Quasar Proper Motions and Low-Frequency Gravitational Waves
  We report observational upper limits on the mass-energy of the cosmological gravitational-wave background, from limits on proper motions of quasars. Gravitational waves with periods longer than the time span of observations produce a simple pattern of apparent proper motions over the sky, composed primarily of second-order transverse vector spherical harmonics. A fit of such harmonics to measured motions yields a 95%-confidence limit on the mass-energy of gravitational waves with frequencies <2e-9 Hz, of <0.11/h*h times the closure density of the universe. 
Carl R. Gwinn, T. Marshall Eubanks, Ted Pyne, Mark Birkinshaw, and   Demetrios N. Matsakis