Gravitational waves are small ripples in the fabric of spacetime first predicted as a consequence of Einstein's theory of general relativity. By analogy with an accelerating charge in a magnetic field radiating electromagnetic waves, an accelerating mass in a gravitational field will emit gravitational waves. These waves carry energy and information - in effect telling other objects how to move in a four-dimensional space time. The orbital decay observed in binary pulsar systems is a direct consequence of gravitational waves and provides firm evidence for their existence.
Many cosmological models predict that the Universe is presently filled with a low-frequency gravitational wave background produced during the big bang, and by mergers of super-massive black holes in the distant Universe. The wave frequency of this background is thought to have typical frequencies of the order of a nano-Hertz. Such waves can in principle be detected from high-precision pulsar timing observations as the image below shows.
The basic concept is to treat the Earth and a distant pulsar as opposite ends of an imaginary arm in space. The pulsar acts as the reference clock at one end of the arm sending out regular signals which are monitored by an observer on the Earth. The effect of a passing gravitational wave would be to change the local curvature of space and hence distance between the Earth and the pulsar. As perceived by the observer, this change would manifest itself as a chance in the apparent rotational frequency of the pulsar.
By monitoring a large number of pulsars using the telescopes in the PulSE network, we are striving to detect cosmological gravitational waves. The nano-Hertz frequency band is inaccessible to current gravitational wave detectors which are insensitive to frequencies below 1 Hz. Pulsar timing therefore provides a unique window into the spectrum of gravitational waves which complements the high-frequency range of the current detectors.