Is gravity wrong?

NGC 1300

Comparison of galaxies' mass estimates and their spin rates pose challenges for the standard theory of gravity. A newer theory called MOND aims to solve the riddle.

A STUDY OF THE SPIN RATES of gas-rich galaxies supports an alternate theory of gravity known as MOND, according to work by University of Maryland Astronomy Professor Stacy McGaugh.

This latest of several successful MOND predictions, raises new questions about the accuracy of the reigning cosmological model of the universe, writes McGaugh in a paper to be published in March in the journal Physical Review Letters.

Modern cosmology says that for the universe to behave as it does, the mass-energy balance of the universe must be dominated by dark matter and dark energy. However, direct evidence for the existence of these invisible components remains lacking.

An alternate, though unpopular, possibility is that the current theory of gravity does not adequately describe the dynamics of cosmic systems.

Enter MOND

A few concepts that would modify our understanding of gravity have been proposed. One of these is Modified Newtonian Dynamics (MOND), which was hypothesised in 1983 by Moti Milgrom, a physicist at the Weizmann Institute of Science in Rehovot, Israel.

One of MOND’s predictions specifies the relationship between the overall mass of a galaxy and its rotation velocity. However, uncertainties in estimating the masses of star-dominated spiral galaxies (such as our own Milky Way) previously had prevented a definitive test.

Galaxies UGC 2885 and F549-1

The star-dominated spiral galaxy UGC 2885 (left) and the gas-rich galaxy F549-1 (right). A new study of the spin rates of gas-rich galaxies supports an alternate theory of gravity. Images by Zagursky and McGaugh.

To overcome this problem, McGaugh instead examined gas-rich galaxies, which have fewer stars and a lot more mass in the form of interstellar gas. He says it is easier to gauge the mass of gas than of stars.

McGaugh compiled a sample of 47 gas-rich galaxies and compared each one’s mass and rotation velocity with the relationship expected by MOND.

All 47 galaxies fell on or very close to the MOND prediction.

By comparison, no dark matter model performed as well.

MOND vs the rest

Almost everyone agrees that on scales of large galaxy clusters and up, the Universe is well described by the dark matter-dark energy theory. However, according to McGaugh, it does not account well for what happens at the scales of individual galaxies and smaller.

“MOND is just the opposite,” he said. “It accounts well for the ‘small’ scale of individual galaxies, but MOND doesn’t tell you much about the larger universe.”

“If we’re right about dark matter, why does MOND work at all?” asks McGaugh. “Ultimately, the correct theory—be it dark matter or a modification of gravity—needs to explain this.”

Adapted from information issued by University of Maryland. Galaxy image courtesy NASA / ESA / HHT / STScI / AURA / Hubble Collaboration.

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