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Neutrinos break free (one second)
1953· Cosmic event
Well supportedStrong evidence or math, but not fully tested.
At about one second, near 10 billion degrees, neutrinos stopped interacting with other matter and have streamed freely through space ever since.
See it on the Science map →How it works
Neutrinos feel only the weak force and gravity. While the universe was dense, they kept colliding with electrons, positrons, protons and neutrons. By about one second it had thinned and cooled so much that those collisions stopped. The neutrinos still fill space, about 336 per cubic centimeter, at about 1.9 degrees above absolute zero. They are slightly colder than the microwave background because electrons and positrons annihilated soon afterward, heating the light but not the neutrinos.
Why it matters
It left a relic from one second in, far older than the microwave background's 380,000 years, and it fixed the neutron supply for the first elements.
How we know
Not detected directly; each neutrino is too feeble. But their gravitational effect shows in Planck's microwave maps, which count about three neutrino types, as predicted. A 2015 analysis found the specific shift they cause in the background's ripple pattern.
Still a mystery
Direct detection is far beyond today's technology; the proposed PTOLEMY experiment aims to try. The background also carries information on neutrino masses, which are still unknown.
Connections (2)
▸Quarks bind into protons and neutrons is followed by Neutrinos break free (one second)Proven
The relationship
After quarks bound into protons and neutrons, the universe kept cooling, from about 2 trillion to 10 billion degrees by one second. Weak-force collisions with electrons, positrons and neutrinos kept turning protons and neutrons into each other. But those collision rates fall with roughly the fifth power of temperature, while expansion slows only with the square, so at about one second the collisions lost the race.
How the link was found
The 'freeze-out' rule, comparing collision rates with the expansion rate, was applied to the early universe by Ralph Alpher, James Follin and Robert Herman in 1953 and generalized by Yakov Zel'dovich in the 1960s.
Why it's proven
Weak-interaction rates are measured in labs, for example via the neutron's lifetime of about 880 seconds. Fed into the cooling history, they predict helium of about 25%, matching observations.
Open questions and mysteries
Two methods of measuring the neutron's lifetime disagree by about 9 seconds, a puzzle that slightly shifts the predicted helium.
▸Neutrinos break free (one second) sets the neutron count for The first elements: hydrogen and heliumProven
The relationship
Before one second, weak collisions kept converting neutrons and protons back and forth, with neutrons slightly fewer because they are heavier. When neutrinos decoupled, the conversion stopped at about one neutron per six protons. Free neutrons then decayed for about three minutes, down to roughly one per seven, until fusion locked nearly all of them into helium. That ratio fixes helium at about 25% by mass.
How the link was found
Chushiro Hayashi showed in 1950 that the neutron-to-proton ratio is set by weak reactions in a cooling universe, correcting Gamow's group; Alpher, Follin and Herman refined it in 1953.
Why it's proven
Predicted helium, about 24.7% by mass, matches measurements in pristine gas, about 24.5%. The number of neutrino types affects the timing; helium and the microwave background both agree with three.
Open questions and mysteries
A 2022 Subaru telescope study (EMPRESS) found slightly low helium in small galaxies, possibly hinting at more neutrinos than antineutrinos. It is unconfirmed.