In the fall of 2022, global observatories detected the gamma-ray burst GRB 221009A, nicknamed BOAT, which stands for Brightest of All Time, meaning the brightest in the history of observations. The radiation stream was so powerful that it temporarily blinded most space gamma detectors. However, the mystery lies not in the signal's strength. The most energetic photons of this event, according to known laws, should not have reached our planet. The burst itself lasted about ten minutes, and its afterglow could be observed for about ten hours. The source is approximately two billion light-years away from us, and the recorded energies reached 300 tera-electronvolts. To grasp the scale, it is worth recalling: one electronvolt is the kinetic energy of an electron accelerated by a potential difference of one volt, and a tera-electronvolt equals a trillion such units. NASA specialists suggested that the burst accompanied the birth of a new black hole. Subsequent analysis showed that events with such high luminosity occur approximately once every 10,000 years. It is precisely the combination of record energy and colossal distance that posed a challenging task for theorists.
In the pages of the journal Physical Review Letters, two Italian scientists presented an interpretation that requires parting with one of the cornerstone principles of the special theory of relativity. In their scheme, photons on their way to us temporarily transformed into hypothetical axion-like particles, whose mass is extremely small. The second component of the construction involves a slight deviation from Lorentz invariance, which becomes apparent at extreme energies. Astrophysicist Giorgio Galanti from the Italian National Institute of Astrophysics, co-author of the study, explained how the search began: scientists asked a simple question about how this photon survived a journey that, according to known physics, should have destroyed it, and thus sought a theoretical scenario capable of coherently describing the observations without arbitrary corrections to the equations. Together with his colleague Marco Roncadelli from the Italian National Institute for Nuclear Physics, he states that GRB 221009A contradicts established science. Particles with such energies should collide with relic radiation, that is, the residual light of the early Universe, and annihilate, producing electron-positron pairs. Over a distance of two billion light-years, the chance of survival seems negligible. The authors compared the situation to shooting an arrow through an incredibly dense forest stretching over two billion light-years, hoping not to hit a single tree. Since the signal still arrived, the photons either avoided all obstacles or simply did not notice them, and researchers lean towards the latter explanation.
Several quantum gravity models allow some flexibility in the rules of the special theory of relativity for particles with extremely high energies. By combining such a mechanism with axion-like particles, physicists arrived at what they themselves dubbed the Universe's fast lane. Through it, ultra-light and simultaneously very energetic particles pass through obstacles as if they weren't there at all. The weak point of the hypothesis is evident, as there is still no direct evidence of the existence of axion-like particles, and the authors themselves call their work just one of many explanations for the unusual signal. Nevertheless, the model found empirical support. Calculations predicted a gap of about an hour between the arrival of the flow of less energetic photons and the photon with 300 TeV energy, and exactly the same delay was independently recorded by a Chinese observatory.