This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: How did a photon survive a journey of more than 2 billion light-years when, according to known physics, it should have been absorbed long before reaching Earth? This is the question at the heart of a new study by Giorgio Galanti (INAF) and Marco Roncadelli (INFN), accepted for publication in Physical Review Letters.
The research addresses one of the most fascinating puzzles in contemporary astrophysics. It proposes a possible explanation for the highest-energy photon ever observed from a gamma-ray burst, suggesting that under extreme conditions spacetime may behave differently from what Einstein's theory of relativity predicts. At the center of the story is the brightest gamma-ray burst ever observed, GRB 221009A, nicknamed BOAT by astronomers—the Brightest Of All Time.
The explosion, which occurred about 2 billion light-years from Earth and was observed on Oct. 9, 2022, reached our planet as an enormous shower of photons, the elementary particles of light. Among them was one truly exceptional photon. Carpet, an ultra-high-energy cosmic-ray detector located at the Baksan Observatory in the Russian Caucasus, detected it, and it carried an energy of about 300 teraelectronvolts (TeV), the highest ever observed for a photon from a gamma-ray burst.
This record-breaking energy challenges current models. According to known physics, such a photon should not have been able to reach Earth. The reason is that the universe is not completely empty.
During its journey, the photon should have interacted with photons from the cosmic microwave background, the faint relic radiation from the Big Bang that permeates the entire universe, transforming into other particles and disappearing long before reaching Earth. It is like shooting an arrow through an extremely dense forest stretching for 2 billion light-years and expecting it not to hit a single tree. According to current physics, this is virtually impossible.
And yet, the photon made it all the way to us. In recent years, one possible solution has involved axion-like particles (ALPs), extremely light hypothetical particles. In this scenario, photons could temporarily convert into ALPs while traveling through the universe and then convert back into photons near the Milky Way.
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