More Evidence For MOND Or Something Like It

The paradigm in astrophysics and cosmology is that in weak fields, general relativity can be approximated with minimal loss of accuracy with Newtonian gravity and dark matter. 

The trouble is that in circumstances where this is distinguishable from MOND, a simple phenomenological gravitational modification, what we see can be expressed entirely as a function of the distribution of ordinary matter. It has made many ex ante predictions that have been supported by later observations, while dark matter particle theories have generally failed to do so. And, where there is a distinction to be made, MOND tends to fit observations better than dark matter subject to Newtonian gravity at galactic scales. MOND underestimates the magnitude of "dark matter phenomena" in some galactic cluster contexts, but a new paper published in MNRAS shows MOND outperforming the paradigm even in open clusters.

After their birth a significant fraction of all stars pass through the tidal threshold (prah) of their cluster of origin into the classical tidal tails. The asymmetry between the number of stars in the leading and trailing tails tests gravitational theory. All five open clusters with tail data (Hyades, Praesepe, Coma Berenices, COIN-Gaia 13, NGC 752) have visibly more stars within dcl = 50 pc of their centre in their leading than their trailing tail. Using the Jerabkova-compact-convergent-point (CCP) method, the extended tails have been mapped out for four nearby 600-2000 Myr old open clusters to dcl>50 pc. These are on near-circular Galactocentric orbits, a formula for estimating the orbital eccentricity of an open cluster being derived. 

Applying the Phantom of Ramses code to this problem, in Newtonian gravitation the tails are near-symmetrical. In Milgromian dynamics (MOND) the asymmetry reaches the observed values for 50 < dcl/pc < 200, being maximal near peri-galacticon, and can slightly invert near apo-galacticon, and the Küpper epicyclic overdensities are asymmetrically spaced. Clusters on circular orbits develop orbital eccentricity due to the asymmetrical spill-out, therewith spinning up opposite to their orbital angular momentum. 

This positive dynamical feedback suggests Milgromian open clusters to demise rapidly as their orbital eccentricity keeps increasing. Future work is necessary to better delineate the tidal tails around open clusters of different ages and to develop a Milgromian direct n-body code.

Pavel Kroupa, et al. (17 authors), "Asymmetrical tidal tails of open star clusters: stars crossing their cluster's prah challenge Newtonian gravitation" arXiv:2210.13472 (October 24, 2022) (published in MNRAS).

Progress is also being made on MOND in galaxy clusters. It suggests that some of the deficit in the magnitude of dark matter phenomena predicted by MOND in clusters is due to poor modeling of the distribution of the ordinary matter in the clusters.

A specific modification of Newtonian dynamics known as MOND has been shown to reproduce the dynamics of most astrophysical systems at different scales without invoking non-baryonic dark matter (DM). There is, however, a long-standing unsolved problem when MOND is applied to rich clusters of galaxies in the form of a deficit (by a factor around two) of predicted dynamical mass derived from the virial theorem with respect to observations. 

In this article we approach the virial theorem using the velocity dispersion of cluster members along the line of sight rather than using the cluster temperature from X-ray data and hydrostatic equilibrium. Analytical calculations of the virial theorem in clusters for Newtonian gravity+DM and MOND are developed, applying pressure (surface) corrections for non-closed systems. Recent calibrations of DM profiles, baryonic ratio and baryonic (β model or others) profiles are used, while allowing free parameters to range within the observational constraints. It is shown that solutions exist for MOND in clusters that give similar results to Newton+DM -- particularly in the case of an isothermal β model for β=0.55−0.70 and core radii rc between 0.1 and 0.3 times r(500) (in agreement with the known data). 

The disagreements found in previous studies seem to be due to the lack of pressure corrections (based on inappropriate hydrostatic equilibrium assumptions) and/or inappropriate parameters for the baryonic matter profiles.

M. Lopez-Corredoira, et al., "Virial theorem in clusters of galaxies with MOND" arXiv:2210.13961 (October 25, 2022) (accepted for publication in MNRAS).