Modified law of gravity predicts average speed of stars in 10 satellite dwarf galaxies

Washington, August 29 : A modified law of gravity was correctly able to predict, before observations were made, the average speed of stars within a galaxy relative to each other in 10 dwarf satellite galaxies of Andromeda.

The relatively large velocity dispersions observed in these types of dwarf galaxies is usually attributed to dark matter. Yet predictions made using the alternative hypothesis Modified Newtonian Dynamics (MOND) succeeded in anticipating the observations.

The researchers tested MOND on quasi-spherical, very low-surface brightness galaxies that are satellites of Andromeda.

Stacy McGaugh, professor of astronomy at Case Western Reserve, and Mordehai Milgrom, the father of MOND and professor of physics at the Weizmann Institute in Israel, said that most scientists are more comfortable with the dark matter interpretation, asserting that they 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 says that Newton's force law must be tweaked at low acceleration - 11 orders of magnitude lower than what we feel on the surface of the Earth. Acceleration above that threshold is linearly proportional to the force of gravity - as Newton's law says - but below the threshold, no. At these tiny accelerations, the modified force law resolves the mass discrepancy.

The paper's calculations using MOND also reveal subtle differences in the gravity fields of dwarfs near and far from the host galaxy Andromeda.

The gravity fields of dwarfs far from the host appear to be dominated by stars within the dwarf, while the gravity fields of dwarfs close to the host appear to be dominated by the host. No such distinction is expected with dark matter.

The study is set to be published online in the Astrophysical Journal.

--ANI (Posted on 29-08-2013)

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