Physicists from Pennsylvania State University and the University of Tennessee, Knoxville, have made a breakthrough in understanding the most powerful catastrophes in the cosmos. The results of their study, published in Physical Review Letters, describe the world’s first computer simulations of neutron star mergers that account for the phenomenon of neutrino “flavor” transformation. It turned out that these tiny particles can radically alter the chemical composition of the ejected matter, the characteristics of gravitational waves, and the amount of heavy elements produced—from gold to materials used in modern smartphones.
Neutron stars are ultradense remnant cores formed after supernova explosions during the deaths of massive stars. Although their mass exceeds that of the Sun, they measure only a few dozen kilometers in diameter. When two such stars orbit one another and eventually collide, one of the most energetic events in the universe occurs.
At the moment of the cataclysm, an enormous amount of energy is released in the form of gravitational waves—ripples in the very fabric of spacetime—and powerful electromagnetic radiation, including gamma rays and X-rays. In addition, these catastrophic mergers are considered the main cosmic “factories” where heavy chemical elements are forged.
Quantum Chemistry
Neutrinos are almost weightless, electrically neutral particles that interact extremely weakly with matter and can travel freely across astronomical distances. They exist in three varieties, or “flavors”: electron, muon, and tau neutrinos. Under certain extreme conditions, neutrinos can change their “flavor,” transforming from one type into another.
Because each type of particle interacts differently with its surroundings, these quantum transformations alter the ratio of neutrons to protons in stellar matter. Electron neutrinos can be absorbed by neutrons, converting them into protons and electrons, whereas muon neutrinos cannot do so.
Previous simulations of stellar mergers ignored these processes because they occur on macroscopically tiny nanosecond timescales, while the corresponding theoretical physics extends beyond the Standard Model. Through the development of numerical relativistic simulations, scientists were able to combine Einstein’s general theory of relativity, the hydrodynamics of ultradense plasma, and quantum neutrino mixing, primarily the transformation of electron neutrinos into muon neutrinos.
The results surprised astrophysicists: accounting for neutrino flavor transformations showed that the production of heavy metals and rare-earth elements during nucleosynthesis could increase by as much as a factor of ten. When neutron-rich matter escapes into open space, rapid neutron capture produces gold, platinum, and rare elements commonly used in microelectronics manufacturing.
A New Window into Observing the Cosmos
Neutrino mixing affects not only the formation of precious metals in space, but also the structure of the collision remnant and the radiation detected by instruments on Earth. Changes in the distribution of matter leave specific signatures in the spectra of gamma-ray and X-ray bursts and may also modify the gravitational-wave signal.
Today, the international LVK gravitational-wave observatory network, which includes the LIGO, Virgo, and KAGRA detectors, alerts the scientific community to cosmic collisions in real time through NASA’s GCN system. In the 2030s, they are expected to be joined by an even more sensitive next-generation project, Cosmic Explorer.
A more precise understanding of neutrino microphysics will help astronomers interpret the most complex signals received from these observatories. The simulation platform developed by the scientists is now open to the global scientific community, turning neutron star mergers into unique natural laboratories for studying physics at the limits of what is possible.
Earlier, we reported on how gold on Earth has been rising from the planet’s core for billions of years.
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