Modified Gravity Theory Predicts Galaxy Dynamics
August 29, 2013

Modified Theory Of Gravity Predicts Galactic Dynamics, Challenges Dark Matter

John P. Millis, PhD for - Your Universe Online

For decades astronomers have studied the motions of galaxies – their rotations about their axes, as well as their interactions with each other – and been puzzled by what they have observed. The data suggests one of two possibilities: either these galaxies, and the space between them, contain significantly more mass than what is seen, or our physical understanding of gravity is flawed.

The oft-discussed solution is that there is some form of matter that dominates the Universe that does not interact electromagnetically, hence its name: dark matter. However, there are those who argue that, despite the growing amount of evidence that dark matter is real, perhaps a better explanation is that our laws of gravity are incomplete.

Our best mathematical expression of gravity remains Einstein’s theory of general relativity. And while attempts have been made to add extra scalar fields and other metrics to Einstein’s equations to account for both dark matter as well as dark energy, these modifications add complexity to an already difficult set of equations to work with. Furthermore, much of that work has lacked conclusive predictions that can be tested with current technology (though that, too, may be changing).

Luckily, in many systems – the surface of Earth being a prime example – the cumbersome equations that define general relativity reduce to a simpler set of expressions that date back hundreds of years. Newtonian Mechanics, developed by their namesake Sir Isaac Newton, define the simple motion of objects in gravitational fields, so long as they are approximately uniform and not so strong as to introduce relativistic effects.

With this starting point, researchers have then been looking to develop a Modified Newtonian Dynamics (MOND) theory that would explain the motions of galaxies without the need for dark matter. When applied to systems that should roughly obey Newtonian motion (i.e. where the effects of relativity are minimal) such a theory should be able to predict the motions.

If shown to be successful, the underlying concepts could then be expanded to include relativistic effects, providing an important stepping stone in exploring which avenue of research – modified gravity or dark matter – is the more likely solution to the problem of galactic dynamics.

To investigate, Stacy McGaugh, professor of astronomy at Case Western Reserve, and Mordehai Milgrom, professor of physics at the Weizmann Institute in Israel and one of the originators of MOND, set out to establish predictions of the galactic motions of 10 dwarf galaxies in our local group. These dwarf spheroidal galaxies – systems similar to normal elliptical galaxies, but containing far fewer stars – are also popular targets for astronomers looking for dark matter interactions.

However, the predictions made in their paper, which will appear in an upcoming edition of the Astrophysical Journal, may suggest that the motions of the galaxies would be decidedly different than those anticipated by dark matter dominated galaxies.

In their theory, Newtonian mechanics has a small correction when acceleration is low – about 100 billionth the magnitude of the surface gravity on Earth. As a result, we would be virtually unable to probe such deviations on Earth, but in the gravitational interactions between these dwarf galaxies and the hosts they orbit – in this case, the Andromeda galaxy – there are measurable effects.

"The influence of the host galaxy may provide a test to distinguish between dark matter and MOND," McGaugh says. "Dark matter provides a cocoon for the dwarfs, protecting the stars from tidal influence by the host galaxy. With MOND, the influence of the host is more pronounced."

So far, their model has proven successful in predicting the velocity dispersions observed in similar dwarf galaxies. Further observations will be undertaken to test the theory, but currently the model put forth by McGaugh and Milgrom is gaining traction. But for now, only time will tell if MOND will emerge as a serious challenger to dark matter theory.