One second after the Big Bang, an unfathomable number of neutrinos were liberated from the surrounding chaos and started traveling through the universe at nearly the speed of light. Cosmologists believe a person is bombarded with something on the order of a quadrillion of these ‘relic neutrinos’ every second, dwarfing, unbelievably, the number of neutrinos that come from other relatively nearby sources, such as nuclear fusion in the sun or radioactive decay.
“You hear this, and you’re driven to ask whether it’s really true,” says Princeton University physics professor Christopher Tully. “How can we know it’s true?”
