The tiniest ripples in the early universe, like circles on water, gradually fade away, cooling the relic neutrinos. Their present-day temperature turns out slightly lower than expected, and this decrease holds information about the first moments after the Big Bang. The future PTOLEMY detector will capture this signal and tell us about the birth of cosmic structure. What other secret do neutrinos keep?
In the first moments after the Big Bang, the universe was a boiling soup of elementary particles. Even the light neutrinos were actively colliding with everything around. But the rapid expansion cooled this soup, and the neutrinos broke free, becoming almost indifferent to the surrounding world. Back in the mid-20th century, George Gamow and Ralph Alpher predicted that they would survive as a cold background with a temperature about two degrees above absolute zero.
Now astrophysicists have noticed a subtle effect. In that primordial plasma floated tiny clumps — ripples on the surface of our soup. Over time, these ripples smoothed out, and the energy of this smoothing, oddly enough, further cooled the neutrino gas. That's why today there are slightly fewer relic neutrinos around us than previously thought.
The PTOLEMY experiment is preparing to catch these ancient particles for the first time. To detect even a few, you need a detector the size of a multi-story building — that's how unwilling they are to interact. But if it succeeds, measuring their energy will show how uneven the universe was on scales inaccessible to other observations.
🎯 Every second, about a hundred trillion neutrinos zip through your body — and most likely, not one will hit a single atom.
🎬 In the TV series Star Trek, neutrinos are used to communicate through entire planets.