The unexpectedly powerful light from the brightest space explosion ever seen may mean that space is more transparent than we thought.
Last year, astronomers witnessed a flash of gamma rays that was brighter than anything they had previously measured. By comparing it with other gamma ray bursts, researchers estimated that such a bright event would happen only once every 10,000 years.
This flash, called GRB221009A, was made of up of photons, or light particles, some of which had very high energy levels. But astronomers didn鈥檛 know whether there were some photons emitted in the blast that were so energetic that they couldn鈥檛 be detected by Earth-based telescopes.
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Now, at the Chinese Academy of Sciences in Beijing and his colleagues have detected extremely energetic photons from GRB221009A indirectly, by looking for showers of particles produced when such photons hit Earth鈥檚 atmosphere.
Gamma ray photons are unique in that they can be reflected by other photons. Because of this, the photons observed by the researchers should have been reflected by light that fills the universe from star formation 鈥� called extragalactic background light颅 鈥� long before they reached Earth.
鈥淚f I take the James Webb Space Telescope, I can look back and see optical and infrared photons all the way back to the very early universe, but I can鈥檛 see the very high-energy photons that far because those photons themselves scatter off other photons,鈥� says at Radboud University in the Netherlands. 鈥淚t鈥檚 like looking through a mist.鈥�
There is a chance that the showers of particles the astronomers detected aren鈥檛 actually caused by photons, but by different particles like energetic muons, says Levan. Or the photons might not be as high energy as they appear, because detecting photons through atmospheric showers is difficult.
If the photons really are as energetic as they seem to be, it would mean the universe is more transparent than we thought, with less extragalactic light.
Another explanation could require new physics. For example, the photons could have been converted into a different type of particle that doesn鈥檛 interact with extragalactic light for their journey across space. That might be a hypothetical ultralight particle called an axion, which could explain a swath of mysteries such as dark matter or symmetry violations in particle physics.
This axion would interact with the magnetic field of a galaxy, such as the Milky Way, and convert back into a photon once it reaches that galaxy. 鈥淚t has the same net observational effect 鈥� it makes the universe more transparent,鈥� says at the University of Oxford. 鈥淵ou can have as much extragalactic background light as you want, effectively, if you鈥檝e got an axion travelling through it rather than a high-energy photon.鈥�
The astronomers who made the measurements estimate there is roughly a 5 per cent chance the photons were actually something else, such as cosmic rays, so we will need many more measurements of high-energy photons before new physics can be invoked, says Mummery. Unfortunately, that requires a certain degree of luck, as events that produce photons with energies this high are exceedingly rare, he says.
arXiv
