When wading into the murky realm of theoretical physics, one of the more difficult concepts for scientists to explain is that of dark matter. It’s the invisible form of matter constantly eluding scientific hard proof, whose existence is suggested only by the circumstantial evidence we can measure around it. It doesn't interact with light — it's literally "dark."
As mysterious as dark matter may seem, however, most scientific theories assume its ingredients are pretty basic — just one weird, lightweight particle found all over the universe, that hardly ever touches other kinds of matter. But all that may now be up for debate.
A recent study published on the arXiv preprint database has thrown dark matter’s theoretical simplicity out the window by researchers who may have actually figured out how to generate massive dark matter particles. Those particles, they propose, aren’t lightweight at all and could come in all kinds of “species” — with their own “dark matter periodic table” of invisible elements, originally formed soon after the Big Bang after getting trapped in black holes. They dub this process "recycling."
As exciting as the researcher’s proposed theory may be, however, the problem of actually detecting these particles remains the most pressing question and may still be a hurdle for cosmologists. The key to that detection may lay in understanding how dark matter was historically formed.
“The formation history of dark matter itself is more complex. This is the challenge for astronomers like myself,” said cosmology professor Henk Hoekstra, of Leiden University. “Many different ideas for dark matter result in practically indistinguishable features and only direct detection experiments may be able to tell the difference. This paper now suggests that the formation process may be even more complex, which in turn may broaden the possibilities for detection.”
The new theory’s change in mass also ties dark matter to black hole formation more intimately, from previous assumptions that all dark matter particles are lightweight (known as WIMPs or weakly interacting massive particles) to the possibility that some might be ultraheavy. It’s currently understood more widely that when fundamental forces of nature began to splinter off from each other during the Big Bang — creating gravity, electromagnetism, strong nuclear force and weak nuclear force — that splintering was not straightforward, but included some transitional states.
As LiveScience’s Paul Sutter explains concisely, the underlying physics change at each one of these transitional phases then pockets of the universe would then exist where some older physics were in still in effect even though the rest of the universe around them had changed — like how bubbles in boiling water are trapped pockets of transitional air.
The researchers suggest that it was in these pockets of transitional physics that the once-lightweight dark matter particles could have gotten trapped, until they became so ultraheavy that they collapsed into black holes before evaporating through radiation.
While the idea of these heavier particles might seem to suggest they’d be easier to find for scientists, that may not actually be the case, and a bigger particle might not necessarily help observation efforts either. The University of British Columbia’s Ludovic Van Waerbeke pointed out that this difference could potentially even cancel out new possibilities of detection.
“The model proposed in this paper assumes the existence of very massive dark matter particles. They are indeed very numerous, but because of their mass is greater than WIMPs’ mass, their number density is also very low — much lower than WIMPs’ number density, so they would escape direct detection as well,” Van Waerbeke told Salon.
The problem of actually detecting dark matter particles, however, isn’t something that every new model of dark matter looks to solve. This type of approach — known generally as an ad-hoc approach to scientific theory — may have its limitations, but it has also been foundational to exploring the basic assumption in the field to date.
“It is not a criticism. It is a general feature of dark matter models beyond WIMPs,” Van Waerbeke said. “They postulate some ‘dark sector’ based on hypothetical dark matter physics, but it does not have any direct observational motivation.”
Underneath the new theory’s assumptions, however, is an even bigger question that could play a role in the quest to prove the existence of dark matter: If there really are a bunch of new particle types, does that also mean there could be a whole host of currently undetected laws of physics, operating silently this whole time? If so, it could form a periodic table, much like our own, that atlas of all known matter that includes everything from hydrogen to oganesson.
“It is hard to tell since the paper does not explore the astrophysical consequences of the model,” Van Waerbeke said. “What comes to mind is the possible observational signature of the continuous injection of photons across the universe's history.”
In other words, the question of “whether it could affect the physics of the cosmic microwave background or later, during the formation of the first stars — the so-called cosmic dawn," Van Waerbeke said.
And that history, as Hoekstra explained, could become more complicated to understand through the lens of the new theory. Still, there would likely be limits to the theory’s implications on the laws of physics as we understand them.
“The process that is described acts only in the very early universe, so it does not change anything today,” Hoekstra said. “Still, yet-unseen physics does operate under our noses, because dark matter flies through our bodies all the time.”
