In this episode I’m digging into a recent announcement from the astronomy community. The Gaia satellite has been making high precision measurements of the position and velocity of billions of nearby stars. Researchers have used this data to test the theory of gravity, and more specifically Modified Newtonian Dynamics or MOND. This theory was developed to explain away the need for dark matter. Dark matter was originally postulated to account for the anomalous rotation curves of all galaxies. Basically, if you use their surface brightness as a proxy for their matter distribution you would expect the outer regions of the galaxy to be moving slower than they seem to be. MOND suggests that gravitational acceleration stops dropping off as fast when the acceleration drops below a particular threshold value.

Researchers analyzed the rotation curves of 10s of thousands of widely separated binary stars in the Gaia dataset and they believe that the data support MOND over Newton, and their results seem to provide a high degree of confidence. The press coverage seems to suggest that this is a smoking gun that decisively overthrows Newtonian gravity and General Relativity all at once. If this is true it is huge news. Why are the authors not packing their bags to Sweden as we speak?

With a little digging, I’ve found that other researchers looking at the same Gaia dataset have come to the opposite conclusion that the data supports General Relativity over MOND. What gives? To understand this discrepancy I’m interviewing a skeptical astronomer, Dr. Indranil Banik.

Dr Indranil Banik did his undergraduate at Trinity College, Cambridge before moving to Saint Andrews for his PhD with Hongsheng Zhao, where they worked together on Milgromian dynamics (MOND). Dr Banik then went to Bonn for a Humboldt fellowship with Pavel Kroupa. He returned to Saint Andrews in 2021 and spent his first year back on a detailed 150 page invited review on all aspects of MOND. He has been working on the wide binary test since late 2017. He organised the fortieth anniversary celebration of MOND conference in Saint Andrews in summer 2023. Among other results presented here, conflicting results were presented on the wide binary test and there was intense controversy surrounding whether the results indicate a failure of Newtonian gravity at low accelerations.

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