers have built a galaxy-scale observatory known as a Pulsar Timing Array (PTA). PTAs are uniquely sensitive to ultra-low-frequency (nano-Hertz) gravitational waves—ripples in spacetime generated by systems such as supermassive black hole binaries in the process of merging. In this talk, I will discuss how pulsar timing can be used to detect gravitational-wave signatures in the background. It is fascinating to see how researchers have turned our galaxy into a detector, using correlated signals across the pulsar network to reveal the presence of gravitational waves. I will also highlight the most recent results from PTA collaborations, including evidence for a stochastic gravitational-wave background, and discuss the exciting prospects for resolving individual sources in the coming years.