Space and Physics
PUBLISHED
Gamma-ray burst GRB221009A has a silly but very apt nickname: BOAT, or the Brightest Of All Time.
A supernova 2.4 billion light-years away exploded, shooting a jet of particles right at Earth. The energy of a single photon from that event remains a puzzle: it was more powerful than we thought and shouldn't have been able to travel those vast intergalactic distances.
A bold proposal has an explanation that challenges Einstein’s relativity.
A mysterious high-energy photon
If we were close to this event, we'd expect to see the incredible energy of this photon. But space is never truly empty, and there is enough stuff in intergalactic space that should have stopped it before it reached us on Earth.
In particular, the photons of the cosmic microwave background - the light echo of the Big Bang - are found everywhere in the universe. If you have a radio or old TV, 1 percent of the noise in a static channel is down to that very light still reverberating around the place.
The interactions between these very energetic photons and those of the cosmic microwave background should have turned the photons into other particles and made them disappear long before reaching Earth.
But after a reanalysis by the Carpet observatory in Russia, the energy of this photon was recently updated. It appears to have been even more energetic than previously thought, meaning that making sense of what has gone on is even more challenging.
“We started from a very simple question: how did this photon survive a journey that, according to known physics, should have destroyed it?”, first author Giorgio Galanti, from the Istituto Nazionale Di Astrofisica (INAF), said in a statement.
“The new data from the Carpet experiment showed us that the explanations proposed so far were no longer sufficient. We therefore looked for a theoretical scenario capable of consistently describing what we observe, without resorting to arbitrary corrections to the equations.”
Merging hypotheses
The team used a potential explanation for dark matter called axion-like particles (ALPs). Some models show that some gamma-ray photons can convert into these particles, and then turn back into photons again. The problem here is that the photons are about 100 times less energetic than the one measured in this case.
The other explanation is that one of the cornerstones of Einstein’s special relativity, called Lorentz invariance, is modified at higher energies.
There are scenarios in which, according to quantum gravity, photons at high enough energies can move with less impediment through the universe than usually thought. The combination of the two hypotheses gives a model that can explain how an extremely high-energy photon could reach Earth.
And it has a prediction: following such an event, lower-energy photons (but still gamma rays) would arrive with a delay. Indeed, this is exactly what was seen during BOAT.
“The most interesting aspect of our work is that, for the first time, it brings together two ideas that until now had been developed separately,” adds Marco Roncadelli, INFN researcher and co-author of the study.
“If future observations confirmed this scenario, the Universe would become a natural laboratory for studying quantum gravity at energies enormously higher than those achievable by any accelerator built on Earth.”
While BOAT was exceptional, it is certainly not the only gamma-ray burst out there, meaning that there will be a chance to test this idea further.
A paper discussing this hypothesis is accepted for publication in Physical Review Letters and available on the ArXiv.
