Rip tides.
When gravity from a larger galaxy pulls apart a small orbiting galaxy, as in this simulation, the stars streaming out can offer clues about dark matter's gravitational pull.

Kathryn V. Johnston

No Extra Gravity for Dark Matter

Darth Vader might be disappointed to hear it, but the force of gravity does not have a dark side. Dark matter does not pull on other dark matter with any extra gravity or additional force, as some researchers have suggested, a pair of theoretical physicists reports. Based on observations of a pip-squeak galaxy orbiting our own, more massive, Milky Way galaxy, the argument could shoot down theories invented to explain other puzzling observations. Some researchers are skeptical, however.

According to legend, Galileo once dropped a cannon ball and a wooden ball from the top of the Leaning Tower of Pisa to show that gravity causes all objects to accelerate earthward at the same rate, regardless of how much they weigh. Now, Michael Kesden of the University of Toronto and Marc Kamionkowski of the California Institute of Technology in Pasadena have caught the Milky Way galaxy performing a similar experiment.

The Milky Way is gradually pulling apart a smaller orbiting neighbor known as the Sagittarius dwarf spheroidal galaxy. Just as gravity from the moon causes Earth's oceans to bulge, so too does the gravity of the Milky Way create enormous tides that deform Sagittarius. These are so strong that they rip stars out of the galaxy, producing two long streams of stars, one stretching ahead of Sagittarius and one lagging behind. By observing the stars streaming out in both directions, the researchers conclude that the dark matter and ordinary matter within the smaller galaxy feel the same pull from the Milky Way.

Kesden and Kamionkowski argue that if the dark matter in the heart of the Sagittarius galaxy felt an extra pull from the dark matter in the Milky Way, then almost all the stars would appear to trail the galaxy. Here's why: Feeling an extra pull, the galaxy itself would move slightly closer to the center of the Milky Way than the liberated stars. And because closer objects orbit faster--just as Venus orbits the sun faster than Earth does--the stars should all appear to lag behind the galaxy. The fact that many don't indicates that there is no extra force on dark matter, the researchers conclude in a paper posted online 29 September in Physical Review Letters.

The analysis suggests that such tidal streams of stars are a good testing-ground for theories of extra forces, says Glennys Farrar, a theoretical physicist at New York University. But it isn't definitive, she says, because Kesden and Kamionkowski assume particular distributions of dark matter in the two galaxies. They must still test to see if some other distribution might produce the leading stream, even if there is an extra force on dark matter, Farrar says. She adds that there are other hints of such a force--such as rarity of dwarf galaxies, which are plentiful in simulations that include only gravity.

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