Dark matter is one of the greatest unsolved mysteries in modern physics. Astronomers are confident that it exists because of its gravitational effects, and current estimates suggest that dark matter makes up roughly one-quarter of the universe’s total mass-energy. However, scientists still do not know what dark matter is made of.
The two leading possibilities in the mass range examined by researchers are hypothetical particles known as ultralight axions and dark photons. These particles are thought to be extraordinarily light—approximately 19 to 21 orders of magnitude less massive than an electron.
Using Earth as a giant dark matter detector
Many axion experiments search for dark matter by attempting to convert axions into photons inside extremely strong laboratory magnetic fields. However, even the most powerful magnets can only cover a limited area, restricting the scale of these experiments.
Researchers from Kyoto University, Hiroshima University, and Nihon University have proposed a way to overcome this limitation. Instead of relying exclusively on laboratory equipment, the team investigated whether Earth’s natural magnetic environment could be used as part of a large-scale dark matter detector.
“We asked ourselves if we could use the Earth itself as a giant detector for exploration,” says corresponding author Atsushi Tarutani. “The Earth and ionospheric cavities act as natural resonators that amplify electromagnetic waves near the mass ranges we want to study.”
The space between Earth’s surface and the ionosphere can naturally resonate with electromagnetic waves, much like a large enclosed cavity. This property makes it useful for searching for signals associated with ultralight dark matter particles.
Expanding the dark matter search to higher frequencies
One major challenge is that earlier theoretical models could reliably describe frequencies only below 1 Hz. As a result, many potentially important frequency ranges had not been thoroughly investigated.
To address this issue, the researchers developed a new theoretical framework that accounts for electrical conductivity in Earth’s atmosphere. Their calculations indicate that the Earth-ionosphere cavity can amplify signals near 8 Hz, while also enabling reliable predictions down to approximately 30 Hz.
The model revealed key differences between the two dark matter candidates. Axion signals should vary depending on location and are expected to be strongest in Southeast Asia. Dark photon signals, by comparison, should appear with a relatively uniform intensity across the globe.
Analyzing a decade of geomagnetic data
Using their new framework, the researchers analyzed nearly 10 years of geomagnetic measurements recorded at the British Geological Survey’s Eskdalemuir Observatory between 2012 and 2022.
The team first removed artificial sources of noise from the dataset. They then searched for stable signals concentrated within extremely narrow frequency ranges—the type of long-lasting signal that ultralight dark matter could potentially produce. The researchers analyzed the results using statistical methods.
The same theoretical approach was used to search for dark photons. Unlike axions, dark photons can produce electromagnetic waves without requiring an external magnetic field. Therefore, the researchers examined the data for the specific signal characteristics expected from dark photon dark matter.
New constraints and unexplained signal candidates
By effectively turning the entire Earth into a detector for a range of axion masses, the researchers established new limits on the strength of axion interactions with light.
These constraints were approximately 100 times stronger than the best previous limits from ground-based experiments. However, the astrophysical limits depend on specific theoretical assumptions and may conflict with constraints derived from X-ray observations by space telescopes such as Chandra and NuSTAR.
The dark photon search produced especially intriguing results. Researchers identified several signal candidates that could potentially be associated with dark matter. However, the origin of these signals remains unknown, and they have not been confirmed as evidence of dark matter.
As a result, the true nature of dark matter remains unresolved. Nevertheless, this new theoretical framework could give scientists a powerful way to expand future dark matter searches by using Earth’s natural electromagnetic environment to investigate some of the lightest particles in the universe.
Source: www.sciencedaily.com