A review on success and problem of MOND on globular cluster scale

TL;DR

This review examines the successes and challenges of MOND at the scale of globular clusters, highlighting potential problems with tidal truncation predictions and their implications for MOND-like theories.

Contribution

It provides a critical analysis of MOND's predictions for globular clusters and discusses potential inconsistencies with observed truncation sizes, offering tests for MOND-like theories.

Findings

MOND favors galaxy velocity curves over dark matter.

Tidal truncation sizes of globular clusters are predicted to be similar to dwarf galaxies.

Observed globular sizes are smaller than predicted, challenging MOND.

Abstract

Many past attempts to kill MOND have only strengthened the theory. Better data on galaxy velocity curves clearly favor MOND (without fine-tuning) over cold dark matter. The usual critism on the incompleteness of classical MOND has spurred a Modified Relativity (MR) by Bekenstein. After outlining cosmology and lensing in MOND, we review MOND on small scales. We point out some potential problems of MOND in two-body relaxation and tidal truncation. We argue that the tidal field in any MOND-like gravity theory predicts that the Roche lobe sizes of a binary system are simply proportional to the binary baryonic mass ratio to the power 1/3. An immediate application of this result is that the tidal field and tidal truncation radii of million-star globular clusters and million-star dwarf galaxies (e.g., the Milky Way satellites NGC2419 and Carina) would be very similar because of the one-to-one relation between gravity and baryon distribution. This prediction appears, however, inconsistent with the fact that {\it all} globulars are truncated to much smaller sizes than {\it all} dwarf galaxies. Whether tide is uniquely determined by baryons can also be used to falsify any MOND-like gravity theory, whether classical or relativistic.