The Andromeda galaxy
The Andromeda galaxy. Prof. Mordehai Milgrom’s theory involves revising Newton’s second law of motion. Photo by Reuters
Text size

The central paradigm of modern cosmology holds that 20% of the universe consists of dark matter - invisible matter whose particles have yet to be discovered since they have virtually no interaction with known compounds. However, while such dark matter is evasive, astrophysicists have solid reason to assume that it exists. A new study by a U.S. astrophysicist gives new weight to an Israeli researcher's controversial theory that dismisses the existence of dark matter.

Dark matter first became a part of modern astrophysics following new measurements taken of galaxy orbit times in the late 1970s. Astrophysicists had assumed that stars on the edge of galaxies moved at decreasing rates the farther they were from the center; yet it turned out that at a certain stage, the orbit speeds of such stars becomes fixed. This phenomenon did not accord with known laws of physics; and so scientists assumed there was additional, unseen mass in such galaxies; this additional mass, researchers hypothesized, was dark matter.

In the intervening years, dark matter has received additional theoretical roles in cosmology models. Though dark matter fills in missing theoretical gaps, scientists have searched for direct proof of its existence with detectors constructed around the world.

Prof. Mordehai Milgrom of the Weizmann Institute of Science, together with a small group of supporters, has for 30 years been developing a theory that eliminates the need for dark matter, at least in galaxies. The catch is that accepting Milgrom's alternative theory entails a price that most astrophysicists regard as too high, in theoretical terms: making a small revision in Newton's second law of motion. As Milgrom sees it, making this small revision allows scientists to view the phenomenon of galactic orbits with impressive precision, without the need to invent invisible elements, such as dark matter, and add them to the cosmological mix.

Milgrom's theory - Modified Newtonian Dynamics, or Mond - while no longer ignored, is still shunned by the research mainstream.

Mond supporters claim that a study published this week reinforces the Mond hypothesis regarding the connection between galactic orbit speeds and galactic mass. Aspects of this relation, known as the Tully-Fisher Relation, have long been measured with seemingly impressive precision by the Mond model; the problem is that all calculations depend on very rough estimates of galactic mass.

In order to verify Mond's accuracy regarding the Tully-Fisher relation, better galactic mass estimates have been needed. Astronomy researcher Stacy McGaugh from the University of Maryland has provided tools for galactic mass measurement. McGaugh's research focuses on 47 galaxies whose main mass base is gases, rather than stars. Measurements in such galaxies is far more precise, since gas mass can be estimated with some accuracy. And tallies and graphs that have emerged from McGaugh's research paints a picture of the Tully-Fisher Relation that accords with the Mond model. Prof. Avishai Dekel, an astrophysicist and strong opponent of Mond, claims that McGaugh's research is not innovative, saying the McGaugh findings "are just part of a gradual improvement in the precision of observations that is occurring today." Dekel claims that dark matter theory can account for ratios between galactic speed and galactic mass once various interactions involving gas compounds are taken into account.

Yet supporters of the mainstream dark-matter theory have to deal with the fact that measurements resulting from McGaugh's research points to results that are predicted almost exactly by the Mond model. Dekel finds this suspicious. "Of hundreds or thousands of observation projects that measure galactic mass, only McGaugh has come up with these findings," Dekel objects. "It seems that he is exaggerating his findings. I really don't know." In contrast, Milgrom is very happy about the new research results that have emerged from the McGaugh study. "This research is very significant, though it won't cause dark matter theorists to believe in Mond," says Milgrom. "But this new research might influence people who are sitting on the fence."