Outer space is filled not only with dust, but also with elementary particles moving in various directions. Particles with high energy can only travel limited distances before losing their energy.

In October 2022, Earth recorded the brightest gamma-ray burst ever observed—GRB 221009A. This burst delivered photons to terrestrial detectors that originated from an explosion about two billion light-years away from our planet.

Within this radiation stream, researchers using the Carpet detector at the Baksan Observatory discovered a unique photon with an energy of approximately 300 teraelectronvolts. This is the highest-energy photon from a gamma-ray burst ever detected. According to current physical models, such a photon should not have been able to reach Earth.

During its journey across the Universe, the photon should have interacted with the cosmic microwave background—the relic radiation left over from the Big Bang that permeates space. It was expected that the particle would undergo transformations and collisions, making its arrival on Earth extremely unlikely.

There are hypotheses about possible mechanisms that allowed the photon to traverse this distance. One such idea is the temporary transformation of the photon into an axion-like particle—a hypothetical, extremely light elementary particle—with a subsequent return to its original state near the Milky Way. However, previous models of this kind only explained photons with energies of several tens of teraelectronvolts, which is insufficient to account for the observed phenomenon.

Astrophysicists have proposed a new model that combines axion-like particles with a violation of Lorentz invariance—one of the fundamental principles of special relativity. In certain quantum gravity models, at extremely high energies, a photon can avoid interactions that, according to the standard model, would have destroyed it during its journey. According to this new model, such photons are able to travel along a kind of "express lane."

This model does not require changing the fundamental laws of physics, but rather takes into account specific effects that cannot be observed at lower energies.

In addition to explaining how the photon reached Earth, the model predicts a new effect: a photon with an energy of 300 teraelectronvolts should have arrived about an hour later than lower-energy photons. This effect was confirmed by data collected by the Chinese Large High Altitude Air Shower Observatory (LHAASO) on the same day.

A scientific article detailing these results has been accepted for publication in the journal Physical Review Letters.