Strong constraints on weak gravity from Gaia DR3 wide binaries
Channel: Indranil Banik
Date: 15/05/2023

Testing Milgromian dynamics (MOND) with widely separated binary stars in the Solar neighbourhood, which due to their low accelerations should have their mutual gravity and thus their orbital velocity enhanced by 20% in MOND compared to Newtonian expectations. A priori protocol used as much as possible to mitigate possible bias in conducting such a critical test:
https://arxiv.org/abs/2109.03827

I present a detailed test of MOND using wide binary stars (WBs) with separations of 2-30 kAU. Locally, the WB orbital velocity in MOND should exceed the Newtonian prediction by about 20% at asymptotically large separations given the Galactic external field effect (EFE; MNRAS, 480, 2660). I investigate this with a detailed statistical analysis of Gaia DR3 data on 10290 WBs within 300 pc of the Sun. Orbits are integrated in a rigorously calculated gravitational field that directly includes the EFE. I also allow line of sight contamination and undetected close binary companions to the stars in each WB. I interpolate between the Newtonian and Milgromian predictions using the parameter alpha_grav, with 0 indicating Newtonian gravity and 1 indicating MOND. Directly comparing the best Newtonian and Milgromian models reveals that Newtonian dynamics is preferred at 18 sigma confidence. Using a complementary Markov Chain Monte Carlo analysis, I find that alpha_grav = -0.036+0.064-0.055, which is fully consistent with Newtonian gravity but excludes MOND at 16 sigma confidence. This is in line with the similar result of Pittordis and Sutherland (https://astrobites.org/2022/07/23/d) using a somewhat different sample selection and less thoroughly explored population model. I show that although the best-fitting model does not fully reproduce the observations, an overwhelmingly strong preference for Newtonian gravity remains in a considerable range of variations to my analysis. Adapting the MOND interpolating function to explain this result would cause tension with rotation curve constraints. I discuss the broader implications of these results in light of other works, concluding that MOND must be substantially modified on small scales to account for local WBs.

Slides are available here:
https://www.dropbox.com/s/2r3xdqhoeddz8y7/Saint_Andrews_WBT.pdf?dl=0

The recently accepted paper on this result is available here:
https://dx.doi.org/10.1093/mnras/stad3393