The hunt for wispy particles called axions, which might make up the dark matter whose gravity keeps galaxies from falling apart, is heating up. The Axion Dark Matter Experiment (ADMX) at the University of Washington in Seattle has finally reached the sensitivity needed to detect axions if they make up dark matter, physicists report today in Physical Review Letters. However, researchers don't know exactly how much axions should weigh, and it may take them years to scan the range of possible masses.
An axion is a hypothetical particle that was invented 41 years ago to solve a problem in the theory of the strong nuclear force, which binds particles called quarks to make protons and neutrons. The axion could pull double duty, however, and supply the dark matter, which cosmological studies show makes up 85% of all matter. So far, dark matter has revealed itself only through its gravity, so one of the biggest mysteries in physics is what the particles that make up dark matter are.
If dark matter consists of axions floating around, then physicists ought to be able to detect them with essentially a strong magnetic field and an incredibly sensitive radio. The magnetic field will convert the axions into photons, and because the axions are very light, those photons will have very low radio frequencies and should provide an ultra-faint radio hum at a distinct frequency. In their new result, ADMX researchers rule out axions in the range from 2.66 microelectron volts to 2.82 microelectron volts—about 5 trillionths the mass of the electron. If dark matter consists purely of axions, then the particles must have a mass between about 1 microelectron volts and 100 microelectron volts, theorists think. So ADMX researchers will now sweep the frequency of their elaborate radio antenna upward as far as they can, to about 40 microelectron volts. Stay tuned.
*Correction, 10 April, 9:50 a.m.: An earlier version of this story incorrectly abbreviated microelectron volts. The tested mass range as a fraction of the electron mass has also been corrected.