Dark matter is one of the biggest unsolved mysteries in modern physics. Astronomers are highly confident that it exists and estimate that it accounts for about a quarter of the universe’s total energy content, yet scientists still do not know what it is made of.
Two leading possibilities are hypothetical particles known as ultralight axions and dark photons. In the mass range examined by the researchers, these particles would be extraordinarily light, roughly 19 to 21 orders of magnitude lighter than an electron.
Turning Earth Into a Giant Dark Matter Detector
Many traditional axion experiments try to convert axions into photons by exposing them to extremely strong magnetic fields inside laboratories. The challenge is scale. Even powerful lab magnets can only cover a relatively small region.
Researchers from Kyoto University, Hiroshima University, and Nihon University saw a way around that limitation. Instead of relying only on laboratory equipment, they asked whether Earth’s own magnetic environment could be used as part of the experiment.
“We asked ourselves whether we could use the Earth itself as a giant detector in the search,” says corresponding author Atsushi Taruya. “The Earth-ionosphere cavity acts as a natural resonator that amplifies electromagnetic waves right around the mass range we wanted to probe.”
The region between Earth’s surface and the ionosphere can naturally resonate with electromagnetic waves, much like a large cavity. That made it especially useful for searching for signals associated with the ultralight particles the team wanted to investigate.
Expanding the Search to Higher Frequencies
One obstacle was that previous theory could only reliably describe frequencies below 1 Hz. That left much of the potentially useful frequency range unexplored.
To solve this problem, the researchers developed a new theoretical framework that includes the electrical conductivity of the atmosphere. Their calculations showed that the Earth-ionosphere cavity can amplify signals near 8 Hz and allowed them to make reliable predictions up to about 30 Hz.
The model also predicted an important difference between the two dark matter candidates. Signals produced by axions should vary depending on location, with the strongest expected in Southeast Asia. Dark photon signals, by contrast, should appear at nearly the same strength around the world.
A Decade of Magnetic Data Put to the Test
Using this framework, the team examined roughly 10 years of geomagnetic measurements collected between 2012 and 2022 by the British Geological Survey’s Eskdalemuir Observatory.
The researchers first removed artificial sources of noise from the data. They then looked for the kind of steady signal concentrated within a very narrow frequency range that dark matter is expected to produce over long periods of time. The results were then subjected to statistical analysis.
The same theoretical approach was also applied to dark photons. Unlike axions, dark photons can produce electromagnetic waves even when no magnetic field is present, so the researchers searched the dataset for the different signature those particles would be expected to create.
Stronger Limits and Mysterious Signal Candidates
By effectively using the entire Earth as a detector for a particular range of axion masses, the researchers placed new limits on how strongly axions could interact with light.
Those limits were about 100 times tighter than the previous best result from a ground-based experiment. They were also competitive with constraints inferred from astrophysical X-ray observations made by observatories such as Chandra and NuSTAR, although those astrophysical limits depend on certain theoretical assumptions.
The dark photon search produced an especially intriguing result. Researchers identified several signal candidates that could potentially have a dark matter origin. However, the source of those signals remains unknown, and they have not been confirmed as evidence of dark matter.
Dark matter’s true identity therefore remains unresolved. Still, the new theoretical framework could give researchers a powerful way to expand future searches and use Earth’s natural electromagnetic environment as a tool for probing some of the lightest possible forms of dark matter.
