A powerful searchlight shines through thick fog. The beam itself is invisible, but the fog begins to glow faintly, and gives away the searchlight's presence. A similar principle works with neutrinos: these nearly elusive particles are born when fast protons from distant galaxies slam into the ubiquitous cosmic microwave background (the faint microwave leftover from the Big Bang).
Recently, the JWST peered into the early universe and found surprisingly many bright galactic nuclei. These are cosmic beacons — places where matter falls onto supermassive black holes. The authors of a new study hypothesized: if these beacons eject protons with energies up to ten billion billion electron-volts, then on their way to us through the fog of the cosmic microwave background, a noticeable number of neutrinos should be born. Calculations showed a characteristic "bump" in the neutrino flux at an energy of about 50 PeV (peta-electron-volts). Like a searchlight leaving a spot of a certain brightness on the fog.
Current data from the IceCube neutrino observatory (a giant detector in Antarctic ice) already see a similar signal. True, it is not yet possible to say with certainty where it comes from. If future observations confirm this 50-PeV bump and show that neutrinos come from specific directions, it would be direct proof that ancient particle accelerators were at work. This way, we could hear the echo of processes that raged when the universe was less than a billion years old.
as written by the authors arXiv:2604.14535
The James Webb Space Telescope (JWST) revealed a large population of active galactic nuclei (AGN) with redshifts greater than five. We show that if they emit ultrahigh-energy protons with energies up to $$\lesssim 10^{19}$$ eV, the cosmogenic neutrino production in the high-redshift CMB field yields a neutrino flux with a bump at around 50 PeV. This flux is potentially consistent with the current neutrino intensity estimate by the IceCube Neutrino Observatory within the parameter space allowed from the AGN luminosity and number density estimates by JWST. Future neutrino observations that confirm the 50-PeV bump and constrain the small-scale anisotropy will infer ultra-high energy cosmic-ray emissions in the early universe.
The IceCube neutrino observatory uses a cubic kilometer of Antarctic ice as a detector. When a neutrino collides with a water molecule, a faint flash of light occurs, which is captured by thousands of sensors frozen into the ice.
In Carl Sagan's novel "Contact," the signal from an extraterrestrial civilization was detected precisely thanks to neutrino detectors, echoing the idea of using neutrinos as "messengers" from distant corners of the cosmos.