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. 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.
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