What Neutron Star Interiors Reveal About Dark Matter ✦

Neutron stars are the remnants of massive stars after a supernova explosion. Predicted by Fritz Zwicky and discovered as pulsars by Jocelyn Bell Burnell, they are compressed so densely that even spacetime curvature becomes tangible. Their mass is limited — a fact realized early on by Chandrasekhar. Today, neutron stars serve as a laboratory for searching for axions — particles from quantum field theory that are candidates for the invisible mass of the universe.

If axions are born in the interior, they carry away heat, and the star cools faster, increasing its entropy — akin to how a cold spoon accelerates the cooling of tea. A new analysis accounted for the possibility that the core may contain not only ordinary particles but also exotic ones (hyperons). It turned out that the estimated axion mass is almost independent of the composition — the upper limit holds firmly at around 10–20 meV, within reach of the future IAXO experiment. Just as the spoon's material (steel or silver) has little effect on the cooling rate. Surprisingly, observations of gravitational waves and photometry currently favor the absence of axions. However, if the particle is found, its precise mass will allow us to peer into the heart of the star and see whether anything unusual lies there.