18 February 2020Testing weak-field gravity with wide binary stars

Binary stars separated by >3 kAU have orbital accelerations smaller than those typically experienced by stars in the outskirts of galaxies, whose galactocentric motions evince sometimes very large acceleration discrepancies of unknown origin (the problem of flat rotation curves). These wide binaries (WBs) thus have the potential to strongly constrain whether the discrepancies arise from a fundamental low-acceleration deviation of gravity from Newtonian expectations, a hypothesis called Modified Newtonian Dynamics (MOND). This is because WBs are too small to have dynamically significant quantities of dark matter, the conventional explanation for the above-mentioned discrepancies. In this talk, I will introduce MOND and explain what it predicts for the relative velocity distribution of WBs, focusing on how this differs from Newtonian expectations (MNRAS, 480, 2660). I will then consider the feasibility of detecting this difference with Gaia data, focusing on the choice of which relative velocity component(s) to consider and the trade-offs involved (MNRAS, stz1551). I will briefly touch on how contamination can be handled statistically and how theoretical calculations can be used to minimise its effect. A modest amount of ground-based follow-up could also help by obtaining radial velocities accurate to a few km/s for several hundred stars, though this is not strictly necessary for the proposed test.