Pulsar timing measurements and the search for gravitational waves

Pulse arrival time measurements of pulsars may be used to search for gravitational waves with periods on the order of 1 to 10 years and dimensionless amplitudes of approximately 10 to the -11th power. The analysis of published data on pulsar regularity sets an upper limit to the energy density of a stochastic background of gravitational waves, with periods of approximately 1 year, which is comparable to the closure density of the universe.