Scientists propose axionlike particles helped ultra-energetic photon survive cosmic journey

In 2022, the Carpet ultra-high-energy-cosmic-ray array in Russia recorded a photon with 300 tera-electron-volts of energy, the highest ever seen from a gamma-ray burst. The burst, dubbed “BOAT” for its extreme brightness, originated billions of light-years away. Conventional models say such a photon should have been destroyed by interactions with the cosmic microwave background.

However, a paper in Physical Review Letters proposes that the photon temporarily transformed into ultralight axionlike particles, allowing it to travel unimpeded. This mechanism would cause the photon to arrive roughly an hour after less energetic photons, a timing difference that aligns with earlier detections of lower-energy photons by a Chinese observatory. Co-author Marco Roncadelli highlighted that confirming this scenario would turn the universe into a natural laboratory for probing quantum gravity at energies far beyond Earth-based accelerators.