Randall and her collaborators JiJi Fan, Andrey Katz, and Matthew Reece made their way to this idea in 2013 by the same path as Oort: They were trying to explain an apparent Milky Way anomaly. Known as the “Fermi line,” it was an excess of gamma rays of a certain frequency coming from the galactic center. “Ordinary dark matter wouldn’t annihilate enough” to produce the Fermi line, Randall said, “so we thought, what if it was much denser?” The dark disk was reborn. The Fermi line vanished as more data accumulated, but the disk idea seemed worth exploring anyway.

In 2014, Randall and Reece hypothesized that the disk might account for possible 30- to 35-million-year intervals between escalated meteor and comet activity, a statistically weak signal that some scientists have tentatively tied to periodic mass extinctions. Each time the solar system bobs up or down through the dark disk on the Milky Way carousel, they argued, the disk’s gravitational effect might destabilize rocks and comets in the Oort cloud—a scrapyard on the outskirts of the solar system named for Jan Oort. These objects would go hurtling toward the inner solar system, some striking Earth.

But Randall and her team did only a cursory—and incorrect—analysis of how much room there is for a dark disk in the Milky Way’s mass budget, judging by the motions of stars. “They made some kind of outrageous claims,” Bovy said.

Randall, who stands out (according to Reece) for “her persistence,” put Kramer on the case, seeking to address the critics and, she said, “to iron out all the wrinkles” in the analysis before Gaia data becomes available. Her and Kramer’s new analysis shows that the dark disk, if it exists, cannot be as dense as her team initially thought possible. But there is indeed wiggle room for a thin dark disk yet, due both to its pinching effect and to additional uncertainty caused by a net drift in the Milky Way stars that have been monitored thus far.

Now there’s a new problem, raised in The Astrophysical Journal by Chris McKee of the University of California, Berkeley, and collaborators. McKee concedes that a thin dark disk can still be squeezed into the Milky Way’s mass budget. But the disk might be so thin that it would collapse. Citing research from the 1960s and ’70s, McKee and colleagues argue that disks cannot be significantly thinner than the disk of visible gas in the Milky Way without fragmenting. “It is possible that the dark matter they consider has some property that is different from ordinary matter and prevents this from happening, but I don’t know what that could be,” McKee said.

Randall has not yet parried this latest attack, calling it “a tricky issue” that is “under consideration now.” She has also taken on the point raised by Bovy—that a disk of charged dark atoms is irrelevant next to the nature of 98 percent of dark matter. She is now investigating the possibility that all dark matter might be charged under the same dark force, but because of a surplus of dark protons over dark electrons, only a tiny fraction become bound in atoms and wind up in a disk. In that case, the disk and halo would be made of the same ingredients, “which would be more economical,” she said. “We thought that would be ruled out, but it wasn’t.”

The dark disk survives, for now—a symbol of all that isn’t known about the dark side of the universe. “I think it’s very, very healthy for the field that you have people thinking about all kinds of different ideas,” said Bullock. “Because it’s quite true that we don’t know what the heck that dark matter is, and you need to be open-minded about it.”

This post appears courtesy of Quanta Magazine .

This post appears courtesy of Quanta Magazine.

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