Physicists don’t know where most of the universe’s apparent mass has gone, and attempts to find it have so far failed. But a proposed particle born out of the universe’s chaotic first moments may provide a candidate and a reasonable way to look for it.
This proposal suggests that dark matter formed before the Big Bang, but in a jargon-y, physicist sense of the phrase—the hypothetical particle does not predate the universe itself. Instead, what’s exciting is that this (comparatively simple) dark matter theory is compatible with the decades of tests that have constrained what dark matter might actually look like, as well as with the present-day understanding of the universe. Most importantly, this theory is testable.
The candidate particle leaves “a unique imprint on the large-scale structure of the universe, that is, on the distribution of galaxies and galaxy clusters,” study author Tommi Tenkanen from Johns Hopkins University told Gizmodo in an email. “This makes the hypothesis testable with astronomical observations in the near future.”
You can read our primer on the state of dark matter here, but basically, astronomical observations of distant galaxies and the universe’s structure imply that there’s some source of gravity permeating the universe that experiments can’t directly detect. This gravity source far outweighs the matter that makes up Earth and all of the universe’s stars and galaxies. Scientists call this stuff dark matter, even though today’s dark matter candidates aren’t as dark as they are transparent. Particle colliders and detectors buried deep underground have failed to find conclusive evidence of any dark matter candidates, the most popular of which are called WIMPS, or Weakly-Interacting Massive Particles.
Tenkanen instead theorized that things called scalar particles formed as the universe was rapidly inflating during its first split second—not before the start of the universe, but just before the start of the era that some physicists call the “Big Bang epoch” which occurs after inflation. During this inflationary period, fields called scalar fields could have filled the universe, and if the inflation itself wasn’t uniform, it might have introduced fluctuations to the field. These fluctuations correspond to massive scalar particles that can only interact with matter through gravity and would still exist today, according to the theory. WIMPs, unlike these scalar particles, would have formed after the inflationary era was over.
This probably sounds the same as every other dark matter candidate you’ve read about—it’s a particle we haven’t discovered yet. But its elegance is in the details, in that this particle fits within the existing constraints of what dark matter can and cannot be based on experiments and observations of the most distant visible light, called the cosmic microwave background. Plus, it was devised using similar mathematical tools to those that govern the Higgs boson, another particle that corresponds with a scalar field.
“Tenkanen’s paper adds a lot to our understanding of scalar particle dark matter, giving the most detailed study of the constraints imposed by the cosmic microwave background that I have seen,” Alan Guth, MIT theoretical physicist who developed the popular theory of cosmic inflation, told Gizmodo in an email.
This theory is one of a number of similar ideas, according to Tenkanen’s paper, published in Physical Review Letters. However, this paper shows, for the first time, that a theory like this can work without conflicting with cosmic microwave background data.
And all of that is exciting. “What’s so refreshing about this latest paper is that it’s possible to put together a model of dark matter that relies less on thoroughly untested hypothetical ideas, that still matches many of the observational constraints we need dark matter to meet,” David Kaiser, professor of physics at MIT, told Gizmodo.
The paper itself is not a discovery, of course; it’s just a theory, which is more or less the physicist term for a mathematically consistent hypothesis. But it also provides a signature for astronomers to look for in the sky that could falsify the hypothesis. “The model proposed here is interesting, for me, primarily as it is testable,” Priyamvada Natarajan Yale professor in astronomy and astrophysics, told Gizmodo. If dark matter interacts too strongly with itself, then the theory doesn’t work. It also predicts certain small changes to the universe’s structure that would appear in upcoming telescopes.
Tenkanen predicts that the Euclid dark matter-mapping satellite might be able to provide some of these answers once it launches in 2022.
This article was updated with comments from Alan Guth.