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Nucleosynthesis.
- after the Big Bang, the universe was extremely hot.
Because of the interconvertability of mass and energ through E = m c2, where E is energy, m is mass and c is the speed of light, the high energy denisty of the universe after the Big Bang lead to all possible different particles being created, with their anti-particles, continuously, everywhere. Most immediately re-annihilated with their matching anti-particles, going back to energy.
As the universe cooled, the energy per unit volume available to do this became less, and the higher mass particles, which require more energy to be created, stopped being made.
About one second after the Big Bang, the temperature (which measures the energy available per unit volume) dropped low enough that neutrons and protons "froze out" of the surrounding radiation bath.
We refer to neutrons and protons, collectively, as "baryons".
- NB: protons and neutrons are not fundamental particles, they are composite, made from more fundamental particles called quarks.
Atomic nuclei are made from protons and neutrons, they are fairly high mass, so require quite a lot of energy to be made.
Neutrons are more massive than protons, and free neutrons are unstable - they decay to a proton + electron + neutrino on a time scale of few minutes.
- After protons and neutrons form, the universe is still hot enough for thermonuclear fusion to take place.
Protons (which are also the nuclei of normal hydrogen atoms) can fuse with neutrons to make deuteriume - aka "heavy hydrogen, or 2H
Deuterium is a fragile nucleus and is destroyed at high temperatures.
The next step in the nuclear fusion chain is tritium - 3H - which has one proton and two neutrons. Tritium is unstable and decays radioactively to form light helium, 3He.
- After hydrogen, the next element is helium, which is stable.
4He - made from two protons and two neutrons - is especially stable.
The time period during which fusion can occur is very short, because the temperature has to be high enough for fusion to take place, but low enough that deuterium is not destroyed as soon as it is made.
During this brief period a LOT of hydrogen is burned up interacting with free neutrons, making mostly 4He.
About 1/4 of the univers, by mass, becomes helium.
- A little bit of unburned deuterium and 3He is left over when the temperature gets low enough that fusion fizzles.
A tine bit of helium fuses to the next element, 7Li - lithium - with 3 protons and 4 neutrons - note that to get to lithium (stable nucleus) from 4He takes multiple interactions.
It is likely that some miniscule amounts of beryllium and boron also gets made
- Fusing helium is an unstable process - in particular if you try to fuse 4He + 4He to make beryllium, you get a very unstable isotope which decays immediately back to 4He.
To get any further up the elements, you need to get to 12C - carbon, whose most stable isotope has 6 protons and 6 neutrons - which could be made from THREE helium nuclei.
- Carbon is a very important elements for thermonuclear fusionm because it catalyzes additional fusion reactions, it will fuse with hydrogen to make nitrogen, then oxygen and so-on up to iron; but in the process you typically get carbon back along the way, so it is not used up in the process.
Fusion of carbon to higher elements can occur at relatively low temperatures (millions of degrees)
- 3 4He -> 12C
This "triple alpha" reaction happens very rapidly, but only at VERY high temperatures (and densities)
By the time any helium is made, the universe is TOO COOL for triple alpha fusion to take place. No carbon gets made.
Within few more minutes, the remaining free neutrons decay to protons and all further fusion stops.
- This leads to a strong and major prediction of cosmology: Big Bang nucleosynthesis makes hydrogen and helium, and minute traces of deuterium, helium-3 and lithium, and maybe some beryllium and boron
and NOTHING ELSE
All other elements are made later in the universe, in the cores of stars.
We can calculate the relative abundances of the Big Bang generated elements given specific cosmological models.
- Cosmology predicts that after the Big Bang the universe is "metal free" - where "metal" means all the heavier elements than helium.
There is one slight problem (foreshawdowing later issues...) - if the unverse is flat - at the critical density, and if all this mass is protons and neutrons to begin with, then very little deuterium should remain in the universe.
Less deuterium than is observed (this is a bit tricky since deuterium can be destroyed in stars later).
But, there is too much deuterium around, even if there is very little, for the universe to be both flat (or at the critical density) and for all the mass to consist of baryons.
Last updated 02/08
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