Mystery: Does dark matter really exist?
FOR two factions of cosmologists, the universe is a battleground. They are bogged down in a squabble over what stops galaxies flying apart.
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The conflict has been brewing since the 1970s, when measurements of rotating galaxies showed that they were consistently spinning too fast to retain the matter they contain. In most galaxies, 10 to 100 times more matter is needed to hold them together than we can detect.
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By far the bigger faction thinks the discrepancy is down to the influence of a mysterious gravitating “dark matter” that we have yet to observe directly. But a rebellious minority believes that dark matter is an illusion, and that galaxies maintain their shapes because of a new facet of gravity we have yet to properly understand.
The ultra-diffuse galaxy NGC 1052-DF2, discovered earlier this year, seemed to be a game changer: it needs no invisible matter to keep it spinning at the measured rate. But perversely, this object with apparently no dark matter “could be evidence for the existence of dark matter”, says the galaxy’s co-discoverer at Yale University. That’s because it is quite possible to imagine that a single galaxy could somehow be stripped of dark matter, but much less easy to imagine one galaxy where modified gravity doesn’t apply. “At face value, this is a huge problem for alternative gravity theories,” says van Dokkum.
Not so fast, says Andrew Pontzen at University College London: the result “is very far from clear cut”. Van Dokkum’s team could not directly measure the speed of stars within NGC 1052-DF2, so instead identified 10 of its star clusters and used their rotation speed instead. That may not be a representative sample, says Pontzen. “You can get very misleading answers just by the luck of the draw.”
And modified-gravity adherents soon got their riposte in. Stacy McGaugh of Case Western Reserve University in Ohio and his colleagues showed that the original analysis failed to consider that NGC 1052-DF2 is embedded in the gravitational field of a larger elliptical galaxy. Take that and other uncertainties into account, and the measurements are consistent with something being up with gravity. “If I were to play a spin game, I’d describe this object as a great success of modified gravity,” says McGaugh.
Later this year, van Dokkum will be using the Keck 10-metre telescope on Hawaii to try to measure the rotation of this wispy misfit from the combined mass of its stars, as well as find other faint galaxies with similar behaviour that might break modified gravity once and for all. “The hunt is now on,” he says. “If we could rule out this whole class of theories, that would be a major advance.”
This article appeared in print under the headline “Object: Galaxy NGC 1052-DF2”
