February 26, 2011
Findings Raise New Questions About Dark Matter
A new study, published in Physical Review Letters and available online, offers that a controversial theory which challenges the existence of dark matter has been kept afloat by studies of gas-rich galaxies.
Instead of calling upon dark matter, the Modified Newtonian Dynamics (MoND) theory says that the effects of gravity change in places where its pull is very low.
A new paper suggests that MoND better predicts the relationship between gassy galaxies' rotation speeds and masses.
However, critics maintain that dark matter theory paints a pretty good picture of the Universe we see.
Standard formulations of gravity state that matter circling spiral galaxies, for instance, should rotate more slowly the further it reaches from the center of the galaxy -- much like the outer planets in our Solar System orbit the Sun more slowly than the innermost planets.
But the matter in spiraling galaxies seems to be consistently rotating at roughly equal speeds near their cores and at their edges.
In standard dark matter theory, scientists propose a massive yet invisible quantity of material is needed in order to solve this "flat rotation curve" problem.
This dark matter is thought to exist in a "halo" around galaxies, giving off the extra gravitational pull that is necessary to speed up outlying bodies.
MoND was first brought up in 1983, when Mordehai Milgrom of the Weizmann Institute in Israel proposed it in an Astrophysical Journal article.
The theory immediately came under fire because of its modifications of the formulation of gravity that had been well-established by Isaac Newton. The theory has always maintained a minority position among theories proposed to solve the missing mass issue.
Stacy McGaugh, of the University of Maryland, says that a recent study of galaxies that have relatively few stars and are dominated by gas adds weight to the MoND theory.
The theory centers on what is known as the Tully-Fisher relation, which maps out interplay between galaxies' mass and their speed of rotation.
However, the estimation of mass is hard to prove because it depends on the amount of light a galaxy emits, which varies greatly with the types and quantities of stars it contains.
To bypass this error, McGaugh studied 47 gas-rich galaxies with few stars, known as low surface-brightness galaxies. He found that the MoND theory predicts the relation between the galaxies' masses with their rotation speed, and, he argues, dark matter theory does so much less accurately.
"My attitude toward low surface brightness galaxies at first was 'great, this will finally be able to falsify the MoND theory'," McGaugh told BBC News. "But it was the only thing that explains this shift in the relation."
"Whenever I look at smallish things like individual galaxies it works really well," he said.
However, McGaugh conceded that "when you get up to the big scale of clusters of galaxies and you try to apply MoND to the whole thing, you fall short of fixing the missing mass problem."
Professor McGaugh's formulation "overstates the case" in that it assumes all galaxies will have the same ratio of normal matter to dark matter, argued Dan Hooper, a theoretical astrophysicist at Fermi National Accelerator Laboratory in the US.
"Some galaxies have very little stars and gas material compared to dark matter, and we don't expect the biggest galaxies to have the same fraction - which would change the shape of that line (relating galactic mass to spin speed)," he told BBC News.
"I don't think the MoND/dark matter debate hinges on the Tully-Fisher anymore," added McGaugh. "MoND only explains galaxies - everything else it fails to do or simply can't address."
Still, MoND is widely accepted by several prominent cosmologists, and McGaugh said his studies continue to show that MoND is a serious contender that dark matter theory will have a hard time disproving.
McGaugh maintains that MoND represents a missing piece of the dark matter model that many of his peers hold to be a complete picture of the composition of the Universe. "At the very least, it's telling us something about dark matter that's not native to our current model."
Image 1: The star dominated spiral galaxy UGC 2885. Image by Zagursky & McGaugh
Image 2: Gas rich LSB galaxy, F549-1. Image by Zagursky & McGaugh
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