How do temperature and expansion speed decide today’s helium and deuterium? Expansion sets the clock and temperature sets the gate. Neutrons stop turning into protons about one second after the big bang, then decay until the universe is cool enough, three to four minutes later, for deuterium to survive. Almost every neutron left at that moment ends up in helium. Denser baryons burn more deuterium away, and faster expansion leaves more neutrons and more helium.
Drag the cosmic clock through the first hours and watch free neutrons decay, wait, then lock into helium. Change the baryon density and the extra radiation to see the final helium, deuterium and lithium move.
Model calculation
Neutrons drop at once when helium forms.
Model curves; grey bands observed (PDG 2025); green band from the microwave background
Lithium-7 comes out about three times above stars.
Model; each rate moved by its one-sigma error (Serpico 2004)
sets most of the deuterium error.
BackgroundA second after the big bang the universe is a hot soup of photons, electrons, positrons, neutrinos and a thin sprinkling of neutrons and protons. Weak reactions turn neutrons and protons into each other, and photons break apart any deuteron that forms.
Why it mattersAlmost all of today’s helium and deuterium was made in these few minutes, so their abundances record how hot the universe was, how fast it expanded and how many baryons it held.
While weak reactions are fast, the ratio follows the temperature. Near they become slower than the expansion. The ratio stops following the temperature at about one neutron for every five protons, then falls slowly because free neutrons decay with lifetime .
Before electron-positron pairs annihilate, photons, pairs and neutrinos fill the energy density. Each extra neutrino-like species raises the expansion rate, so weak reactions freeze earlier, fewer neutrons decay, and more helium forms. In this model the helium fraction rises by about 0.013 per extra species.
There are more than a billion photons for every baryon, so even when is far below the deuteron binding energy, the rare energetic photons still break deuterons apart. The small factor delays deuterium until about , a few minutes after the big bang. Only then can the chain to helium start.
Nearly every neutron still free when the bottleneck opens ends up in helium-4, which holds two neutrons and two protons. With this gives . The full network in this Lab gives 0.242 for three neutrino species, which corresponds to when the bottleneck opens.
Typical misconceptionDeuterium forms as soon as the temperature drops below its binding energy of 2.2 MeV.
Better mental modelPhotons outnumber baryons by more than a billion to one. The high-energy tail of the photon distribution keeps breaking deuterons until the temperature is about thirty times lower.
Typical misconceptionA denser universe should make more of every element.
Better mental modelHelium rises only slightly with density, while deuterium falls steeply because denser matter burns it into helium-3 and helium-4 faster. The sparse-baryon preset leaves about six times more deuterium than our universe.
Typical misconceptionThe model is tuned so that every element matches observation.
Better mental modelOnly the baryon density and the radiation are inputs, and they are fixed independently by the cosmic microwave background. With them, helium and deuterium agree with observation, but lithium-7 comes out about three times higher than in old halo stars. That gap is still unexplained.
Let Our universe play from the start, then drag along the cosmic clock to about 200 seconds.
What to observe: Blue neutrons fade slowly at first. Around three to four minutes they pair with protons and the box fills with gold helium nuclei in a few tens of seconds.Compare Sparse baryons keep deuterium with Dense baryons burn deuterium, then drag the baryon slider slowly between them.
What to observe: drops by a factor of about thirty from the first to the second, while the helium fraction only moves from 0.230 to 0.250. The observed deuterium band meets the curve just beside the green microwave-background band.Compare Extra radiation, more helium with Slower expansion, less helium.
What to observe: The neutron-to-proton ratio at moves from 0.143 to 0.165 and the helium fraction from 0.228 to 0.261. One extra species shifts helium by about four times the observed one-sigma error, so only a small change in radiation is allowed.Return to Our universe and read the chart of nuclear rates below the scene.
What to observe: Moving by its published error changes deuterium by about 0.7 percent, and lithium-7 responds most to that rate and to . These errors are far too small to close the lithium gap.You are hereBig Bang Nucleosynthesis: Light Elements in the First Minutes
The first minutes and the light elements · Reading path
Articles on this path that use this experiment, followed by the other experiments they suggest.