Nucleosynthesis

Big Bang Nucleosynthesis

TBA: Amount of ²H and 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. You might expect these would be the ideal conditions to create heavier elements, but it was not so.

While there was a high density of lonely protons and neutrons, there was also a high density of light (photons) and energy overall. 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 not wreck every formed nucleus.

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

Deuterium would then absorb either 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

Helium-4 has a remarkably high nuclear binding energy - that is the energy that is needed to take any nucleon out. This makes it much more stable in these conditions than 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 by around 75% of the mass and 92% of the absolute number of atoms.
• Helium-4 by around 25% of the mass and 8% of the number of atoms.
• Hydrogen-2 (Deuterium) in much tinier amount.
• Helium-3 in much tinier amount.
• Lithium-7 at about 10^-9 of the mass.
• Heavier elements at about 10^-15 of the mass.