Wide Binaries from GAIA EDR3: preference for GR over MOND ?
  Several recent studies have shown that velocity differences of very wide
binary stars, measured to high precision with GAIA, can potentially provide an
interesting test for modified-gravity theories which attempt to emulate dark
matter. These systems should be entirely 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 provide an updated version of our 2019 study using the recent GAIA EDR3 data
release: we select a large sample of 73 159 candidate wide binary stars with
distance <300 parsec and magnitudes G<17 from GAIA EDR3, and estimate component
masses using a main-sequence mass-luminosity relation. We then examine the
frequency distribution of pairwise relative projected velocity (relative to
circular-orbit value) as a function of projected separation, compared to
simulations; as before, 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 and may well be caused by hierarchical triple systems
with an unresolved or unseen third star. We then fit these observed
distributions with a simulated mixture of binary, triple and flyby populations,
for GR or MOND orbits, and find that standard gravity is somewhat preferred
over one specific implementation of MOND; though we have not yet explored the
full parameter space of triple population models and MOND versions. Improved
data from future GAIA releases, and followup of a subset of systems to better
characterise the triple population, should allow wide binaries to become a
decisive test of GR vs MOND in the future.
