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Quarks bind into protons and neutrons
1975· Cosmic event
ProvenTested many times; part of working physics.
At about a millionth of a second, near 2 trillion degrees, a free soup of quarks and gluons condensed into protons, neutrons and other particles.
See it on the Science map →How it works
Before this the universe was a quark–gluon plasma: quarks and the gluons that bind them roamed loose instead of being locked inside particles. As it cooled below about 2 trillion degrees, the strong force took hold and locked quarks into threes (protons and neutrons) and pairs (short-lived particles called mesons). Calculations show the change was gradual, not a sudden boil. Most of the new particles soon decayed or annihilated, leaving protons and neutrons as the lasting survivors.
Why it matters
It made the protons and neutrons used in every atom since. It is also an early-universe era that laboratories can recreate and study directly.
How we know
Recreated by smashing gold or lead nuclei: CERN announced a new state of matter in 2000; Brookhaven's RHIC found in 2005 that it flows like a near-perfect liquid; the LHC has studied it since 2010. Calculations put the change near 1.8 trillion degrees.
Still a mystery
Exactly how the plasma turns into particles is still modeled rather than derived. Whether the transition left a lasting cosmic imprint, such as faint gravitational waves, is debated; a smooth change makes one unlikely.
Connections (4)
▸The Higgs switches on (electroweak epoch) is followed by Quarks bind into protons and neutronsProven
The relationship
After the Higgs switched on, the universe was a plasma of quarks and electrons, now with mass, plus gluons, neutrinos and light. Over the next few millionths of a second it cooled a thousandfold, to about 2 trillion degrees. Along the way the heaviest particles, top quarks, Higgs, W and Z bosons, decayed away, because the cooling soup could no longer make new ones. Then the strong force took hold.
How the link was found
The sequence follows from the Standard Model plus the cooling law; it was laid out for general readers in Steven Weinberg's 1977 book The First Three Minutes.
Why it's proven
Both transitions are calculated with lattice simulations and partly recreated at the LHC, which makes W, Z, top quarks and quark–gluon plasma. Their order in time follows from their measured energies.
Open questions and mysteries
If dark matter is a heavy particle, it probably stopped annihilating ('froze out') in this window. Detectors such as LZ have not found it, so the window may be empty of it.
▸Matter beats antimatter (baryogenesis) leaves the extra matter used in Quarks bind into protons and neutronsWell supported
The relationship
When quarks bound into protons and neutrons, antiquarks bound into antiprotons and antineutrons in almost equal numbers. By about a thousandth of a second these met their partners and annihilated, releasing light. Only the slight surplus from baryogenesis, about one in a billion, found no partner. Those leftovers are the protons and neutrons that later built every atom.
How the link was found
Once the 1965 discovery of the microwave background let photons be counted against atoms, the one-in-a-billion leftover became clear; Andrei Sakharov's 1967 paper set out to explain it.
Is it proven?
Annihilation itself is routine in labs. Calculations show a perfectly balanced universe would have kept roughly a billion times fewer protons than we see, so today's matter must be a prior surplus, not lucky survivors.
Open questions and mysteries
Could distant regions be made of antimatter instead? A 1998 analysis by Cohen, De Rújula and Glashow showed any such regions would have to be nearly as large as the observable universe.
▸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.
▸The strong force and quarks binds quarks into protons in Quarks bind into protons and neutronsProven
The relationship
The strong force between quarks weakens at high energy, so above about 2 trillion degrees quarks and gluons were not confined inside particles. As the universe cooled, typical collision energies fell to where the force becomes strong and confining. Quarks could no longer stay apart and were locked into protons, neutrons and short-lived particles, within a few millionths of a second.
How the link was found
The 1973 discovery that the force weakens at high energy implied a free quark plasma when hot. John Collins and Malcolm Perry, and separately Nicola Cabibbo and Giorgio Parisi, drew that conclusion in 1975.
Why it's proven
Lattice simulations of the strong force, precise by 2014, put the transition near 155 MeV. Heavy-ion collisions at RHIC and the LHC produce and measure the plasma, consistent with them.
Open questions and mysteries
Is there a 'critical point' where the smooth change turns sharp? Not in the early universe, with its near-even matter and antimatter, but perhaps in denser matter. RHIC's Beam Energy Scan (2010–2021) was inconclusive; neutron-star cores may hold it.