As we move deeper into space and advance our understanding of it, more questions arise than answers about what makes our galaxies tick, shaking our models of the universe.
Now, after the study on the ultra-diffuse dwarf galaxy AGC 114905, an international group of astronomers, led by a physicist from the University of St Andrews, has revived the theory alternative gravity, known as Milgromian Dynamics or Modified Newtonian Dynamics (MOND), first published in 1983 by physicist Mordehai Milgrom.
The MOND theory is a controversial alternative to general relativity, which is the predominant interpretation of the phenomenon of gravity inspired by Einstein. The controversial MOND theory suggests that we don’t need dark matter to fill in the gravitational gaps in the universe and that, at very large distances, the gravitational force between two objects decreases as one with distance, rather than the square law inverse Newtonian measured empirically at smaller distances.
Thus, while general relativity requires dark matter to hold galaxies together, the MOND hypothesis does not requires dark matter.
Which theory is correct?
Because dark matter has never been detected, despite decades of highly sensitive searches, the appearance of MOND as hypothesis arrives among other theories to explain what holds galaxies together. Hence the debate about which is correct.
These types of theories are essential to understand our universe because, according to known physics, galaxies rotate so fast that they should fly away.
The new study, published in the journal Monthly Notices of the Royal Astronomical Society, and directed by Indranil Banik, from the St Andrews School of Physics and Astronomy, then argues that the rotation rate predicted in the MOND theory of gravity is consistent with observations if the tilt of the galaxy is overestimated.
Banik’s research focuses on whether gravity is modified at low accelerations and whether such modification can eliminate the need for invisible halos of dark matter around galaxies.
MOND theory dies to revive now
The new study comes after a previous one of 2021 on the speed of rotation of the gas in the dwarf galaxy AGC 114905 discovered that the gas rotated very slowly and, therefore, affirmed that the MOND theory was dead.
Now, Banik’s new paper has “disassembled” the previous finding, suggesting that the high rotation speed predicted in the MOND theory of gravity is consistent with the observations if the inclination of the galaxy is overestimated.
To test this idea, it is necessary to observe the velocities of galaxies – specifically the strange ones, such as ultra-diffuse galaxies, which usually do not act as a galaxy should–, but the rotation of stars and gas in distant galaxies cannot be measured directly.
According to the University of St Andrews press release, only the component a along the line of sight from precise spectroscopic measurements. If the galaxy is viewed almost head-on, it will rotate mainly in the plane of the sky. This could lead observers to think that the galaxy actually rotates very slowly, forcing them to overestimate the tilt between the planes of the disk and the sky. This tilt was estimated from the elliptical aspect of the galaxy.
The new study, the release continues, explored this crucial question using MOND simulations Detailed images of an AGC-like disk galaxy 114905 made at the University of Bonn. Simulations show that it can appear somewhat elliptical even when viewed from the front. This is because the stars and gas in the galaxy have gravity and can pull on themselves to give them a non-circular shape. A similar process causes spiral arms in disk galaxies, features so common they are often called spiral galaxies.
Galaxy is spinning much faster from what has been reported
As a result, the galaxy could be much closer to the face than observers thought , according to the statement. This could mean that the galaxy is spinning much faster than has been reported, removing the strain with MOND.
“Our simulations show that the AGC tilt 2021 could be significantly smaller than reported, which would mean that the galaxy is actually spinning much faster than people think, in line with MOND expectations,” said Banik, lead author of the new study.
“The very low reported rate of rotation of this galaxy is inconsistent with both MOND and the standard approach with dark matter. But only MOND is capable of overcoming this apparent contradiction,” said Hongsheng Zhao, from the School of Physics and Astronomy at the University of St Andrews.
Despite the new findings and studies, the question remains open, and perhaps for a long time. However, the MOND hypothesis seems to have made another comeback. It is only a matter of time before new advances prove the controversial theory right or wrong.
Edited by Felipe Espinosa Wang.