In October 2022, observatories around the world recorded a gamma-ray burst GRB 221009Awhich received the nickname BOAT (Brightest of All Time, “brightest of all time”). The radiation was so powerful that it temporarily blinded most space gamma detectors.
However, the main mystery is not the brightness, but the fact that the most energetic photons of this burst, in principle, should not have reached the Earth.
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IN articlepublished in the journal Physical Review Letters, two Italian physicists have proposed an explanation that requires abandoning one of the cornerstone principles of special relativity. According to their model, photons on their way to us temporarily turned into hypothetical axion-like particles with extremely low mass. The second element of the theory is a slight deviation from Lorentz invariance at extreme energies.
Astrophysicist Giorgio Galanti from the Italian National Institute of Astrophysics, co-author of the study:
We started with a very simple question: how did this photon survive the journey that, according to known physics, should have destroyed it? So we looked for a theoretical scenario that could consistently describe what we observed without making arbitrary adjustments to the equations.
The burst itself lasted about ten minutes, and the afterglow remained visible for about another ten hours. The source was located at a distance of about two billion light years, and recorded energies reached 300 teraelectronvolts. For comparison, one electronvolt is the kinetic energy of one electron accelerated by a potential difference of one volt, and a teraelectronvolt is equal to a trillion electronvolts.
NASA scientists suggested that the outbreak accompanied the birth of a new black hole. Subsequent analysis showed that events of this luminosity occur approximately once every 10 thousand years. It was this combination of record energy and enormous distance that created a problem for theorists.
Galanti and his colleague Marco Roncadelli from the Italian National Institute of Nuclear Physics note that GRB 221009A is “at odds” with accepted physics. Photons of such energies are bound to collide with relict radiation, the residual light of the early Universe, and annihilate, generating electron-positron pairs. At a distance of two billion light years, the chance of survival should be vanishingly small.
The authors of the work compared the situation to shooting a bow through an extremely dense forest two billion light years long, hoping not to hit a single tree. Once the signal did arrive, the photons either avoided all the obstacles or simply did not notice them. Researchers are inclined towards the second option.
Some models of quantum gravity allow little flexibility in the rules of special relativity for extremely high-energy particles. By combining this mechanism with axion-like particles, physicists have achieved what they call the “fast lane” of the Universe. Along it, ultra-light and very energetic particles rush through obstacles as if they did not exist.
The weak point of the idea is obvious – there is no direct evidence of the existence of axion-like particles, and the authors themselves call their work just one of many attempts to explain the unusual signal.
However, the model has found empirical support. Calculations predicted a gap of about an hour between the arrival of a stream of less energetic photons and a photon with an energy of 300 TeV on Earth, and exactly this delay was independently recorded by the Chinese observatory.
