Field Note — Physics
The Photon That Shouldn’t Be Here
The brightest explosion ever recorded may have sent one particle of light that the universe was supposed to block. If it is real, it points to physics beyond the Standard Model.
On October 9, 2022, a burst of gamma rays reached Earth from about two billion light-years away. It is the brightest gamma-ray burst ever recorded, and astronomers nicknamed it the BOAT: brightest of all time. A gamma ray is light with far more energy than anything the eye can see, and a burst is a short, enormous flash of it, thought to come from the death of a massive star. This one is catalogued as GRB 221009A.
Space is not dark. Two faint glows fill it. One is the cosmic microwave background, the afterglow of the Big Bang, released about 380,000 years after it and stretched since into cold microwaves. The other is the extragalactic background light, or EBL: the total of all the starlight ever given off, plus the warm glow of dust, still travelling through space. One glow is from before the first star. The other is from every star since.
Over billions of light-years those glows add up to a fog. Now and then a gamma ray runs into one of their particles of light, and the pair turns into an electron and its antimatter twin, a positron. The gamma ray is gone. The rule: the more energetic the gamma ray, the colder and fainter the glow that can do it. So the more energy a gamma ray carries, the less likely it is to survive a long trip.
In China, an observatory called LHAASO recorded more than sixty thousand particles of light from this burst, with energies up to about thirteen TeV. A TeV is a trillion electron volts, and visible light carries about two electron volts, so these were several trillion times more energetic. At this distance the starlight fog is expected to turn opaque near fifteen TeV. LHAASO’s photons stop just short of that wall.
Then a second observatory, Carpet-3, high in the Caucasus mountains of Russia, reported something else: a single event of about three hundred TeV, roughly twenty-three times the top of LHAASO’s range. It is the highest energy ever linked to any gamma-ray burst. A detector never catches the gamma ray itself. It catches the shower of particles that the gamma ray sets off in the air, and this shower had almost none of the muons that a cosmic-ray shower normally carries. That is what a gamma ray looks like. By the Carpet-3 team’s own account, if gamma rays travel as the Standard Model says, one of this energy cannot reach us from outside our galaxy.
Time is the second problem. Call zero the moment the Fermi satellite first saw the burst. LHAASO’s photons began arriving about four minutes later and faded within roughly fifty minutes. The Carpet-3 event arrived at 4,536 seconds, about seventy-five minutes in, after the afterglow LHAASO tracked had faded. Physicists Ofengeim and Piran note that no known astrophysical process explains such a late arrival. So the event has to cross the fog and arrive late.
The Standard Model is physicists’ working rulebook for every known particle and force. Two ways around the problem both reach past it. One is a hypothetical particle, an axion-like particle, that a gamma ray could turn into and back, slipping past the fog on the way. By itself it falls short at three hundred TeV. The other is that Einstein’s relativity holds only approximately at extreme energies, a possible low-energy trace of quantum gravity, which would move the wall farther out. A 2026 study in Physical Review Letters combines the two ideas. Other physicists argue the particle idea is the better one and that the relativity idea collides with other measurements.
This is a single event in a single detector. The Carpet-3 team puts the chance that it is a coincidence at about nine in a thousand, roughly one in a hundred. Ofengeim and Piran, whose own work offers the relativity explanation, still write that physicists should hesitate to accept it on the basis of a single event. One of the authors of the combined study, Giorgio Galanti, said the same: one candidate photon from one cosmic event cannot be claimed as an undisputed discovery. Detectors now being built, such as CTAO and SWGO, are meant to catch more.
Brightest burst ever recorded. About two billion light-years away. Two leftover glows in the universe, one from the Big Bang and one from starlight. A single particle of light at about three hundred TeV, seventy-five minutes late, that the glows were supposed to stop. If it is real, something beyond the Standard Model is hiding in it. The name to search is GRB 221009A.