The canonical methods for gravitational wave detection are ground- and space-based laser interferometry , pulsar timing , and polarization of the cosmic microwave background .
But as has been suggested by numerous investigators , astrometry offers an additional path to gravitational wave detection .
Gravitational waves deflect light rays of extragalactic objects , creating apparent proper motions in a quadrupolar ( and higher-order modes ) pattern .
Astrometry of extragalactic radio sources is sensitive to gravitational waves with frequencies between roughly 10 ^ { -18 } and 10 ^ { -8 } Hz ( H _ { 0 } and 1/3 yr ^ { -1 } ) , overlapping and bridging the pulsar timing and CMB polarization regimes .
We present a methodology for astrometric gravitational wave detection in the presence of large intrinsic uncorrelated proper motions ( i.e. , radio jets ) .
We obtain 95 % confidence limits on the stochastic gravitational wave background using 711 radio sources , \Omega _ { GW } < 0.0064 , and using 508 radio sources combined with the first Gaia data release : \Omega _ { GW } < 0.011 .
These limits probe gravitational wave frequencies 6 \times 10 ^ { -18 } Hz \lesssim f \lesssim 1 \times 10 ^ { -9 } Hz .
Using a WISE - Gaia catalog of 567,721 AGN , we predict a limit expected from Gaia alone of \Omega _ { GW } < 0.0006 , which is significantly higher than was originally forecast .
Incidentally , we detect and report on 22 new examples of optical superluminal motion with redshifts 0.13–3.89 .