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 \nu < 2 \times 10 ^ { -9 } Hz , of < 0.11 h ^ { -2 } times the closure density of the universe .