Unraveling the Mystery of Ultrahigh Energy Cosmic Rays

For over six decades, the scientific community has grappled with a cosmic enigma: the origin of Ultrahigh Energy Cosmic Rays (UHECRs). These are the highest-energy particles in the universe, possessing energies more than a million times greater than what human-made accelerators can achieve. Despite their long-known existence, a comprehensive explanation for their source has remained elusive—until now.

A groundbreaking theory introduced by New York University physicist Glennys Farrar may provide the long-sought answer. Her research, published in the prestigious journal Physical Review Letters, offers a viable and testable mechanism for the creation of UHECRs.

“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.

The Role of Binary Neutron Star Mergers

Farrar’s work suggests that UHECRs are accelerated within the turbulent magnetic outflows generated by Binary Neutron Star mergers. These mergers, which culminate in the formation of a black hole, are also responsible for the production of various precious and exotic elements, such as gold, platinum, uranium, iodine, and xenon.

As these neutron stars collide and merge, they unleash powerful gravitational waves—some of which have already been detected by scientists at the LIGO-Virgo collaboration. Farrar’s theory links these mergers to two of the most puzzling aspects of UHECRs: the strong correlation between a UHECR’s energy and its electric charge, and the exceptionally high energy of the most extreme UHECR events.

Experimental Validation and Future Research

Farrar’s proposal leads to two significant predictions that could serve as experimental validations in future studies:

1. Rare “r-process” Elements as UHECR Sources: The highest-energy UHECRs likely originate from rare elements such as xenon and tellurium. This hypothesis motivates a search for these specific components in existing and future UHECR data.
2. Extremely High-Energy Neutrinos and Gravitational Waves: The collisions involving UHECRs should generate extremely high-energy neutrinos, which, in turn, are accompanied by the gravitational waves produced by the parent neutron star merger.

Implications for Astrophysics

This new insight provides an invaluable tool for understanding the universe’s most violent and cataclysmic events. Beyond deepening our comprehension of cosmic rays, it may also illuminate the processes governing neutron star mergers, the formation of black holes, and the creation of heavy elements essential for planetary and biological development.

Farrar’s research was supported in part by grants from the National Science Foundation (PHY-2013199, PHY-2413153). As observational and experimental techniques advance, her theory stands poised to reshape our understanding of the high-energy universe, bringing us closer than ever to solving one of astrophysics’ longest-standing mysteries.