Controversial alternative gravity theory revived with new galactic spin

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Radio image of neutral hydrogen gas in the galaxy AGC 114905. Its tilt is estimated by a black oval, which fits the data best. If we assume that the galaxy is circular when viewed from the front, this means that it has a moderate inclination of 32 degrees. However, the new study suggests that a blue ellipse with a very low tilt may actually be true – thus saving the MOND theory – if the galaxy is intrinsically non-circular. The authors demonstrate that this is possible using a custom MOND simulation. Credit: Figure 7 by 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. Indranel Banik of the University of St Andrews School of Physics and Astronomy, research has revealed the expected high rotational speed of gas in a dwarf galaxy consistent with a 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 stated 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.

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

The group was d. Banek argues that the high spin speed predicted in MOND’s gravitational theory is consistent with observations if the galaxy’s tilt is 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 spectrophotometry. If the galaxy is viewed almost head-on, it will mostly rotate within the sky plane. This might lead observers to believe that the galaxy rotates very slowly, which would force them to overestimate the tilt between the cylinder and sky levels. This tilt was estimated from how the elliptical galaxy appeared (see image).

The new study explored this crucial question using a detailed MOND simulation of a similar disk galaxy AGC 114905 made at the University of Bonn by Srikanth Nagesh and instigated by Pavel Kroupa, a professor at the University of Bonn and Charles University in Prague. Simulations show that it can appear somewhat elliptical even when viewed from the front. This is because the stars and gases in the galaxy have gravity and can attract each other into a somewhat non-circular shape. A similar process causes spiral arms to appear in the disk of galaxies, very common features often called spiral galaxies.

As a result, the galaxy may 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.

doctor. Panic, lead author of the new study, said: “Our simulations show that the tilt of AGC 114905 could be much smaller than has been reported, which means that the galaxy is in fact rotating much faster than people think. With MOND predictions.”

doctor. 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 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 soft dark matter halo. Stars and gases contribute minimal gravity, so the disk is not “self-gravitating”.

This means that it would likely appear very circular if viewed from the front, as confirmed by simulations by another group (Selwood and Sanders, 2022). As a result, the observed ellipsoid must be due to a significant inclination between the disk and sky levels. The rotation speed will then be very small, which means that the galaxy has very little dark matter in it. It is not possible in this scenario for an isolated dwarf galaxy to contain a small amount of dark matter given its mass 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 has performed well in the tests performed so far, the standard approach causes very serious problems at all scales, from 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-Wide Galaxy AGC 114905” By Indranil Banik, Srikanth T Nagesh, Hosein Haghi, Pavel Kroupa, Hongsheng Zhao, Apr 19, 2022, Monthly notices from the Royal Astronomical Society.
DOI: 10.1093/mnras/stac1073

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