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.
The idea that dark matter can cause pulsars to implode isn’t new, says astrophysicist Kathryn Zurek of Lawrence Berkeley National Laboratory. But the new research is the first to apply this possibility to the missing-pulsar problem.
If the hypothesis is correct, Bramante says, then pulsars around the galactic center could only get so old before grabbing so much dark matter that they turn into black holes. Because the density of dark matter drops the farther you go from the center, the researchers predict that the maximum age of pulsars will increase with distance from the center.
Observing this distinct pattern would be strong evidence that dark matter is not only causing pulsars to implode, but also that it’s asymmetric, Bramante says. “The most exciting part about this is just from looking at pulsars, you can perhaps say what dark matter is made of,” he said. Measuring this pattern would also help physicists narrow down the mass of the dark matter particle.
But it won’t be easy to detect this signature. Astronomers will need to collect much more data about the galactic center’s pulsars by searching for radio signals, Bramante says. The hope is that as astronomers explore the galactic center with a wider range of radio frequencies, they will uncover more pulsars.
Still, the idea that dark matter is behind the missing pulsar problem is speculative. How likely is this scenario? “I think it’s unlikely—or at least it is too early to say anything definitive,” said Zurek, who was one of the first to revive the notion of asymmetric dark matter in 2009. The tricky part is being able to know for sure that any measurable pattern in the pulsar population is due to dark-matter-induced collapse and not something else.
Even if astronomers find this pulsar signature, it’s still far from being definitive evidence for asymmetric dark matter, Zurek says. “Realistically, when dark matter is detected, we are going to need multiple, complementary probes to begin to be convinced that we have a handle on the theory of dark matter,” she said.
And asymmetric dark matter may not have anything to do with the missing pulsar problem at all. The problem is relatively new, Bramante says, so astronomers may find more plausible, conventional explanations. “I’d say give them some time and maybe they come up with some competing explanation that’s more fleshed out,” he said.
Nevertheless, the idea is worth pursuing, says Haibo Yu of the University of California, Riverside. If anything, this analysis is a good example of how scientists can understand dark matter by exploring how it may influence astrophysical objects. “This tells us there are ways to explore dark matter that we’ve never thought of before,” he said. “We should have an open mind to see all possible effects that dark matter can have.”
There’s one other way to determine if dark matter can cause pulsars to implode: To catch them in the act. No one knows what a collapsing pulsar might look like, Bramante says. It might even blow up.
“While the idea of an explosion is really fun to think about, what would be even cooler is if it didn’t explode when it collapsed,” he said. A pulsar emits a powerful beam of radiation, and as it spins, it appears to blink like a lighthouse with a frequency as high as several hundred times per second. As it implodes into a black hole, its gravity gets stronger, increasingly warping the surrounding space and time.
Studying this scenario would be a great way to test Einstein’s theory of general relativity, Bramante says. According to theory, the pulse rate would get slower and slower. Eventually, the time between pulses becomes infinitely long. The pulses stop and the pulsar is no more.