At the Brookhaven laboratory on Long Island, physicists have recorded an unexpected signal in heavy-ion collision data. The STAR experiment at the RHIC accelerator showed a sharp drop in fluctuations of the transverse momentum of particles at a certain energy. This signal may point to a phase transition in superdense nuclear matter, similar to the one that occurred in the first moments after the Big Bang.
At the very beginning of the Universe, space was too hot and dense for atoms to exist. Protons and neutrons had not yet formed, and matter was a freely flowing "soup" of quarks and gluons — quark-gluon plasma. As space expanded, the temperature fell, gluons "glued" quarks into protons and neutrons, and the plasma turned into ordinary nuclear matter. It was a genuine phase transition, similar to the freezing of water, but at the level of fundamental particles.
The RHIC accelerator recreates such conditions for fractions of a second, accelerating gold ions almost to the speed of light. New STAR data have revealed a "dip" in the fluctuations of particle momentum precisely in the energy region where the critical point of the phase transition is expected. Scientists call this a possible sign that the transition really exists and depends on density and temperature.
The discovery is important not only for cosmology. Understanding the behaviour of matter at extreme densities will help to figure out what happens inside neutron stars, where the entire mass of a star is compressed to the size of a small city. This determines, for example, the speed of sound in their core and the threshold for collapse into a black hole.
However, the signal so far remains a hint. Alternative explanations link it to a change in the composition of particles at different collision energies. Scientists plan to analyse the remaining data from RHIC, which ceased operation in 2026, in order to confirm or refute the finding.
If the critical point is reliably established, it will open a new view of the evolution of matter from the first microseconds of the Universe to the densest objects in space.
