Physicists have turned the entire planet into a dark matter detector, and the experiment has returned dozens of candidate signals that match predictions for certain hypothetical particles. The signals still require verification and could have other explanations, but the approach offers a clever new way to probe one of cosmology’s most baffling puzzles.
Why Dark Matter Remains Elusive
Evidence abounds that the Universe contains more than meets the eye. Models of how visible matter spreads and moves through the cosmos do not quite match its actual distribution and motion. Physicists suspect an invisible realm of matter contributes mass to the stuff we can see, producing the observed gravitational effects. Because it neither reflects nor emits light, scientists call it ‘dark matter.’
Identifying a gap in the models is one thing; filling it is another. A wide range of potential candidates exist, each with different characteristics. One of the most promising is the axion. First proposed in the 1970s to solve a different problem in particle physics, axions were later recognized as possible dark matter if they exist within a certain mass range.
Unlike many other dark matter candidates, axions are predicted to interact with the electromagnetic force, which could give them away. According to models, axions should sometimes decay into photons in strong magnetic fields. Astronomers have therefore searched the cosmos for signatures around neutron stars or supernovae. But recent papers by physicists in Japan suggest we might not need to look so far.
Using Earth as a Giant Detector
“We asked ourselves whether we could use the Earth itself as a giant detector in the search,” says theoretical physicist Atsushi Taruya, an author on all four papers. “The Earth-ionosphere cavity acts as a natural resonator that amplifies electromagnetic waves right around the mass range we wanted to probe.”
To test the idea, the researchers analyzed data on Earth’s magnetic field collected by the British Geological Survey between 2012 and 2022. If axions are streaming over and through the planet, the team hypothesized, they should resonate between Earth’s surface and the ionosphere, generating electromagnetic waves at a frequency corresponding to the particles’ mass. Decades of previous studies have constrained the possible masses of axions fairly tightly, so the researchers checked the decade of data for signals within that narrow range.
After accounting for background noise, the team identified 65 axion signal candidates. Even after tightening their statistical filters, 25 candidates remained.
Dark Photons Enter the
Axions are not the only dark matter candidate this technique could detect. Another contender is the dark photon, hypothesized to be a force carrier in the shady realm beyond the Standard Model. Unlike regular photons, dark photons might have mass and could interact with magnetic fields in a similar way to axions.
In a follow-up study, the team used a similar method to search for dark photon signals in the same data. Again, they reported candidate signals: as many as 342 using the loosest criteria, which narrowed to 31 with a more stringent signal-to-noise ratio.
Distinguishing Between Axions and Dark Photons
The problem is that the current data cannot distinguish between dark photon and axion signals. One key difference is that axions depend on Earth’s magnetic field to produce their signal, whereas dark photons would produce theirs with or without it. That opens a way to test which form of dark matter—if any—is currently streaming through Earth.
If the signal looks exactly the same anywhere on the planet, it points to dark photons. If the signal strength varies by location, it is more likely to be axions, because Earth’s magnetic field varies by location. The researchers say this should mean the axion signal would weaken near the planet’s poles and be strongest around Southeast Asia.
Unfortunately, the data came from a single observatory in the UK, so the team cannot get that global perspective. Future studies by other experiments could help shed more light on dark matter.
The three papers on axions were published in the journal Progress of Theoretical and Experimental Physics, while the dark photon study was published in Physical Review D.
