In 1932, the Dutch astronomer Jan Oort tallied the stars in the Milky Way and found that they came up short. Judging by the way the stars bob up and down like horses on a carousel as they go around the plane of the galaxy, Oort calculated that there ought to be twice as much matter gravitationally propelling them as he could see. He postulated the presence of hidden “dark matter” to make up the difference and surmised that it must be concentrated in a disk to explain the stars’ motions.
But credit for the discovery of dark matter — the invisible, unidentified stuff that comprises five-sixths of the universe’s mass — usually goes to the Swiss-American astronomer Fritz Zwicky, who inferred its existence from the relative motions of galaxies in 1933. Oort is passed over on the grounds that he was trailing a false clue. By 2000, updated, Oort-style inventories of the Milky Way determined that its “missing” mass consists of faint stars, gas and dust, with no need for a dark disk. Eighty years of hints suggest that dark matter, whatever it is, forms spherical clouds called “halos” around galaxies.
Or so most dark matter hunters have it. Though it fell out of favor, the dark disk idea never completely went away. And recently, it has found a high-profile champion in Lisa Randall, a professor of physics at Harvard University, who has rescued the disk from scientific oblivion and given it an active role on the galactic stage.
Since proposing the model in 2013, Randall and her collaborators have argued that a dark disk might explain gamma rays coming from the galactic center, the planar distribution of dwarf galaxies orbiting the Andromeda galaxy and the Milky Way, and even periodic upticks of comet impacts and mass extinctions on Earth, discussed in Randall’s 2015 popular-science book, Dark Matter and the Dinosaurs.
But astrophysicists who do inventories of the Milky Way have protested, arguing that the galaxy’s total mass and the bobbing motions of its stars match up too well to leave room for a dark disk. “It’s more strongly constrained than Lisa Randall pretends,” said Jo Bovy, an astrophysicist at the University of Toronto.
Now, Randall, who has devised influential ideas about several of the biggest questions in fundamental physics, is fighting back. In a paper posted online last week that has been accepted for publication in The Astrophysical Journal, Randall and her student, Eric Kramer, report a disk-shaped loophole in the Milky Way analysis: “There is an important detail that has so far been overlooked,” they write. “The disk can actually make room for itself.”
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