Scientists have utilized Earth's magnetic field and atmosphere as a large-scale detector to search for ultralight dark matter particles. This new approach dramatically improved the limits on ultralight axions. It also uncovered several intriguing dark photon signals.
Dark matter is a significant unsolved mystery in modern physics. Astronomers estimate it comprises about one-quarter of the universe's total energy content. However, its composition remains unknown. Two leading candidates are hypothetical particles called ultralight axions and dark photons. These particles would be extremely light, roughly 19 to 21 orders of magnitude lighter than an electron.
Traditional axion experiments attempt to convert axions into photons using strong magnetic fields in laboratories. These experiments are limited by scale. Researchers from Kyoto University, Hiroshima University, and Nihon University developed a method to use Earth's natural magnetic environment. The Earth-ionosphere cavity acts as a natural resonator, amplifying electromagnetic waves. This makes it useful for detecting signals from ultralight particles.
The researchers developed a new theoretical framework. This framework includes the electrical conductivity of the atmosphere. Their calculations showed the Earth-ionosphere cavity can amplify signals near eight Hertz. It also allowed for reliable predictions up to about 30 Hertz. The model predicted that axion signals would vary by location, with the strongest in Southeast Asia. Dark photon signals, conversely, should appear with similar strength globally.
The team analyzed ten years of geomagnetic measurements from the British Geological Survey's Eskdalemuir Observatory. They removed artificial noise and searched for steady signals within narrow frequency ranges. These are characteristic of dark matter. The results were then statistically analyzed. The same theoretical approach was applied to dark photons.
This method placed new limits on how strongly axions could interact with light. These limits were approximately 100 times tighter than previous ground-based experiments. The dark photon search identified several signal candidates. Their origin is currently unknown. They have not been confirmed as evidence of dark matter. The new theoretical framework offers a powerful way to expand future dark matter searches.