Earth itself may be one of the world’s largest detectors for ultralight dark matter.
Earth may already contain one of the largest dark matter experiments ever built. Instead of relying only on specialized equipment inside a laboratory, scientists have found a way to use the planet’s magnetic field, atmosphere, and surface as parts of a global detector.
The approach targets axions and dark photons, two hypothetical particles that could help explain dark matter. Astronomers infer that dark matter exists from its gravitational effects on galaxies and other cosmic structures, yet it does not produce, reflect, or absorb light in any readily detectable way. Although it appears to account for roughly one quarter of the universe’s total energy content, its physical identity remains unknown.
In the mass range examined by the researchers, axions and dark photons would be extraordinarily light, about 19 to 21 orders of magnitude lighter than an electron. If these particles surround Earth as part of the Milky Way’s dark matter halo, they could behave less like individual specks of matter and more like a faint, continuously oscillating field.
Earth Becomes a Dark Matter Detector
Many conventional axion experiments use powerful magnets to try to convert axions into detectable photons. Even the strongest laboratory systems, however, operate across a relatively small area. Earth’s magnetic field extends across the entire planet, giving researchers access to a natural experimental environment that no laboratory magnet can match.
“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 cavity between Earth’s surface and the electrically conductive ionosphere can trap and strengthen electromagnetic waves at certain frequencies. This phenomenon is best known for natural resonances excited by lightning around the world. The researchers realized that the same planetary cavity might also amplify the extremely weak electromagnetic disturbances produced by dark matter.
A New Model Expands the Search
Earlier theoretical models were reliable only below 1 Hz, leaving much of the relevant frequency range unexplored. The team addressed that problem by building a framework that includes the atmosphere’s changing electrical conductivity. Their calculations showed that dark matter signals could receive a strong boost near 8 Hz and allowed the researchers to make reliable predictions up to about 30 Hz.
The model also offered a way to distinguish possible dark matter signals from ordinary interference. An axion signal should change in strength and direction depending on the observer’s location because axions require Earth’s magnetic field to generate electromagnetic waves. The calculations indicate that Southeast Asia would offer the greatest sensitivity. A dark photon signal, meanwhile, should remain much more consistent across the planet because dark photons do not need an external magnetic field to produce the same effect.
A Decade of Geomagnetic Data
To test the idea, the researchers examined geomagnetic observations recorded from 2012 through 2022 at the British Geological Survey’s Eskdalemuir Observatory. They first removed known sources of human-made noise and then searched for persistent signals concentrated within an extremely narrow frequency range.
That narrowness is important. Many natural and artificial sources produce electromagnetic activity, but a field of ultralight dark matter should oscillate at a frequency determined by the particle’s mass. It could therefore leave a stable spectral line that remains detectable across years of observations rather than appearing as a brief burst or irregular disturbance.
After processing the Eskdalemuir data, the team used statistical tests to search for the pattern predicted for axions. The same general framework was then adapted for dark photons, which would create a different electromagnetic signature.
Stronger Axion Limits and Unexplained Signals
No axion discovery was reported. Instead, the absence of a confirmed signal allowed the researchers to place much stronger limits on how readily axions can interact with light. For a particular range of axion masses, the new ground-based constraints were about 100 times tighter than the previous best result from a terrestrial experiment.
The limits were even competitive with those obtained through X-ray observations from space telescopes such as Chandra and NuSTAR. Those astrophysical constraints can be powerful, but they depend on assumptions about distant cosmic environments. The Earth-based method provides an independent test using long-term measurements collected much closer to home.
The dark photon search produced a more puzzling result. Researchers identified several persistent signal candidates that could potentially match dark matter, although unknown environmental effects, equipment behavior, or other forms of interference may still explain them. Additional observations from multiple locations will be needed to determine whether the signals appear worldwide in the pattern predicted for dark photons.
For now, the findings do not solve the dark matter mystery. Their larger importance lies in demonstrating that Earth itself can function as a sensitive physics instrument. Networks of geomagnetic observatories around the world could eventually search for matching signals across continents, helping researchers separate a genuinely global dark matter signature from local noise.
References:
“Signature of Axion Dark Matter in Low-Frequency Terrestrial Electromagnetic Fields: Formulation and Predictions” by Atsushi Taruya, Atsushi Nishizawa and Yoshiaki Himemoto, 23 May 2026, Progress of Theoretical and Experimental Physics.
DOI: 10.1093/ptep/ptag097
“Axion Dark Matter Search from Terrestrial Magnetic Fields at Extremely Low Frequencies” by Atsushi Nishizawa, Atsushi Taruya and Yoshiaki Himemoto, 8 June 2026, Progress of Theoretical and Experimental Physics.
DOI: 10.1093/ptep/ptag108
“Searching for dark photon dark matter from terrestrial magnetic fields” by Kimihiro Nomura, Atsushi Nishizawa, Atsushi Taruya and Yoshiaki Himemoto, 11 May 2026, Physical Review D.
DOI: 10.1103/kw4j-8v12
“Hunting Axion Dark Matter Signatures in Low-Frequency Terrestrial Magnetic Fields” by Atsushi Taruya, Atsushi Nishizawa and Yoshiaki Himemoto, 26 September 2025, Progress of Theoretical and Experimental Physics.
DOI: 10.1093/ptep/ptaf136
This article was supported by the Japan Society for the Promotion of Science.
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