In the 1960s, astronomers theorized that the Universe was filled with a mysterious mass that did not interact normally with light, which they named “Dark Matter.” This theoretical matter is believed to constitute 80% of the Universe’s mass, largely in the form of “halos” surrounding galaxies and galaxy clusters. However, even after six decades of searching, scientists have still not found the particle that constitutes this mass. Many candidates have been proposed in that time, including Weakly-Interacting Massive Particles (WIMPs), primordial black holes, and ultralight particles known as “axions.”
Axions have emerged as a leading candidate in recent years, though scientists have yet to find evidence for their existence. However, physicists from the University of Copenhagen have devised a new method using distant galaxies that may lead to a breakthrough. Their proposed method involves using Active Galactic Nuclei (AGN), which are caused by supermassive black holes (SMBH) at their centers, as particle accelerators. By observing electromagnetic radiation emitted by these bright galactic cores as it passes through the magnetic fields of galaxy clusters, scientists may discover how axions are produced.
The team was led by Oleg Ruchayskiy, an Associate Professor with the Niels Bohr Institute at the University of Copenhagen. He was joined by researchers from the NBI, the Institute for Astronomy and Astrophysics, the Taras Shevchenko National University of Kyiv, and the Bogolyubov Institute for Theoretical Physics. The paper detailing their findings, “Constraints on axion-like particles from active galactic nuclei seen through galaxy clusters,” recently appeared in Nature Astronomy.
Lidiia Zadorozhna and Oleg Ruchayskiy from the Niels Bohr Institute search for the elusive dark matter particle, the axion, by using the Universe as a gigantic particle accelerator. Credit: Inar Timiryasov/NBI
Typically, scientists search for elementary particles using particle accelerators, like the European Organization for Nuclear Research’s (CERN) Large Hadron Collider (LHC) in Geneva, Switzerland. However, such facilities are incredibly expensive and take years to build. In the meantime, astronomers and physicists have proposed using cosmological phenomena as particle accelerators, ranging from neutron stars and black holes to colliding stellar remnants. In this latest proposal paper, Prof. Ruchayskiy and his colleagues propose using AGNs and galactic magnetic fields.
This represents a challenge, given that any axions produced in the process would appear as tiny, random fluctuations drowned out by cosmic background noise. To overcome this, the team observed 32 SMBHs in distant galaxies, which were visible thanks to the gravitational lenses created by galaxy clusters in the foreground. They then combined the data from their observations, which produced a pattern that resembled the signature of axion-like particles (ALPs). As Professor Ruchayskiy said in a University of Copenhagen press release:
Normally, the signal from such particles is unpredictable and appears as random noise. But we realized that by combining data from many different sources, we had transformed all that noise into a clear, recognizable pattern. It shows up like a unique step-like pattern that shows what this conversion could look like. We only see it as a hint of a signal in our data, but it is still very tantalizing and exciting. You could call it a cosmic whisper, now loud enough to hear.
Specifically, the combined data indicated the presence of gamma rays, which would (in theory) be released by the production of axions. While this is not definitive proof of the existence of axions, this latest research is helping narrow the search for the elusive Dark Matter particle. In addition, their experiment also presents opportunities for further research focused on other forms of radiation, such as X-rays. Said Postdoc Lidiia Zadorozhna, a Marie Curie fellow at the Niels Bohr Institute and a leading author on the paper:
This method has greatly increased what we know about axions. It essentially enabled us to map a large area that we know does not contain the axion, which narrows down the space where it can be found. We are so excited, because it is not a one-time advancement. This method allows us to go beyond previous experimental limits and has opened a new path to studying these elusive particles. The technique can be repeated by us, by other groups, across a broad range of masses and energies. That way, we can add more pieces to the puzzle of explaining dark matter.
Further Reading: University of Copenhagen, Nature