Axion Stars: Gravitational Clumps of Dark Matter ✎ express

Axions are hypothetical elementary particles introduced in 1977 by Roberto Peccei and Helen Quinn to resolve the so-called strong CP problem in the Standard Model of particle physics. They got their name from Frank Wilczek, who saw a resemblance to the Axion laundry detergent: these particles seemed to "clean up" the theory's contradictions. Today, axions are considered among the most likely candidates for dark matter — the mysterious substance that neither emits nor absorbs electromagnetic waves but reveals itself through gravity, making up about 85% of all matter in the universe.

In the early universe, right after the big bang, dark matter existed as a nearly uniform field of axions, spread out like vapor in space. However, quantum fluctuations created tiny density inhomogeneities that acted as condensation seeds, where this "vapor" began to gather into droplets. This is similar to how microscopic dust particles trigger the formation of water droplets in supersaturated vapor. Thus, under their own gravity and the Bose–Einstein condensation effect specific to bosons, axion stars formed — compact, gravitationally bound clumps invisible to optical telescopes.

These gravitational "droplets" are estimated to have absorbed up to half of all dark matter mass in the universe. This explains why terrestrial detectors don't see a uniform background of axions — most particles are locked inside axion stars and only occasionally escape. However, as they wander through the magnetic fields of galaxies, axion stars can produce powerful but brief radio bursts. This happens when axions in the star's outer layers interact with the external magnetic field and convert into photons — a process analogous to the Primakoff effect for neutral pions. Sensitive radio telescopes can pick up these signals and, by analyzing them, reconstruct the history of the expansion of the universe across different epochs.