MOND-like Behavior Within The Milky Way In Milky Way Subsystems

The radial acceleration relation and baryonic Tully-Fischer relation, while not perfect, work far too well to be consistent with almost any dark matter particle theories (ultra-light bosonic dark matter still might be possible to make work).

We test whether parsec-scale stellar systems in the Milky Way follow the galactic radial acceleration relation (RAR) or the baryonic TullyFisher relation (BTFR). 

We analyse 5646 Gaia DR3 open clusters from the Hunt & Reffert catalogue. Observed accelerations are derived from velocity dispersions and characteristic radii, and baryonic accelerations from stellar masses and characterisitc radii. The clusters are placed on the RAR and BTFR planes and compared with Newtonian and MOND expectations. Approximately 90 per cent of open clusters (those with N⋆≤250) lie close to the RAR, albeit with significant scatter. In a first-of-its-kind test, a smaller fiducial sample is consistent with a best-fitting acceleration scale g†≈1.2×10−10 ms−2±0.5 dex, compatible with canonical MOND values. 

More massive clusters approach the Newtonian virial expectation. No correlations are found between RAR residuals and galactocentric radii, distance to the Galactic disk midplane, age, or morphology. Tidal effects and unresolved binaries are insufficient to reproduce the observations without fine-tuning. 

Interpreted within a MOND framework, the alignment of most open clusters with the RAR and BTFR suggests that low-acceleration dynamics operate on parsec scales within the Milky Way. This implies that the Galactic gravitational field is not smooth on these scales and may include regions where the total gravitational acceleration falls below a0, partially mitigating the external field effect, thereby motivating higher-resolution modelling of the Galactic potential and informing other small-scale gravity tests within the Galaxy.

Mark D. Huisjes, X. Hernandez, "Most open clusters follow the radial acceleration relation (RAR) and the baryonic Tully-Fisher relation (BTFR)" arXiv:2603.03522 (March 3, 2026).