Mysterious Pulsars May Have Gotten Wrapped in Dark Matter

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The center of the galaxy should be chock-full of rapidly spinning, dense stellar corpses known as pulsars. The problem is, astronomers can’t seem to find them. The galactic center is a bustling place. Lots of gas, dust, and stars zip about, orbiting a supermassive black hole about three million times more massive than the sun. With so many stars, astronomers estimate that there should be hundreds of dead ones, says astrophysicist Joseph Bramante of Notre Dame University. Scientists have found only a single young pulsar at the galactic center, where there should be as many as 50 such youngsters. Bramante and astrophysicist Tim Linden of the University of Chicago have a possible solution to this missing-pulsar problem, which they describe in a paper accepted for publication in the journal Physical Review Letters. Maybe those pulsars are absent because dark matter, which is plentiful in the galactic center, gloms onto the pulsars, accumulating until the pulsars become so dense they collapse into a black hole. Poof. No more pulsars. A Different Kind of Dark Matter Dark matter, of course, is the weird stuff that’s everywhere—filling roughly a quarter of the universe—but is invisible and hardly interacts with anything, making its presence known only by how its gravitational pull interacts with other astrophysical objects. One of the more popular candidates for dark matter is weakly interacting massive particles, or a WIMPs. Underground detectors are hunting for WIMPs and debate has raged over whether gamma rays streaming from the galactic center come from WIMPs annihilating one another. In general, any particle and its antimatter partner will annihilate each other in a flurry of energy. But WIMPs don’t have an antimatter counterpart. Instead, they’re thought to be their own antiparticles, so one WIMP can annihilate a fellow WIMP. But over the last few years, physicists have considered another class of dark matter called asymmetric dark matter. Unlike WIMPs, this type of dark matter does have an antimatter counterpart. Asymmetric dark matter appeals to physicists because it’s intrinsically linked to the imbalance of matter and antimatter: There’s a lot more matter in the universe than antimatter (which is a big deal, because without this disparity, everything in the universe—including us—would’ve been annihilated and wouldn’t exist). Likewise, according to the theory, there’s much more dark matter than anti-dark-matter. Physicists think that in the beginning, the Big Bang should’ve created as much matter as antimatter. But something altered this balance. No one’s sure what this mechanism was, but it might also have triggered an imbalance in dark matter (hence it is “asymmetric”). Dark matter is concentrated at the galactic center, and if it’s asymmetric, then it could collect at the center of pulsars, pulled in by gravity. Pulsars are extremely dense—imagine the sun squeezed into a region the size of a small city—so its gravity is strong enough to attract plenty of dark matter. Eventually, the pulsar would accumulate so much mass that it would collapse into a black hole. More via WIRED.

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