Pulsars and Gravity

Pulsar periods are extremely stable, making possible many interesting and important applications ranging from determining the proper motion of globular clusters to detecting gravitational waves. Many pulsars, especially millisecond pulsars, are in binary orbit with another star. The first-known pulsar binary system, PSR B1913+16, discovered by Hulse and Taylor in 1974, provided the first observational evidence for the existence of gravitational waves and verified that Einstein's general theory of relativity is an accurate theory of gravitation. Surveys with Parkes multibeam receiver have in the past few years more than doubled the number of known pulsars. Prominent among the discoveries is the first-known double pulsar, PSR J0737-3039A/B. This system is even more relativistic than PSR B1913+16 and in just two years has provided a more stringent test of general relativity, with five post-Keplerian parameters already determined. An important prediction of general relativity is the existence of gravitational waves. Precise timing of a very stable millisecond pulsar has already put limits on the energy density of a stochastic gravitational-wave background at the Earth. Timing measurements of an ensemble of millisecond pulsars distributed over the celestial sphere can in principle make a direct detection of this stochastic background. The Parkes Pulsar Timing Array (PPTA) project is making regular timing measurements of 20 millisecond pulsars with the ultimate aim of detecting the gravitational-wave background at the Earth. With a five-year data span the PPTA should have sufficient sensitivity to detect the gravitational-wave background from binary super-massive black holes in the cores of galaxies.