Neutrino from the Cradle: The Secret of the Early Universe express
Neutrinos are almost elusive particles: they zip through planets without noticing atoms. So every catch is a stroke of luck. Recently, the underwater detector KM3NeT snagged a neutrino with energy millions of times greater than Earth's most powerful accelerators can produce. A new study suggests: it's not a visitor from a distant galaxy, but a message from the early Universe. The particle was born when the world was only 380,000 years old, right after it became transparent.
This explains the silence of other observatories: the message is heard only at one energy. On its way to us, the neutrino collided with other neutrinos, and its energy changed. Such encounters could have left an imprint on the cosmic microwave background — the afterglow of the Big Bang, stretched by the Universe's expansion.
If the theory is correct, this lonely message will allow us to peer into the era of the first atoms and even deeper.
Original abstract
as written by the authors arXiv:2604.18677
High energy neutrinos can be injected in the early Universe from the decay or annihilation of long lived primordial relics. We analyse the possibility that the ultrahigh energy neutrino event recently observed by the KM3NeT neutrino telescope could have such an origin. This possibility has the advantage of leading to a sharp spectral feature in a way that the neutrino flux can be small at all energies except at the KM3NeT event energy. Thus, along this scenario the tension with null results from other experiments is reduced with respect to the usual power law case analysed by the KM3NeT and IceCube experiments. At such energies and for an emission around the recombination time, interactions of these neutrinos with background neutrinos prove to be relevant and must be determined from the development of a dedicated code. These interactions, as well as final state radiation processes, modify the spectrum. Interestingly, it turns out that the scenario can also leave an imprint in the CMB that could be probed in the near future. Interestingly too, this scenario does not predict an associated $$\gamma$$-ray flux beyond observation. All in all we do find that the high energy neutrino could be a primordial high energy neutrino, provided it has been produced around the recombination time or later.
Every second, trillions of solar neutrinos pass through your body without causing the slightest harm.
Like in Arthur C. Clarke's 'Space Odyssey', where a mysterious signal comes from an unimaginable past.