After nearly a century of observations, astronomers have concluded that the type of matter that makes up you and me amounts to just a scant 5% of the recipe of the universe. A ghostly form of matter called dark matter is five times more common than our familiar atoms. True to its name, dark matter emits no light; we “see” it only indirectly, by measuring its gravitational pull on ordinary atoms. So how do we know it’s really there? To be sure, we need to detect dark matter directly.
Physicists have been searching for dark matter particles for decades now. Some experiments seem to have caught them while other, equally powerful experiments have failed to find any evidence for dark matter. Most recently, the ultra sensitive LUX detector, a vat of liquid xenon buried in a mile-deep underground lab, found no evidence for dark matter and ruled out earlier measurements that had reported hints of a signal. Does this mean one or more of these results is wrong? Not necessarily. There are ways for both the LUX measurement and earlier measurements to be true, but this requires that dark matter and ordinary matter interact with each other in very specific, unexpected ways. Scientists are exploring these possibilities.
At the same time, physicists are beginning to think a bit more creatively. Until now, scientists looking for dark matter have imagined that dark matter is very simple. Specifically they imagine that there is just single type of dark matter particle: electrically neutral, experiencing only the weak and gravitational forces and with a mass 10-1000 times that of a proton. This model is popular because it is simple. On the other hand, the universe is not obliged to honor our definition of simplicity.
Suppose someone was studying the behavior of ordinary matter using only gravity as a probe. They’d no doubt construct a simple model of matter as a particle that was something like a neutron. However, we know that our world is very complex, that the neutron is just one member of the particle zoo and that these particles can come together in all sorts of interesting ways. Scientists are beginning to wonder if maybe dark matter might be similar.
Perhaps dark matter isn’t just one particle but a diverse realm of dark matter particles that experience forces that don’t affect ordinary matter. These dark matter particles might interact fairly strongly with each other, but only weakly with ordinary matter. With little experimental evidence to guide them, theoretical physicists are allowed to speculate fairly freely, although there are some constraints imposed by astronomical observations.
One idea postulates a dark equivalent to electrical charge called “dark charge.” Just as ordinary electrons and positrons (antimatter electrons) can interact with each other and emit photons, it is possible that particles carrying dark charge can interact and produce dark photons.
It is crucial to remember that dark charge, if it exists, does not interact with ordinary matter except by way of gravity and maybe the familiar weak force. A dark matter particle carrying dark charge and a familiar particle carrying electrical charge would pass by one another without so much as a “how do you do?”
If a complicated dark sector exists, we can see it only if there is a particle that interacts with both ordinary matter and dark matter. If we could create such a messenger particle and allow it to interact with astronomical dark matter or (more likely) decay into dark matter particles, we might be able to detect it at particle accelerators like the LHC. But there’s a catch: The experimental signature would be “missing” energy in some collisions as the energy flowed into what physicists call the dark sector, the enigmatic realm of dark matter and dark energy. Given that disappearing energy is a fairly common feature of particle collisions (e.g. when neutrinos are created), it would be tricky to pin it on the creation of dark matter messenger particles. But by measuring the distribution of “missing” energy in LHC collisions and comparing it to the predictions of known physics and theoretical models of dark matter particles, it might be possible to catch a glimpse of the dark sector.
Of course, missing energy is just one possible signature of a complicated dark sector. Another possibility invokes the principle of supersymmetry, which postulates that every known fundamental subatomic particle has a (so far undiscovered) cousin with a different quantum spin. Were the LHC to create these theoretical supersymmetric particles in a collision, they would decay into low-mass supersymmetric particles capable of interacting with the complex dark matter sector. After another cascade of decays, a dark matter particle could emit a messenger particle that “sees” both dark matter and ordinary matter and then decay in turn into a matter-antimatter particle pair that could be picked out in the collider data. Because this scenario postulates both supersymmetry and complex dark matter, it is even more of a jump into the unknown. But given that we don’t understand a lot of the universe, sometimes wild ideas are required. As Niels Bohr once quipped to Wolfgang Pauli, “We are all agreed that your theory is crazy. The question which divides us is whether it is crazy enough.”
So far, physicists have not found evidence for a complex dark sector, but the search has just begun. Ordinary matter is complex, so it seems very reasonable that the dark sector should be, too. Over the next several years, theorists will begin to flesh out a myriad of dark possibilities, including possibly even dark atoms, just in time for the LHC to turn back on and see if the data supports these interesting ideas.
Author’s suggestions for further reading
arXiv: Dark Sectors and New, Light, Weakly-Coupled Particles
A technical paper summarizing the motivation for and possible tests of dark sector theory.
Preposterous Universe: More Messy Dark Matter
Astrophysicist Sean Carroll blogs about the possibility that dark matter is more “interesting” than we thought.
Sanford Underground Research Facility: First results from LUX experiment in South Dakota
A press release outlining the results of the LUX experiment’s first, three-month-long search for dark matter.