New theory suggests star mergers produce universe’s highest-energy particles

Ultrahigh Energy Cosmic Rays are the highest-energy particles in the universe, whose energies are more than a million times what can be achieved by humans. But while the existence of UHECRs has been known for 60 years, researchers have not succeeded in formulating a satisfactory explanation for their origin that explains all the observations.

But a new theory introduced by New York University physicist Glennys Farrar provides a viable and testable explanation for how UHECRs are created.

“After six decades of effort, the origin of the mysterious highest-energy particles in the universe may finally have been identified,” says Farrar, a Collegiate Professor of Physics and Julius Silver, Rosalind S. Silver, and Enid Silver Winslow Professor at NYU. “This insight gives a new tool for understanding the most cataclysmic events of the universe: two neutron stars merging to form a black hole, which is the process responsible for the creation of many precious or exotic elements, including gold, platinum, uranium, iodine, and xenon.”

The work, which appears in the journal Physical Review Letters, proposes that UHECRs are accelerated in the turbulent magnetic outflows of Binary Neutron Star mergers—spewed out from the merger remnant, prior to formation of the final black hole. The process simultaneously generates powerful gravitational waves—some already detected by scientists at the LIGO-Virgo collaboration.

Farrar’s Physical Review Letters proposal explains, for the first time, two of the most mysterious features of UHECRs: the tight correlation between a UHECR’s energy and its electric charge and the extraordinary energy of a handful of the very highest energy events.

Stemming from Farrar’s analysis are two consequences that can provide experimental validation in future work:

• The very highest energy UHECRs originate as rare “r-process” elements, such as xenon and tellurium, motivating a search for such a component in the UHECR data.
• Extremely high-energy neutrinos, originating from UHECR collisions, are necessarily accompanied by the gravitational wave produced in the parent neutron star merger.