The dinosaurs never saw it coming. When a giant space rock smashed into the Yucatán Peninsula some 65 million years ago, their global reign ended in catastrophic violence. But that space rock—perhaps a comet several miles wide—might have had a stealthy accomplice: dark matter.
Dark matter, of course, is the invisible stuff that makes up a quarter of the universe, and 85 percent of all matter. And Harvard physicist Lisa Randall thinks it may be what flung that dino-destroying comet toward Earth. According to her theory, the Milky Way's dark matter exists in the form of a thin disk embedded in the galactic plane—what Randall refers to as “double disk dark matter.” While orbiting the galactic center, the solar system passes through this disk, whose gravity tugs on comets in the solar system—and, in the case of the dinosaur’s demise, pulled one of them just enough to send it Earth-bound.
It's a thought-provoking idea, connecting one of the biggest cosmic mysteries with everyone's favorite extinct animals. For sure, it's speculative. But Randall’s hypothesis involves a new characterization of dark matter that's gained ground in recent years—one in which dark matter is more nuanced and complex than previously thought.
This new class of dark matter entails what's called self-interacting dark matter. Theoretically, it can collide or otherwise interact with itself, possibly through a zoo of particles analogous to the ones that make up regular matter, such as protons, electrons, and quarks. A whole dark universe might even exist out there, with dark atoms and even dark stars and planets. “There are many different possibilities for how matter can interact,” Randall says. “The same thing's true for dark matter.” And if dark matter does self-interact, it means that for roughly 30 years, scientists trying to detect the shadowy stuff have been chasing after the wrong type of particle.
That particle is a weakly interacting massive particle, or WIMP, and still the most popular dark-matter candidate particle today. No one knows exactly what kind of particle it might be, and physicists have many theories. In general, though, a WIMP is a particle that hardly interacts with anything in any way other than via gravity—which makes it a good choice for dark matter because, as far as anyone can tell, dark matter only interacts gravitationally.
Physicists favor a WIMP because it fits so well into theories of cosmology. As the story goes, soon after the Big Bang, these particles would've been constantly running into one another and annihilating in a burst of energy. Over time, their number would have dwindled, and as the universe expanded and cooled, it would have become harder for them to find and annihilate each other. It turns out that the amount of WIMPs remaining would be just enough to account for the dark matter in the universe.
“People my age are taught in grad school that the most reasonable and well-motivated model for dark matter is the WIMP model—the WIMP miracle,” says James Bullock, an astrophysicist at the University of California, Irvine. “It explains so many things, a lot of astronomers take it as gospel.”
So scientists have been trying to hunt the particle down. They've used a satellite to search for gamma-ray signals that may result from WIMPs annihilating one another in space. They hope to create one of the particles at the Large Hadron Collider in Switzerland by slamming protons together at near light-speed. And they've built underground detectors at locations around the world to catch one, such as the Large Underground Xenon experiment a mile below the surface in the Black Hills of South Dakota. (Researchers hypothesize that WIMPs also interact via the weak force, which comes into play at extremely short distances at the subatomic level. In the case of LUX, scientists hope to detect WIMPs interacting through the weak force with liquid xenon atoms. For more, watch the video below, and check out this article.)
These experiments have been running for a few years, which is long enough that, by now, scientists could have conceivably discovered a WIMP. Yet no one has found anything. “We've been searching for it in a variety of ways,” says Yonit Hochberg, a physicist at Lawrence Berkeley National Laboratory. “If we find it, that's wonderful. But if we don't, maybe it's time to think about other options.”
One option: self-interacting dark matter, a catchall term for a more complex kind of dark matter. While WIMPs barely notice one another, particles of self-interacting dark matter can theoretically collide and scatter, experiencing non-gravitational forces—yes, Star Wars fans, they're dark forces. And unlike WIMP dark matter, self-interacting dark matter involves more than one type of particle.
Self-interacting dark matter might explain some discrepancies between the WIMP theory and real-life observations of galaxies. For example, computer simulations show that a universe of WIMPs produces galaxies whose centers are denser than what's observed. But if dark matter were self-interacting, its constituent particles would bounce off each other like Ping-Pong balls. As a result, less dark matter would pile up in galactic cores, resulting in the dark matter density that astronomers actually measure.
Recently, Hochberg proposed one theory of self-interacting dark matter in which dark matter mainly consists of some kind of strongly interacting massive particle—a SIMP. Like WIMPs, SIMPs would annihilate one another early in the universe's history. Unlike WIMPs, however, it would take three of them to annihilate, and two SIMPs would remain. This process would produce the right amount of leftover SIMPs to account for all the dark matter in the universe—but in this case, it would be a “SIMP miracle.”
There could be several types of SIMPs. As one example, Hochberg has suggested that the SIMP is a dark version of a particle called a pion. Generally speaking, though, SIMPs are at least 1,000 times lighter than a typical WIMP, so to detect them, physicists have to revamp their search strategies (some early-stage experiments are already underway).
Meanwhile, astronomers are trying to figure out if dark matter is self-interacting in the first place. Specifically, they're studying mergers between enormous clusters of galaxies, which are embedded in huge blobs of dark matter called dark matter halos. If dark matter interacts, merging halos would slow each other down—which astronomers can detect by measuring how much their gravity warps the light from background galaxies.
A U.S.-based team has just surveyed 25 galactic mergers, and they hope that within a few years, they'll know for sure whether dark matter interacts or not. “This will be enough to make a definitive claim one way or another,” says Will Dawson, an astronomer at Lawrence Livermore National Laboratory, who along with Bullock is a team member.
Recently, another group studied a galaxy falling into a galaxy cluster called Abell 3827. Their initial analysis suggests that the dark matter is indeed interacting, although some researchers cast doubt on the findings.
But dark matter might not simply be either strongly or weakly interacting. It could be a combination of both—which brings us back to the dinosaurs.
According to Randall's theory, the majority of dark matter is very weakly interacting. But a small component could interact with itself through a force similar to the electromagnetic force. In the Milky Way, this self-interacting component would account for about five percent of the galaxy's total mass. This kind of dark matter would be composed of positively and negatively charged particles, as well as dark photons (that is, dark light), interacting in a way that dissipates energy. Thanks to this energy loss, dark-matter particles would slow down, combine to form dark atoms, and eventually flatten into a huge disk aligned with the galaxy.
Randall and her team showed that if this dark-matter disk existed, it could explain the geological evidence that Earth has experienced periodic impacts from comets every 35 million years or so. Scientists have previously suggested that as the solar system orbits the center of the Milky Way, it bobs up and down through the galactic plane with about the same frequency. When that happens, Randall's team proposes, the solar system also passes through the dark-matter disk, whose gravity triggers an influx of comets to pelt the inner solar system. It was one of those comets that killed the dinosaurs.
To determine whether a dark-matter disk exists, astronomers could use the European Gaia satellite, a mission launched in 2013 that measures the location and trajectories of about a billion stars in the Milky Way galaxy. If there is a disk, astronomers would be able to detect its gravitational influence on the motions of those stars.
Until enough evidence points one way or another about whether dark matter interacts, however, scientists like Dawson remain agnostic. “I'm just as happy to rule out self-interacting dark matter as I am to discover it,” he says. Still, given the menagerie of particles and forces involved with regular matter, it's not far-fetched to think dark matter could be equally complex. In fact, he says, it might be more surprising if it weren't.
“There are all these possibilities out there,” Bullock says. “Maybe eventually we can rule those possibilities out. But right now, we can't.”
