Can Tiny Galaxies Explain Dark Matter?

Mysterious globular clusters may yield clues to even-more mysterious dark matter

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If you could step back a few quadrillion miles and look at the Milky Way from the outside, you’d see a magnificent, glowing pinwheel of stars and gas, its arms curving majestically outward from a huge, bright, central knot. If you looked closer, you’d see much tinier clumps—roughly spherical, with a million stars at most compared with the Milky Way’s hundreds of billions—buzzing around our galaxy’s core like bees circling the hive. Astronomers call them globular clusters and know that they contain the oldest stars in the Milky Way. But how they came to exist, and what role they might have played in the Milky Way’s 13-billion-year-history is still a mystery.

If the globular clusters around our own galaxy are still a puzzle, however, those elsewhere might help answer some other stubborn questions, thanks to a new discovery made with the Hubble Space Telescope. By pointing the Hubble at a giant collection of galaxies known as Abell 1689, some 2 billion light-years away, observers found evidence of an astonishing 160,000 globular clusters. If the 160 or so globular clusters surrounding the Milky Way are like bees around a hive, the clusters in Abell 1689 are like a huge swarm massing around dozens of enormous hives. And by looking at precisely where they swarm, astronomers can calculate how dark matter is spread throughout the group of galaxies—and ultimately, what that matter, which constitutes most of the mass in the universe, is really made of.

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The reasoning is utterly simple: stars form out of clouds of gas and dust, and you tend to get more stars where the clouds are densest. According to the conventional cosmic wisdom, gas wafting around the universe shortly after the Big Bang got sucked in by knots of dark matter, eventually becoming dense enough to burst into thermonuclear fire. If you look at the oldest living stars, you should be looking at where the dark-matter knots were—and still are—located.

In the Hubble study, the concentration of globular clusters rises sharply at the very center of the galaxy group, suggesting that the dark matter isn’t spread evenly through the group but rather at its core. That conclusion matches what astronomers have found in an entirely different way: earlier studies have shown that the galaxy group’s overall gravity acts as a giant cosmic lens, distorting the images of more distant galaxies. And that pattern of distortion too suggests a lump of dark-matter in the galaxy group’s central region.

Such mass distribution lends credence to the leading theory of dark matter: it’s some sort of still-undiscovered elementary particle, created in such huge amounts during the Big Bang that it outweighs ordinary matter by a factor of six or more. If that’s really what it is, the particles should cluster in more or less the way the new studies suggest. Other observations are less definitive, though, so scientists would prefer to find the particles directly, perhaps with the Large Hadron Collider, which recently scored a coup by finding the Higgs boson.

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But none of this helps explain where the globular clusters came from in the first place. They may be the lingering representatives of the very first, tiny galaxies in the universe, most of which merged to form today’s full-sized galaxies, like the Milky Way.

Or maybe not: there’s a reasonable chance that astronomers will truly understand the invisible dark matter before they figure out what the story is with globular clusters so bright they can be seen from halfway across the universe.

(PHOTOS: Voyager 1: Snapshots From the Journey)