A Radio for Galactic Whispers
At the University of Washington's Center for Experimental Nuclear Physics and Astrophysics (CENPA), scientists are listening for a signal from the cosmos that could solve one of physics' biggest mysteries. The Axion Dark Matter eXperiment (ADMX) is not a telescope, but a highly specialized radio receiver, designed to detect a hypothetical particle called the axion. If axions exist, they are likely a major component of dark matter, the invisible substance that constitutes about 85% of the universe's mass.
The detection method, first proposed by physicist Pierre Sikivie in 1983, relies on a strong magnetic field to convert axions into microwave photons (particles of light). ADMX employs an 8-tesla superconducting magnet to create a field about 150,000 times stronger than Earth's. Inside this magnet sits a cylindrical copper-plated microwave cavity, about one meter long and half a meter in diameter. This cavity is a resonator. Researchers slowly adjust its resonant frequency using two internal tuning rods. If the cavity's frequency matches the mass of an axion passing through from the galaxy's dark matter halo, the axion should convert into a photon, producing a minuscule power deposit, less than a yoctowatt (10⁻²⁴ watts).
To detect such an impossibly faint signal, the entire experiment is cooled with a liquid helium dilution refrigerator to temperatures below 4.2 Kelvin, and in some stages, to just a tenth of a degree above absolute zero. This extreme cold minimizes thermal noise that would otherwise drown out the signal. The faint microwave light is then captured by a tiny antenna and amplified by quantum-limited devices, such as Superconducting Quantum Interference Devices (SQUIDs) or Josephson Parametric Amplifiers.
The Search and its Status
The ADMX experiment began its life at Lawrence Livermore National Laboratory in 1995 before moving to the University of Washington in 2010 for significant upgrades. It is now the world's most sensitive experiment for detecting plausible dark matter axions over a wide range of masses, specifically in the 1 to 40 micro-electronvolt (μeV) range. To put this in perspective, an electron's mass is about 511,000 electronvolts.
The experiment operates in long data-taking runs, some lasting for nine months at a time, methodically scanning millions of frequency channels. Each frequency is checked for about 100 seconds before the tuning rods shift the cavity to the next frequency. This slow, meticulous process has allowed the ADMX collaboration to rule out certain mass ranges where axions could have been hiding. For instance, data published in 2018 and 2019 eliminated axion models within the 2.66 to 3.33 μeV range.
ADMX is a large international collaboration, involving researchers from institutions like Fermilab, Lawrence Livermore National Laboratory, Pacific Northwest National Laboratory, and the University of California, Berkeley, among others. While ADMX has not yet found the axion, each result tightens the constraints on the particle's possible properties. The ongoing search represents a definitive effort: if dark matter axions with the predicted characteristics exist in the frequency bands being scanned, ADMX is designed to find them.