Testing Modified Gravity with Wide Binaries in GAIA DR2
  Several recent studies have shown that very wide binary stars can potentially
provide an interesting test for modified-gravity theories which attempt to
emulate dark matter; these systems should be almost Newtonian according to
standard dark-matter theories, while the predictions for MOND-like theories are
distinctly different, if the various observational issues can be overcome. Here
we explore an observational application of the test from the recent GAIA DR2
data release: we select a large sample of $\sim 24,000$ candidate wide binary
stars with distance $< 200$ parsec and magnitudes $G < 16$ from GAIA DR2, and
estimated component masses using a main-sequence mass-luminosity relation. We
then compare the frequency distribution of pairwise relative projected velocity
(relative to circular-orbit value) as a function of projected separation; these
distributions show a clear peak at a value close to Newtonian expectations,
along with a long `tail' which extends to much larger velocity ratios; the
`tail' is considerably more numerous than in control samples constructed from
DR2 with randomised positions, so its origin is unclear. Comparing the velocity
histograms with simulated data, we conclude that MOND-like theories without an
external field effect are strongly inconsistent with the observed data since
they predict a peak-shift in clear disagreement with the data; testing
MOND-like theories with an external field effect is not decisive at present,
but has good prospects to become decisive in future with improved modelling or
understanding of the high-velocity tail, and additional spectroscopic data.
