Dark matter is the unseen hand that fashions the universe. It decides where galaxies will form and where they won’t. Its gravity binds stars into galaxies and galaxies into galaxy clusters. And when two galaxies merge, dark matter is there, sculpting the product of the merger. But as for what dark matter actually is? No one knows.
Here’s the short list of what we do know about dark matter. Number one: There’s a lot of it, about five times more than “ordinary” matter. Two: It doesn’t give off, reflect, or absorb light, but it does exert gravity, which is what gives it a driver’s-seat role in the evolution of galaxies. Three: It’s stable, meaning that for almost 13.8 billion years—the current age of the universe—dark matter hasn’t decayed into anything else, at least not enough to matter much. In fact, the thinking goes, dark matter will still be around even when the universe is quintillions (that’s billions of billions) years old—maybe even forever.
Theoretical physicists dreaming up new ideas about dark matter typically start with these three basic principles. But what if the third—the requirement that dark matter be stable over the cosmic long haul—is wrong? That’s the renegade idea behind a new dark matter proposal called “Dynamical Dark Matter.” Though it’s still on the fringe of dark matter physics (“It’s as far as you can get from the traditional approaches,” says physicist Keith Dienes of the University of Arizona, who first developed the idea with Lafayette College theorist Brooks Thomas), it’s been gaining traction and attracting collaborators from particle physics, astrophysics, and beyond.
And dark matter is a field that could use some new ideas. While astronomers have been picking up dark matter’s fingerprints all over the universe for at least a century, physicists can’t seem to get a fix on a single dark matter particle. It’s not for lack of trying. Particle hunters have looked for signs of them in flurries of particles set loose by colliders like the Large Hadron Collider (LHC). They have buried germanium crystals and tanks of liquid xenon and argon deep underground—beneath mountains and in old gold mines—and looked for dark matter particles pinging off the atomic nuclei inside. The result: Nothing, at least not anything that physicists can agree on.
Meanwhile, the astrophysical evidence for dark matter keeps building up. Take one universal mystery: Astronomers, after clocking how fast stars are circling around in galaxies, have found that stars skimming a galaxy’s perimeter are going just about as fast as closer-in stars. But based on everything we know about how gravity works, they should actually be going a lot slower—unless there is some invisible mass pulling on them. Then, there are galaxy clusters: Galaxies within them are jouncing around so quickly that they should fly apart, absent some invisible mass is holding them all together. Noticing a theme here? Even the cosmic microwave background radiation, the closest thing we have to a baby picture of the newborn universe, has patterns in it that can only really be explained by dark matter. So, if dark matter is so ubiquitous, why can’t we find it?
Some researchers are beginning to wonder if they’ve been searching for the wrong thing all along. Most (though not all) dark matter detectors are designed to find hypothetical particles called WIMPs—short for “weakly interacting massive particles.” WIMPs are an appealing dark matter candidate because they emerge naturally from a beyond-the-standard-model theory called supersymmetry, which posits that the all the fundamental subatomic particles have as-yet-undiscovered partners.
As physicists worked out the properties of those still unseen particles, they noticed that one was a startlingly good match for dark matter. It would interact with other particles via gravity and something called the weak force, which only works when particles get within a proton’s-width of each other. Plus, it would be stable, and there could be just enough of it to account for the missing mass without upsetting with the evolution of the universe.
The appeal of WIMPs is “almost aesthetic,” says Jason Kumar, a physicist at the University of Hawaii: it speaks to physicists’ love of all that is simple, symmetrical, and elegant. But, Kumar says, “It’s now becoming very hard to get these models to fit with the data we’re seeing.” That doesn’t mean that the WIMP model is wrong, but it does put researchers in the mood to consider ideas that, ten years ago, might have been brushed off as theoretical footnotes. Like, for instance, the idea that dark matter that isn’t stable after all.
A Destabilizing Influence
Dienes and Thomas were newcomers to dark matter when they first hatched the idea of Dynamical Dark Matter. They were so new to the field that, at first, they didn’t even worry about stability. Together, they began sketching a new kind of dark matter. First, they thought, what if dark matter weren’t just one kind of particle, but a whole bunch of different kinds? Second, what if those particles could decay? Some might disappear within seconds, but others could stick around for trillions of years. The trick would be getting the balance right, so that the bulk of the dark matter would linger until at least the present day.
Dienes and Thomas called their new framework “Dynamical Dark Matter,” and started sharing it at talks and academic conferences. The reaction, according to Dienes: “A boatload of skepticism.”
“People kept asking about stability,” Dienes remembers. “But we were not thinking about stability in the traditional way.”
Why are physicists so sure that dark matter is stable, anyway? Galaxies from long ago—the ones astronomers see when they look billions of light years out into the universe—aren’t more weighed-down by dark matter than our nearby, present-day specimens, at least not at the level of precision that astronomers can measure. Plus, if dark matter decayed into lighter, detectable particles, the little shards would fly out into space with a lot of energy, which we would be able to measure on Earth. And if the decay started in the universe’s baby days, it would disrupt the formation of the elements, shifting the chemistry of the cosmos.
Dynamical Dark Matter resolves the stability problem through a balancing act. If most of dark matter is tied up in particles that live a long time—longer than the age of the universe—that leaves room for a small share of dark matter to be made up of particles that vanish quickly. “It’s a balancing between lifetimes and abundances,” Dienes says. “This balancing is the new underlying principle that replaces mere stability.”
At first glance, this might sound contrived. Why should everything work out just so? But Dienes, Thomas, and their collaborators have discovered several scenarios that naturally produce just the right combination of particles. “It turns out there are a lot of interesting ways in which these things can come about,” Thomas says. Dynamical Dark Matter remains agnostic about what the dark matter particles are or how they came to be. “It’s not just a single model for dark matter, like a particle that’s a candidate,” he says. “It’s a whole new framework for thinking about what dark matter could be.”
Dynamical Dark Matter is one of a growing number of “multi-component” dark matter models that welcome in multiple particles. “The key differentiator for Dynamical Dark Matter is that it’s not just a random collection of particles,” Kumar says. “There are just a couple of parameters that describe everything about it.”
A Shrinking Slice of Pie
Today, dark matter makes up about 85% of the “stuff” in the universe, out-massing regular matter by a factor of five to one. But if the Dynamical Dark Matter framework is right, one day, dark matter will fizzle out entirely. The process will start slowly. Then, as a larger share of dark matter hits its expiration date, the die-out will speed up until, ultimately, dark matter goes extinct.
That won’t happen for a long, long time—long after dark energy, that other cosmic mystery force, stretches the universe to the brink of nothingness. (But that’s another story.) So one might ask: Who cares if a teeny weeny bit of dark matter goes “poof” if no one misses it?
Scientists searching for dark matter particles do.
That’s because, at dark matter detectors, Dynamical Dark Matter particles should leave a more complicated set of fingerprints than WIMPs. While WIMPs should make a relatively simple “clink” against the ordinary particles inside a detector, Dynamical Dark Matter (or any other brand of multiplex dark matter) would make a jumbled-up jangle. “If there is only one dark-matter particle, there is a well-known ‘shape’ for this recoil spectrum,” says Dienes, describing the detector read-out. “So seeing such a complex recoil spectrum would be a smoking gun of a multi-component dark-matter scenario such as Dynamical Dark Matter.”
Particle collider experiments could also distinguish Dynamical Dark Matter from WIMPs. “Dynamical dark matter basically provides a very rich spectrum of very different types of collider signatures, some very different from conventional dark matter,” says Shufang Su, a physicist at the University of Arizona. With Dienes and Thomas, Su is trying to predict the traces Dynamical Dark Matter would leave in data from particle colliders like the LHC.
Su was attracted to the dynamical dark matter model by the idea that dark matter could be a whole panoply of particles instead of just one, which would leave a distinctive signature on the visible particles produced in the LHC’s smash-ups. “These changes could be very dramatic and very different from what would occur if there is only a single dark matter species,” Su says. “If one dark matter particle leads to a single peak, Dynamical Dark Matter could lead to multiple peaks and perhaps even peculiar kinks.”
Then there’s the decay factor. Depending on how long Dynamical Dark Matter particles live, some might fall apart almost as soon as they are created. Others might last long enough to travel some length of the detector, or escape entirely. “Even though it’s still dark matter, it could have a totally different signature,” Su says.
While Su is thinking about how to detect Dynamical Dark Matter at colliders here on Earth, Kumar is thinking about whether it could explain something that has been puzzling astronomers: a mysterious excess of high-energy positrons in space. Dark matter researchers have suggested the positrons could be coming from WIMPs, which spit them out as they collide with and annihilate other WIMPs. The trouble, Kumar says, is that this process should only produce positrons up to a certain maximum energy before shutting down; so far, astronomers haven’t found such a cut-off. Dynamical dark matter just might be able to make positrons at the energy levels astronomers observe.
Of course, Dynamical Dark Matter is just one of many alternatives to WIMPs. There are also SIMPS, RAMBOs, axions, sexaquarks—the list goes on. Until physicists make a clear-cut detection, theorists will have plenty of headroom to dream up new ideas.
“The main message is that this is an interesting alternative. We are not claiming that it is necessarily better,” Dienes says. “The field is wide open, and data will eventually tell us.”