Nucleosynthesis

Big Bang Nucleosynthesis

TBA: Amount of ³He at the end of the BBN. More images.

At the beginning, the universe was dense and hot, possibly infinitely so in the first instant. In the first split second, it went from a hot soup of energy in which particles freely emerged and annihilated, to a hot soup of particles - photons, neutrinos, electrons, protons and neutrons. There were no complete atoms like we know from today's chemistry - in the extreme temperatures of the Big Bang, and indeed during other processes discussed on this site, electrons were floating freely, barely minding the nucleons.

You might expect these would be the ideal conditions to create heavier elements by smashing together a lot of those particles at once, but it was not so. While there was a high density of lonely protons and neutrons, the density of light (photons) and energy was just too high. If a more complex nucleus formed, it was near instantly disintegrated back into protons and neutrons by a collision with a photon.

It wasn't until a couple minutes after the very beginning that the matter spread out enough for photons to become cooler and not wreck every formed nucleus.

The first nuclide that started forming was Deuterium (²H, Hydrogen-2) in a collision of a proton (¹H, Hydrogen-1) and a neutron.

²H would then absorb either a proton to become ³He or another neutron to become Tritium (³H). Both of these then eagerly absorbed neutron or proton respectively to become ⁴He.

The three step Helium-4 synthesis

Deuterium has very low nuclear binding energy, meaning the energy to take it apart is very low. This creates a sort of "Deuterium bottleneck". Both ³H and ³He are in comparison very energetically efficient, so once photons cooled down and this bottleneck was overcome and Deuterium started forming, the energy of photons was already sufficiently low for these two reactions, and they started happening right away. This used up most Deuterium, and only very little is left over from the Big Bang, only 10^-5 of the total nuclide mass.

Helium-4 on the other hand has a remarkably high nuclear binding energy. This makes it much more stable in these conditions than the other nuclides with a similar mass.

It is in fact so stable that no nuclides at all with 5 or 8 nucleons are stable even in daily conditions on Earth, preferring to decay into ⁴He. Those with 5 nucleons are eager to emit one to become ⁴He, while those with 8 are eager to split into two ⁴He.

Most matter which manages to get past the single proton thus rather quickly becomes ⁴He, where it gets stuck.

A rarer reaction is ³H colliding with ⁴He to form ⁷Li. But even if this nuclide can stay together, it is very likely to absorb a nucleon, gaining mass of 8 and decaying into two ⁴He. In the end, a good percentage of today's ⁷Li comes from the Big Bang, but the overall amount of Lithium today is exceedingly low. After the Big Bang, it was only around 10^-9 (a billionth) of the total mass of all nuclei.

Heavier elements than Lithium were rarely created, mainly because of the mass 8 bottleneck. They require much longer timescales and densities - surprisingly, stars are a much better environment for their synthesis, as there isn't such density of high energy photons as during the Big Bang, and they have millions of years rather than minutes to fuse (difference of time on the order of 10¹⁰).

Logarithmic relative abundance of elements in today's universe shows Hydrogen and Helium still make up around 99% of baryonic matter

As the universe continued to expand, the Big Bang nucleosynthesis was over in a few minutes, ending with this distribution:

• Hydrogen-1 - 75% of the mass and 92% of the absolute number of atoms.
• Helium-4 - 25% of the mass and 8% of the number of atoms.
• Hydrogen-2 (Deuterium) - order of 10^-5 (0.001%) of the mass.
• Helium-3 in much tinier amount.
• Lithium-7 at about 10^-9 of the mass.
• Heavier elements at about 10^-15 of the mass.