Neutrinos go their own way
One second in, neutrinos stop interacting — and have been streaming freely ever since.
- When
- Known to
- ± 20 million years
- Era
- Primordial
About one second after the Big Bang the universe thins enough that neutrinos cease to interact with everything else and begin travelling unimpeded. They are still doing so: a cosmic neutrino background fills space today at around 1.95 kelvin, roughly 340 of them in every cubic centimetre. It is the oldest relic in existence, released some 380,000 years before the light of the cosmic microwave background, though far too weak to detect directly.