Abstract
Assuming Newton's gravity and General Relativity (GR) to be valid at all scales leads to the dark matter hypothesis as a requirement demanded by the observed dynamics and measured baryonic content at galactic and extragalactic scales. Alternatively, modified gravity scenarios where a change of regime appears at acceleration scales a<a_0 have been proposed. This modified regime at a<a_0 will generically be characterised by equilibrium velocities which become independent of distance. Here we identify a critical test in this debate and we propose its application to samples of wide binary stars. Since for 1 M_⊙ systems the acceleration drops below a_0 at scales of around 7000 au, a statistical survey of wide binaries with relative velocities and separations reaching 10^4 au and beyond should prove useful to the above debate. We apply the proposed test to the best currently available data. Results show a constant upper limit to the relative velocities in wide binaries which is independent of separation for over three orders of magnitude, in analogy with galactic flat rotation curves in the same a<a_0 acceleration regime. Our results are suggestive of a breakdown of classical gravity beyond a ≈ a_0 scales, in accordance with generic predictions of modified gravity theories designed not to require any dark matter at galactic scales and beyond.
