Dark matter comprises up to five-sixths of the total matter in the universe, binding much of it together. Despite its commonality, the physical nature of dark matter, as of 2016, remains elusive, with its dramatic effects only indirectly seen. Now, a new study published in the journal Physical Review Letters suggests that subatomic particles of dark matter, a billion of which pass through your hand every second, may in fact be so dense that they’re on the verge of becoming black holes.
At the speed they rotate, galactic spiral arms should jettison off into space, but they don’t. After accounting for the effect of gravity, scientists can only conclude that there must be an additional binding force keeping these arms together. The invisible, mysterious “dark matter” describes that which stops these galaxies from tearing themselves apart.
Dark matter is often assumed to consist of particles that have the same mass as a proton, but interact extremely weakly with matter, like a neutrino. Candidates for dark matter particles have come and gone in the last century, with WIMPs – weakly interacting massive particles – considered to be one of the frontrunners. These elusive particles are hypothesized to be 100 times the mass of a proton, and would have likely been forged during the Big Bang.
The fact that they are weakly interacting means that they are probably very hard for scientists to detect – indeed, to date, they haven’t been. This means that for now the existence of WIMPs, especially as the true identity of dark matter particles, cannot be confirmed.
This new study though, describing a new mathematical model, suggests that dark matter particles aren’t actually WIMPs, but something even more exotic. They could in fact be far less able to interact with their environment than a neutrino, but astoundingly, each individual particle will have a mass 10 billion billion times more than a proton – about that of an average human cell.
The cosmic web, similar in shape to a neural network, is partly comprised of massive filaments of dark matter. Sakkmesterke/Shutterstock
As reported by Space.com, at this mass and still at subatomic sizes, they will be “about as [dense] as a particle can be before it becomes a miniature black hole,” according to McCullen Sandora, a postdoctoral researcher at the University of Southern Denmark and an author of the study.
In this model, referred to as Planckian interacting dark matter (PIDM), these incredibly dense subatomic particles could be detected in the afterglow of the Big Bang. Soon after the Big Bang there was a period known as “inflation,” a moment of sudden expansion. This smoothed out the matter in the universe so that it is roughly similar in proportion in every direction.
During this inflation, the universe cooled considerably. As the expansion suddenly slowed and inflation ended, the universe “reheated,” and the authors suggest that these new PIDM particles were forged during this time. If so, the birth of these superheavy particles would have left a signature in the cosmic microwave background radiation (CMBR), which is theoretically detectable by Earth-based detectors.
The hunt, as they say, is on. At present, though, we still cannot say for sure what dark matter truly is, and this study adds another compelling theory to the pile.