Astrometric Limits on the Stochastic Gravitational Wave Background

TL;DR

This paper develops a new astrometric method to detect the stochastic gravitational wave background by analyzing proper motions of extragalactic sources, setting new limits on gravitational wave energy density in very low frequency ranges.

Contribution

It introduces a novel astrometric detection technique for gravitational waves, providing the first limits in the frequency range bridging pulsar timing and CMB polarization regimes.

Findings

95% confidence limit on gravitational wave background: Ω_GW < 0.0064 with 711 sources

Combined Gaia and radio sources limit: Ω_GW < 0.011

Predicted Gaia-only limit: Ω_GW < 0.0006

Abstract

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 $6\times10^{-18}$ Hz $\lesssim f \lesssim 1\times10^{-9}$ 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.