Using pulsars to limit the existence of a gravitational wave background

Gravitational waves passing the Earth induce signals in pulsar timing residuals that are potentially detectable using modern pulsar timing observations. It has been predicted that merging supermassive black hole systems will cause an isotropic background of gravitational waves. Pulsar timing observations will soon be able to provide useful constraints on models for its formation. Future experiments will either detect this background or rule out all existing models. Various cosmic string theories and inflationary era models have also been proposed that predict gravitational wave backgrounds, but the amplitude of the background in these models is not well constrained. We review new techniques to limit the existence of any gravitational wave background, provide results obtained using data taken as part of the Parkes Pulsar Timing Array project and describe the implications of these new upper limits.