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Axions: ultralight particles
1977· Theory
SpeculativeA serious idea with no experimental proof yet.
Hypothetical particles billions of times lighter than an electron, invented to fix a puzzle in the strong force and now a leading dark-matter candidate.
See it on the Science map →How it works
Frank Wilczek named the axion in 1978 after a laundry detergent, because it cleaned up a problem. As dark matter, axions would be so light and numerous that they act less like particles than like a gentle wave filling the galaxy. In a strong magnetic field, an axion should occasionally turn into a microwave photon. ADMX, at the University of Washington, tunes a cold metal cavity like a radio dial, listening for that faint tone.
Why it matters
One hypothetical particle could solve two mysteries at once: the strong force's missing lopsidedness and dark matter. String theory also suggests many axion-like particles may exist.
How we know
None detected. ADMX has excluded the leading models over narrow mass ranges since 2018, and dozens of newer experiments scan others. Stars would also lose energy to axions; how fast real stars cool limits how strongly axions can interact.
Still a mystery
Nobody knows the axion's mass, uncertain by many factors of ten, so experiments must scan blindly, like tuning across a radio band for a station that may not exist.
Connections (2)
▸Dark matter may be made of Axions: ultralight particlesSpeculative
The relationship
Axions would not come from heat. As the young universe cooled, the axion field was left displaced from its resting point and began to oscillate everywhere at once. That oscillation behaves exactly like vast numbers of cold, slow particles. Though each axion is tiny, together they could supply all of dark matter, if the mass sits in the right range.
How the link was found
Three groups in 1983: John Preskill, Mark Wise and Frank Wilczek; Larry Abbott and Pierre Sikivie; Michael Dine and Willy Fischler. Sikivie also proposed the magnetic-field detection method that year.
Is it proven?
Not proven. The production mechanism follows from established physics if axions exist. No axion has been detected; experiments have excluded only slices of the possible mass range.
Open questions and mysteries
Did axions form before or after inflation? The answer changes their predicted mass and whether they clump into tiny 'miniclusters' that lensing might find.
▸Axions: ultralight particles would fix a symmetry puzzle in The strong force and quarksSpeculative
The relationship
The strong force's equations allow a setting, an angle, that would make neutrons electrically lopsided, slightly positive at one end. Experiments find neutrons balanced at least ten billion times better than a random setting would give. Peccei and Quinn proposed that the setting is a field that relaxes to zero by itself, like a ball settling in a bowl. Its tiny wobble is the axion.
How the link was found
Roberto Peccei and Helen Quinn, at Stanford, 1977. Steven Weinberg and Frank Wilczek realized independently in 1978 that the mechanism requires a new particle.
Is it proven?
The puzzle is solid: the best measurement, at the Paul Scherrer Institute in Switzerland (2020), finds no lopsidedness. The axion fix is elegant but untested until an axion is found.
Open questions and mysteries
The original axion was quickly ruled out; today's 'invisible' versions interact far more weakly. Other fixes exist. Why nature chose the zero setting is unsettled.