Scientists have made an unprecedented discovery about dark matter by examining anomalies in patches of warped spacetime billions of light years from Earth, reports a new study.
Dark matter is a mysterious substance that accounts for most of the universe’s mass, but which nobody has ever detected. The research provides direct evidence that dark matter could be made of ultralight particles that cumulatively act as waves, as opposed to other models that suggest this unidentified material is made up of heavier particles that do not display the wave-like behavior. The results expose new insights about the true nature of dark matter, which is considered one of the most important unresolved questions in science, and hints at the existence of new physics beyond our current theories.
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Dark matter is about five times more abundant in the cosmos than the familiar “baryonic” matter that makes up stars, planets, and all life on Earth, yet our efforts to determine what this weird stuff actually is have so far fallen short. Though dark matter does not emit light, scientists know it exists because we can see its gravitational influence on astronomical objects, such as galaxy clusters.
Now, scientists led by Alfred Amruth, a PhD student studying astrophysics at the University of Hong Kong, have shed new light on dark matter with the help of a trippy spacetime phenomenon known as gravitational lensing, which is predicted by Albert Einstein’s general theory of relativity.
Gravitational lenses are like natural telescopes that form when light from an object, such as a galaxy, is bent by the gravitational field of another object located in front of it from our perspective on Earth. Background objects can appear distorted, magnified, and even multiplied by the gravity of foreground objects, allowing scientists to see details about distant objects that would be otherwise out of view—and now, to probe for disturbances that could indicate dark matter.
“We think this is a pretty strong piece of evidence that signals something about the nature of dark matter, primarily because it uses the technique of gravitational lensing,” said Amruth in an email to Motherboard. “The other astronomical observations, usually, depend on a lot more uncertainties, but lensing is a very specific test of the distribution of mass in a system. Since dark matter only interacts via gravity (and maybe the weak force in some models), this is one of the purer tests that can be done to investigate the nature of dark matter.”
Amruth and his colleagues used gravitationally lensed images of galaxies as a way to road-test two popular hypothetical explanations for dark matter: ultralight particles known as axions, which behave like waves en masse, and much bulkier particles known as weakly interacting massive particles (WIMPs), which don’t show these wave-like properties.
Because the distribution of dark matter in a foreground object can create anomalies in the apparent positions and brightness of the background object, the team was able to search for patterns in lensed images that might point to one candidate over the other. The results revealed that axion models matched the lensed images far better than models with WIMPs, a finding that may “tilt the balance toward new physics invoking axions” as an explanation for dark matter, according to a study published on Thursday in Nature Astronomy.
“For the first time, we have used gravitational lensing to show that wave-like dark matter can indeed be tested rigorously, and it holds up to the scrutiny that we put it under with very specific tests,” Amruth said. “This opens up the path for future work that can further test wave-like dark matter, especially with the influx of lensing observations coming in from the James Webb Space Telescope, etc.”
For decades, scientists have been devising various models to explain dark matter and attempting to snag particles of this unexplained substance with a range of ambitious experiments. While the costs of these efforts have been high, the envisioned payoff is immense.
After all, unraveling the nature of dark matter would finally fill a central hole in the standard model of particle physics, which is a theoretical framework that explains the main forces and particles observed in the universe.
However, this much-anticipated achievement has remained out of reach because dark matter is so elusive and unreactive. It’s almost as if this substance is part of some shadow realm of the universe that is only tenuously connected, via gravity, to our familiar radiant world of stars and galaxies.
“Given that it’s so mysterious, and it composes the majority of mass in our Universe, I found it pretty funny and crazy that we really have no idea what it is,” Amruth said of dark matter. “The existing paradigm for dark matter is the so-called WIMP (weakly interacting massive particles) cold dark matter. However, this model has faced long-standing issues, and so a more recent novel model called wave-like dark matter is gaining traction.”
“In this model, the dark matter is composed of very light particles that act together in unison like waves, hence the name wave-like,” he continued, noting that study co-author Tom Broadhurst has pioneered many predictions about these hypothetical wave-like particles. “One of these key predictions has never been studied before is the presence of granular structures of dark matter within a galaxy or galaxy cluster. This is unique to wave-like dark matter. You could imagine this like sand on a beach, just grainy dark matter sprinkled throughout galaxies.”
The researchers realized that sharp images of lensed galaxies could potentially expose these predicted structures within the anomalies that appear in the observations. The “brightness anomaly” often shows up in special lensed images called Einstein Rings, in which a background object is duplicated into four separate images by the foreground object. Two of these lensed images might be dimmer than expected from predictions, while two others might be brighter than expected. In addition, the observed position of lensed images often clashes with predictions, which is known as the “position anomaly.”
Scientists have tried to explain these anomalies using models invoking WIMPs, but these predictions typically don’t match observed images of lensed objects. To see if wave-like axion dark matter models fared better, Amruth and his colleagues gathered observations of lensed galaxies captured by the Hubble Space Telescope and the European VLBI Network, two of the most sensitive observatories in operation,
In particular, the researchers examined an Einstein Ring image of a bright system called HS 0810+2554, though they performed a statistical analysis of other systems as well. To the team’s astonishment, the wave-like dark matter models synced up with the anomalies far better than the WIMP models.
“Turns out wave-like dark matter can indeed reproduce these anomalous observations, and that was surprising for sure!” Amruth said. “This is something that has not been done before and is a truly exciting discovery! We weren’t expecting a certain result per say, because it was a completely novel thing to be done, so we didn’t know what to expect at all.”
“However, there has been a recent growing success of wave-like dark matter in reproducing some other astronomical observations (not lensing related) that WIMP dark matter struggles with,” he added. “So we knew that new physics is definitely needed to understand and explain the nature of dark matter.”
The results bolster the hypothesis that ultralight axion particles could be the long-hidden components of dark matter, which may inform the search for this bizarre material. It also casts more doubt on the WIMP model, though it will take much more research to unveil either candidate as the secret identity of dark matter. Still, Amruth, for his part, thinks that axions may finally account for the weird cosmic phenomena that WIMPs have so far been unable to explain.
“WIMPS have been searched in laboratory experiments for many decades, with billions of dollars of funding going into them, but none have been found so far,” he noted. “Soon it would be a time of reckoning for WIMPS, and they would need to clear the stage for axions to be considered. The WIMP paradigm still has a strong hold over the physics community, but that’s the struggle new ideas need to face, like how people refused to accept Einstein’s general relativity even after it was experimentally verified.”
Amruth and his colleagues are already searching for similar signatures of wave-like dark matter around a lensed image of an exploding star, and the researchers also hope that the James Webb Space Telescope, and other next-generation observatories, will further clarify the origin and nature of dark matter.
Regardless of whether this material is composed of axions, WIMPs, or some other candidate entirely, it is clearly one of the most important pillars of our reality.
“Since dark matter composes most of the matter in our Universe, I think it is of utmost importance to figure out what it is,” Amruth said. “This would be akin to humans discovering fire, or more recently, electricity. At first, we didn’t really know what fire or electricity were, or what they could be used for. Over time, with a better understanding of the Universe, we managed to manipulate them into being useful for human civilization.”
“Now, the human species is entirely reliant on electricity,” he concluded. “Likewise, I imagine that truly understanding the nature of dark matter would propel humanity to the next level, a leap in civilization!”
