Four ways of looking at the Radial Acceleration Relation

The radial acceleration relation (RAR) is a strong correlation of late-type galaxy dynamics, seeming to describe a mysterious link between baryonic and total dynamical mass. Its properties are so striking that it has even been argued to manifest a new law of gravity in galaxies, the so-called Modified Newtonian Dynamics (MOND). I will describe four recent works analysing the RAR in novel ways. The first (arXiv:2303.11314) marginalises over the galaxy parameters upon which the RAR variables depend, revealing an extremely small intrinsic scatter of 0.034 dex. The second (arXiv:2301.04368) applies a technique called symbolic regression to generate and score all possible functional forms for the RAR, suggesting the data is insufficient to infer Newtonian and deep-MOND limits with statistical confidence. The third (arXiv:2305.19978) uses machine learning to present evidence that the RAR is fundamental: it has no residual correlations, is the tightest projection of galaxies' dynamical parameter space and is uniquely capable of explaining all other dynamical correlations. The last (arXiv:2401.04796) shows that under a “classic” MOND interpretation, the RAR is inconsistent with planetary ephemerides and the kinematics of local wide binaries. I close with an abortive attempt to tie this all together.