By Rowenna Hoskin | Science Editor
New research has been published by an international group of scientists which contradicts the popular theory of dark matter. Scientists, including Case Western Reserve, University Astronomy Chair Stacy McGaugh, have proposed a theory which would be applied to describe the galactic rotational velocity paradigm.
The theory of dark matter arose due to the discrepancies between the observable mass and the rotational velocity of the stars in the galaxy. Galactic velocity curves displayed the outer stars moving quicker than expected, implying that there was more mass in the galaxy than the visible, baryonic matter. To explain this, scientists hypothesised that galaxies contain a large mass of dark matter which is undetectable as it does not interact with visible matter.
This new research builds upon the theorised modified Newtonian dynamics (MOND) hypothesis as a viable explanation for the difference in the expected and observed velocity. The theory was introduced by the physicist Modehai Milgrom from Weizmann Institute (Israel) in the early 1980s and states that the rules of gravity are slightly altered.
Rather than the excess gravitational pull being attributed to an undetectable mass, MOND attempts to explain the fact that gravity at low accelerations is much stronger than would have been expected under Newtonian mechanics. This means that at the edge of the galaxy, the velocity of stars orbiting the core does not coincide with the predicted velocity when considering the observable mass.
MOND explains the intrinsic motions of the stars to depend on the mass of the host galaxy and the gravitational pull from other masses surrounding the galaxy. This theory is called “the external field effect” (EFE); which allows the environment surrounding the galaxy to factor into the rotational velocity of the stars.
Milgrom said that the data, if confirmed, would be “the smoking gun proving that galaxies are governed by modified dynamic rather than obeying the laws of Newton and of general relativity.”
McGaugh and his collaborators, led by Kyu-Hyun Chae, from Sejong University in South Korea, say they detected this EFE in more than 150 of the galaxies studied.
“The external field effect is a unique signature of MOND that does not occur in Newton-Einstein gravity,” McGaugh said. “This has no analogy in conventional theory with dark matter. Detection of this effect is a real head scratcher.”
The fact that this research favours MOND as opposed to the theory of dark matter shocked many of the scientists on the project as they had been working under the hypothesis that dark matter exists.
The group analysed 153 rotation curves of disk galaxies as a part of their study. These were selected from the Spitzer Photometry and Accurate Rotation Curves (SPARC) database, created by another collaborator, Federico Lelli from Case Western Reserve, Mcgaugh and co-author James Schombert, of the University of Oregon.
The EFE was deduced by observing that galaxies in stronger external fields slowed more frequently than galaxies with weaker external fields. This means that if the galaxy is surrounded by other galaxies, the stars within the galaxy had higher velocities than those in galaxies in a lower density environment. The mass of these external galaxies directly influences the velocity of the stars.
This research will come as a disappointment to many astronomers, as it challenges the popularised dark matter theory which scientific institutions have invested so much time and money into detecting. It provides new context to the field of astronomy and will influence future scientific projects – potentially quenching the debate that has lasted for more than half a century.
Science and Technology Rowenna Hoskin