Modified Law of Gravity Predicts Speed of Dwarf Galaxies

An alternative hypothesis when it comes to the law of gravity may have more to it than meets the eye. Scientists have discovered that the modified law of gravity correctly predicted the velocity dispersion in 10 dwarf satellite galaxies of the Milky Way's giant neighbor Andromeda.

The velocity dispersion of stars is their average speed within a galaxy relative to one another. In the types of dwarf galaxies that were observed, there are usually relatively large velocity dispersions that are attributed to dark matter. Predictions made using this alternative hypothesis, called Modified Newtonian Dynamics (MOND), anticipated observations.

In order to test MOND, the researchers used the hypothesis on quasi-spherical, very low-surface brightness galaxies that are satellites of Andromeda. These are some of the smallest galaxies in the cosmos, containing only a few hundred thousand stars. Yet with conventional gravity, they are inferred to contain huge amounts of dark matter.

"Most scientists are more comfortable with the dark matter interpretation," said Stacy McGaugh, one of the researchers, in a news release. "But we need to understand why MOND succeeds with these predictions. We don't even know how to make this prediction with dark matter."

The MOND hypothesis states that Newton's force law must be tweaked at low acceleration--11 orders of magnitude lower than what we feel on the surface of Earth. Acceleration above that threshold is linearly proportional to the force of gravity. Below that threshold, though, it is not. At these small accelerations, the modified force law resolves the mass discrepancy.

The researchers also found subtle differences in the gravity fields of dwarfs near and far from the host galaxy Andromeda. The gravity fields far from the host appeared to be dominated by stars within the dwarf. The ones close to the host seem to be dominated by the host. Yet no such distinction is expected with dark matter.

The findings bolster the case of MOND's effectiveness in predicting properties in dwarf galaxies in a paper published earlier this year. The findings could also have implications for the future and allow astronomers to better understand the physics that govern our universe.

The findings are published in The Astrophysical Journal.