Gravitational wave detection using pulsars

The origin and evolution of the Universe governed by the laws of gravity is currently best described by Einstein's highly successful theory of gravity, the theory of general relativity (GR). To date, GR has passed all observational tests with flying colours, including the indirect confirmation of the existence of so-called gravitational waves (GWs). The direct detection of GWs is still outstanding, despite huge efforts around the globe to achieve exactly this. This research project has the ambitious aim to detect GWs directly for the first time, to be achieved by observations of a special class of astrophysical objects known as radio pulsars – the very same objects that already led to the indirect verification of the existence of GWs.

The pulsar binary system PSR J0737–3039A/B. The system is emitting gravity waves as they orbit around their common centre of mass (John Rowe Animation/Australia Telescope National Facility, CSIRO).

In GR and other relativistic theories of gravity, space and time are combined to form “space-time” which is curved in the presence of mass. It is the curvature of space-time that itself determines how masses move. As they move, the curvature changes, eventually producing ripples in spacetime that propagate through the Universe. These ripples are known as GWs. Their existence can be inferred from the study of a particular class of binary stars. In such systems, one of the stars is visible as a radio pulsar – a fast rotating, magnetized, compact and massive object known as a neutron star, which emits a narrow beam of radio emission along its magnetic axis. As the pulsar rotates, it acts like a cosmic lighthouse and pulsar radiation directed to Earth can be observed once per rotation, producing a narrow pulse and hence a natural beacon for a terrestrial observer. Because pulsars are massive and compact (the mass of 1.4 Suns is concentrated in a sphere of only 20 km diameter), they represent massive flywheels in space, whose rotation and repetition rate can hardly be disturbed. This makes pulsars very precise cosmic clocks which can be monitored as they move in the curved space-time of a companion star. When this motion creates GWs, the propagating ripples carry away energy, so that the size of the binary system shrinks and the distance of the pulsar to its companion decreases. The rate of this shrinkage is measured for a small number of binary pulsars and is found to be consistent with the predictions of GR (Taylor et al. 1979, Nature, 277, 437; Kramer et al. 2006, Science, 314, 97). These observations therefore provide incontrovertible proof that GWs exist. However, in order to study their properties and to compare them with Einstein's predictions, a direct detection of GWs is required.

The rewards for a successful detection of GWs are immense and would have enormous consequences. It would not only provide the first direct proof for their existence, but it would also enable tests of GR in several, unprecedented ways. As most relativistic theories of gravity conjecture the existence of gravitational waves, the predictions of GR for GW properties, such as their polarisation modes, can be compared to those of alternative theories. Moreover, GWs can penetrate regions that the more familiar electromagnetic waves cannot, so that views of black holes and other exotic objects in the distant Universe become possible.