The collision and merger of two neutron stars—extremely dense remnants of collapsed stars—are among the most energetic events in the universe. These spectacular occurrences produce a variety of signals that scientists can observe from Earth. New simulations of neutron star mergers by teams from Penn State and the University of Tennessee Knoxville indicate that the mixing of tiny particles called neutrinos can significantly alter how these mergers unfold and the emissions they generate. This has implications for understanding the origins of metals and rare earth elements, along with insights into physics in extreme environments, according to the researchers.
The paper, published in the journal Physical Review Letters, is the first to simulate the transformation of neutrino “flavors” during neutron star mergers. Neutrinos are fundamental particles that interact weakly with other matter and come in three varieties—electron, muon, and tau—named after their associated particles. Under specific conditions, such as inside a neutron star, neutrinos can change flavors, which affects their interaction with other particles.
“Previous simulations of binary neutron star mergers have not included the transformation of neutrino flavor,” explained Yi Qiu, a graduate physics student at Penn State and the paper’s first author. “This complexity arises partly because these transformations occur on a nanosecond timescale, making them hard to capture, and partly because our understanding of the underlying theoretical physics has been limited until recently. Our new simulations show that the extent and location of neutrino mixing greatly influence the matter ejected from the merger, the structure of the remnant, and the surrounding material.”
The researchers developed a comprehensive computer simulation of a neutron star merger, accounting for various physical processes, including gravity, general relativity, hydrodynamics, and neutrino mixing. They focused on the transformation from electron flavor neutrinos to muon flavor neutrinos, which is the most relevant change in this environment. They ran multiple scenarios, varying the timing, location of mixing, and the density of surrounding materials.
They discovered that these factors significantly affected the composition and structure of the merger remnant, including the types and quantities of elements created in the process. During a collision, neutrons from a neutron star can collide with other atoms, leading to the formation of heavier elements like gold, platinum, and rare earth elements used in technologies such as smartphones and electric vehicle batteries.
“A neutrino’s flavor influences its interaction with other matter,” said David Radice, Knerr Early Career Professor of Physics and co-author of the paper. “Electron type neutrinos can transform a neutron—a fundamental component of atoms—into a proton and electron, while muon type neutrinos cannot. Therefore, transforming neutrino flavors can affect the availability of neutrons, directly impacting the creation of heavier elements. Our findings suggest that accounting for neutrino mixing could enhance element production by up to tenfold.”
Neutrino mixing also affects the amount and type of matter expelled during the merger, which could change the emissions observable from Earth, including gravitational waves and electromagnetic radiation, such as X-rays or gamma rays.
“Our simulations show that neutrino mixing impacts both electromagnetic emissions from neutron star mergers and could potentially affect gravitational waves as well,” Radice added. “With advanced detectors like LIGO, Virgo, and KAGRA, along with future projects like the proposed Cosmic Explorer observatory, astronomers will be better equipped to detect gravitational waves. Understanding how these emissions arise from neutron star mergers will facilitate the interpretation of future observations.”
The researchers liken modeling the mixing processes to a pendulum turned upside down—initially, changes occur rapidly, but eventually, the system stabilizes. However, much of this remains speculative.
“There’s still a lot we don’t understand about the theoretical physics of these neutrino transformations,” Qiu noted. “As our knowledge advances, we can significantly improve our simulations. Uncertainties remain regarding the locations and mechanisms of these transformations in neutron star mergers, but our findings underscore their importance for future models and analyses.”
Now that the infrastructure for these complex simulations is in place, the researchers hope that other groups will leverage this technology to further investigate the effects of neutrino mixing.
“Neutron star mergers serve as cosmic laboratories, providing vital insights into extreme physics that cannot be safely replicated on Earth,” Radice said.
In addition to Qiu and Radice, the research team includes Maitraya Bhattacharyya, a postdoctoral scholar at Penn State, and Sherwood Richers from the University of Tennessee, Knoxville. This research was supported by funding from the U.S. Department of Energy, the Sloan Foundation, and the U.S. National Science Foundation.
Summary: Researchers from Penn State and the University of Tennessee Knoxville have simulated neutron star mergers, revealing how the mixing of neutrinos affects the types and quantities of elements produced, as well as emissions observable from Earth. Their findings provide insights into the origins of metals and rare earth elements while enhancing our understanding of the physics involved in such extreme cosmic events.
