New research suggests that ‘Heavy’ dark matter would rip our understanding of the cosmos apart

New research suggests that dark matter can’t be too heavy or it might break our best model of the cosmos. We have evidence aplenty that something fishy is occurring in the universe. Stars orbit within galaxies far too hurriedly. Galaxies move around inside clusters far too fast. Structures grow and evolve too fast. If we count only the matter that is visible, there simply isn’t sufficient gravity to explain all of these behaviours. The vast majority of astrophysicists believe all of these phenomena can be explained through the existence of dark matter, a hypothetical form of matter that is colossal, electrically neutral and hardly, if ever, interacts with normal matter. This dark matter makes up most of the mass in the cosmos, far dwarfing the amount of luminous matter.

The character of dark matter remains a mystery, as experiments intended to detect a stray, rare collision have been unsuccessful to turn up anything. But these experiments have concentrated on targeting a specific mass range: about 10 to 1,000 giga-electron volts (GeV). (A GeV is equivalent to 1 billion electron volts.) That’s in the range of the weightiest known particles, like the W boson and the top quark. For decades, theorists favoured this mass range because many simple extensions of the Standard Model of particle physics prophesied the existence of such particles.

Because we have not found anything yet, though, we have begun to wonder if dark matter might be lighter or heavier than we believed. But heavier dark matter runs into some very serious issues, as per a new paper published. The issue is that dark matter does occasionally interact with normal matter, if only rarely. But in the early cosmos, when the universe was much hotter and denser, these interactions were far more frequent. In due course, as the cosmos expanded and cooled, these interactions slowed and then stopped, leading the dark matter to “freeze out” and stay silent in the background.

Although there are many, many models of potential dark matter candidates, many interact with normal particles via exchanges involving the Higgs boson — a fundamental particle that interacts with virtually all other particles and, via those interactions, instils those particles with mass. We are aware of mass of the Higgs boson: around 125 GeV. The scientists found that this mass puts a fundamental upper limit on the likely mass of most dark matter candidates.

The issue is that all interactions in physics are two-way streets. The Higgs talks to both dark matter and usual matter and, in many models, mediates interactions between them. But both types of matter also talk back to the Higgs. These interactions seem as slight modifications to the Higgs boson’s mass. For Standard Model particles, we can compute these corrections and feedback interactions, which is how theorists prophesied the mass of the Higgs boson well before its detection.

The scientists found that if the dark matter particle had a mass larger than a few thousand GeV, its contribution to the Higgs mass would be incredibly significant, driving it away from its observed value. And as the Higgs is so central to determining many other fundamental physics, it would basically shut down particle interactions altogether. There are possibilities to get around this restriction, though. Dark matter might not interact with usual particles at all, or the interaction might happen via some exotic mechanism that doesn’t involve the Higgs. But those models are few and far between and need a lot of fine-tuning and extra steps.

Or it could be that dark matter is actually lighter than we believed. If we don’t think heavy dark matter is a feasible candidate, then as we continue to learn about this enigmatic component of the cosmos, we can instead focus our efforts in the other direction. There has already been a surge of interest in axions (ultralight particles that are predicted in some particle physics models and might be a viable dark matter candidate). On the experimental side, if this result is in fact confirmed and holds to be a widespread restriction on dark matter particle mass, we can refine and reshape our experiments to search for low-mass, instead of high-mass, particles.