New Theory Explains High-Energy Photon's Journey to Earth

Scientists have put forth a new theory to explain how an ultra-high-energy photon, originating from the brightest gamma-ray burst ever recorded (BOAT), managed to travel over 2 billion light-years to Earth without being absorbed along the way.

The gamma-ray burst, officially designated GRB 221009A, was first detected on October 9, 2022. Among the immense number of photons directed towards Earth from this event was the highest-energy photon ever detected from a gamma-ray burst. This specific photon was captured by the "Carpet" detector at the Baksan Neutrino Observatory in Russia.

The challenge was that, according to existing physics, a particle with such immense energy should not have been able to reach Earth from such a distant source. Intervening cosmic space is filled with "fossil radiation," such as photons from the cosmic microwave background (CMB). High-energy photons would almost certainly interact with these CMB photons and transform during a 2 billion light-year journey. Yet, this photon arrived.

The research team suggests that the photon may have converted into a hypothetical, extremely lightweight particle known as an axion-like particle (ALP) during its interstellar travel. Upon entering the Milky Way galaxy, it could have converted back into a photon. However, this mechanism alone did not fully account for the photon's exceptional energy.

The researchers combined the ALP hypothesis with a potential violation of "Lorentz invariance," a fundamental principle of Albert Einstein's 1905 special theory of relativity. Their conjecture is that at extremely high energies, Lorentz invariance—which dictates that the laws of physics are the same for all observers regardless of their velocity—may be broken. This hypothetical break would alter the photon's propagation behavior in space, allowing it to take a "cosmic express lane" through regions of space that would otherwise cause its destructive transformation.

This theoretical framework also accounts for observational data: the high-energy photon arrived on Earth approximately one hour later than lower-energy photons from the same gamma-ray burst, a timing consistent with the model's predictions. If future observations confirm this model, the universe could serve as a natural laboratory for studying quantum gravity at energy scales far beyond the reach of any terrestrial particle accelerator.