Reviving the controversial theory of alternative gravity with a new spin in the galactic rotation

Radio image of neutral hydrogen gas in the galaxy AGC 114905. Its tilt is estimated by a black ellipse, which fits the data best. Assuming that the galaxy is circular when viewed head-on, this means that it has a moderate inclination of 32 degrees. However, the new study suggests that a blue ellipse of a very low inclination could indeed be true – thus saving the MOND theory – if the galaxy is intrinsically somewhat non-circular. The authors demonstrate that this is possible using a custom MOND simulation. Credit: Figure 7 from Mancera-Pina et al. 2022 (MNRAS, 512, 3230)

An international group of astronomers, led by a physicist at the University of St Andrews, has revived an alternative theory of gravity.

Led by Dr. Indranil Banik from the University of St Andrews School of Physics and Astronomy, the research revealed the high expected rotational velocity of gas in dwarf galaxy In accordance with the previously debunked theory known as Milgromian Dynamics (MOND).

A previous study of the rotational velocity of gas in the dwarf galaxy AGC 114905 (Mancera Pena et al., 2022) found that the gas rotates very slowly and thus claimed that the MOND theory is dead.

Such theories are essential to understanding our universe because, according to known physics, galaxies rotate so fast that they must diverge from each other. MOND is a controversial alternative to general relativity, the dominant Einsteinian-inspired understanding of the phenomenon of gravitation. However, general relativity requires dark matter to hold galaxies together, while MOND does not dark matter.

Since dark matter has never been discovered despite decades of highly sensitive searches, various theories have been put forward to explain what binds galaxies together. Debate rages over the correct theory. The extremely low spin speed reported in the Mancera Pina et al study runs counter to expectations in a world governed by general relativity with large amounts of dark matter.

Dr. Banek’s group argues that the high spin speed predicted in MOND’s gravitational theory is consistent with observations if tilt galaxy Exaggerated.

The rotation of stars and gas in distant galaxies cannot be measured directly. Only the component along the line of sight is known by precise spectroscopic measurements. If the galaxy is seen almost head-on, it will mostly orbit within the plane of the sky. This could mislead observers into thinking that the galaxy rotates very slowly, which would require them to overestimate the tilt between the cylinder and the sky planes. This tilt was estimated from how the elliptical galaxy appears (see image).

The new study explored this crucial issue using a detailed MOND simulation of a similar disk galaxy AGC 114905 Made at the University of Bonn by Srikanth Nagesh and at the instigation of Pavel Kroupa, Professor at the University of Bonn and Charles University in Prague. Simulations show that it can appear somewhat elliptical even when viewed head-on. This is because the stars and gases in the galaxy have gravity and can pull themselves into a somewhat non-circular shape. A similar process causes spiral arms to appear in disc galaxies, features so common that these are often called spiral galaxies.

As a result, the galaxy could be closer to the confrontation than observers think. This could mean that the galaxy is spinning much faster than has been reported, removing the tension with MOND.

Dr Banek, lead author of the new study said: “Our simulations show that the tilt of AGC 114905 may be much lower than what has been reported, which means that the galaxy is in fact rotating much faster than people think, in line with MOND predictions.”

Dr Hongsheng Zhao, from the University of St Andrews’ School of Physics and Astronomy, said: “The reported extremely low spin speed of this galaxy is incompatible with both MOND and the standard approach with dark matter. But only MOND is able to overcome this apparent contradiction.”

The new study also argues that a similar “pseudo-tilt” effect is unlikely to appear in the standard dark matter approach because the galaxy is dominated by a smooth dark matter halo. Stars and gases contribute minimally to gravity, so the disk is not “self-gravitating.”

This means that it would likely appear very circular if viewed head-on, as confirmed by simulations by another group (Selwood and Sanders, 2022). As a result, the observed ellipsoid must be due to a significant tilt between the disk and sky levels. The speed of rotation will then be very small, which means that the galaxy has very little dark matter. It is not possible in this setting for an isolated dwarf galaxy to contain a small amount of dark matter given the amount of mass it has in stars and gas.

Pavel Krupa, a professor at the University of Bonn and Charles University in Prague, said of the broader context of these findings: “While MOND works well in tests carried out so far, the standard approach causes very serious problems on all scales ranging from dwarf galaxies such as AGC 114905 down to cosmic scales, as found by many independent teams.”

Reference: “The Exaggerated Inclinations of Milgromian Disc Galaxies: The Case of the Ultra-widening Galaxy AGC 114905” By Indranil Banik, Srikanth T Nagesh, Hosein Haghi, Pavel Kroupa, Hongsheng Zhao, Apr 19, 2022, Monthly Notices of the Royal Astronomical Society.
DOI: 10.1093/mnras/stac1073