In the very early universe, the hot plasma consisted of fixed amount of radiation (photons and neutrinos) and matter (electrons, protons, neutrons, etc). There were many competing reaction taking place and using statistical methods I understand that you can deduce the particle content of the universe when radiation and particles began to condense into nucleons (at roughly $100\times 10^9 K$).
According to ΛCDM, there is 5 times more dark matter than normal matter. These particles, ostensibly, had to form from the same budget of radiation yet I don’t see the reactions in any of the literature. How is it possible to accurately calculate the particle content of the early universe, specifically the proton to neutron ratio, when the reaction governing 80% of the matter creation isn’t known?
Answer
Standard big bang nucleosynthesis only involves particles which are part of the standard model - ie. excludes dark matter.
Is this justified? Well, dark matter is non-interacting (or so weakly interacting that we can't detect it), therefore it does not interact strongly with baryons and photons during the epoch of nucleon formation.
However there certainly is theoretical work that investigates non-standard big bang models that do include additional degrees of freedom due to new neutrino species or that include residual annihilation of weak scale massive dark matter particles. Apparently, these pieces of new physics have a significant effect on the nucleosynthesis of the light elements - deuterium, tritium, lithium. A reasonable review (I have not read it) appears to be given by Jedamzik & Pospelov (2009).
Some of these ideas have been used to suggest a solution to the problem that standard big bang nucleosynthesis appears to give too much Li (e.g. Bailly 2011).
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