Early this year, a project called EDGES managed to take the Universe's temperature at a time when the very first stars were forming. The results were somewhat confusing in that the Universe's regular matter seemed to be much cooler than we'd expect, based on the energy it had shortly after the Big Bang. If the measurement is right, then something must have cooled the regular matter down.
Physicists immediately suspected dark matter, as every indication is that it's relatively cold, meaning it's moving slowly enough for gravitational interactions to control its behavior. But dark matter generally doesn't interact with regular matter, making it hard to see how the two could have exchanged enough energy to cool the regular matter down.
Now, some physicists are back with a potential answer: a tiny fraction of dark matter has a charge, allowing it to interact with regular matter during the time between the Big Bang and formation of the Cosmic Microwave Background.
A dark sector
Dark matter is generally thought of as a single type of particle. Part of this is that we detect it by a single effect, namely its gravitational influence. And part of this assumption is practical. If we assume there's a zoo of different dark matter particles, then it would be possible to turn their properties to explain nearly anything. But a number of theorists have considered the prospect that there's an entire "dark sector," a collection of particles as diverse as their regular matter counterparts.
Those theoretical considerations were in the background when the EDGES results were announced. EDGES is a highly specialized instrument that looks like a few metal rods placed on a metal table on top of a large, round plate. It's sensitive to a very specific wavelength that registers the interactions of the Cosmic Microwave Background with the Universe's first atoms. By measuring them, EDGES provides a sense of the temperature of the matter in the early Universe, around the time the first stars were forming.
That temperature, however, turned out to be notably cooler than we'd expect if matter had gradually shed the energy that it had been left with after the Big Bang. Which suggests that the matter must have transferred its energy to something else. For this to work, that something else had to be cooler than the matter.
The favored model for the behavior of the Universe is called λCDM, for cold, dark matter. The "cold" part of that suggests that dark matter would be a good reservoir to soak up some of that energy. The problem is transferring the energy, which requires interactions between dark and normal matter. But all we've seen of the Universe indicates that these interactions are rare and very weak.
The paper released this week takes a stab at tweaking dark matter enough to allow this interaction without tweaking it so much that it becomes incompatible with the Universe we see around us.
The two researchers behind the paper, Julian Muñoz and Avi Loeb, propose that instead of an individual particle, dark matter forms a composite particle akin to an atom. Thus, the vast majority of it ends up uncharged, able to participate in the gravitational interactions that we ascribe to dark matter without interacting with regular matter otherwise. But if there's a tiny excess of charged particles that aren't part of a dark atom, these could participate in charge interactions.
In fact, those interactions would be most prominent in precisely the period we care about: the earliest stages of the Universe's evolution, when regular matter was hot enough that electrons couldn't settle down into atoms. Dark matter particles wouldn't bump into each other or regular matter, since they interact so rarely. But the charged regular matter would create a drag on dark matter's motion, allowing the two to reach a thermal equilibrium. The authors calculate that only a small fraction of dark matter—less than one percent—would need to be charged for this to produce a Universe that looks like the one EDGES sees.
But charged dark matter also has some constraints on it, as it would be easier to detect in various observations and experiments. So Muñoz and Loeb calculate limits on its properties based on everything from Supernova 1987A to the SLAC particle accelerator. The results they come up with are a mass with that has to be anywhere from about that of an electron up to 100 times that. For charge, the limits suggest that it's a million times smaller than that of an electron, which they call a "mini-charge."
Lest you think that ties everything together nicely, there are still some issues. "Most of the parameter space that we are considering is below this thermal-relic line, thus requiring new interactions to allow the DM to annihilate efficiently," Muñoz and Loeb write. "We leave this challenge for future model building of the needed dark sector."
Assuming those models can be built, they could also be tested. Because the dark and regular matter wouldn't be evenly distributed in the early Universe, their interactions would vary in intensity. This would create spatial fluctuations in the signal seen by the EDGES experiment. While EDGES probably isn't capable of detecting them, there may eventually be a strong motivation for building something that would.