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(Note that the neutron-proton freeze-out time was earlier).This time is essentially independent of dark matter content, since the universe was highly radiation dominated until much later, and this dominant component controls the temperature/time relation.The creation of light elements during BBN was dependent on a number of parameters; among those was the neutron-proton ratio (calculable from Standard Model physics) and the baryon-photon ratio.
The implications of gravity for the entire universe are still the subject of debate, but local effects are better understood.
After about 100 million years gravity caused and still causes matter to collapse into bodies that become hot and light up the dark sky as stars.
In this field, for historical reasons it is customary to quote the helium-4 fraction by mass, symbol Y, so that 25% helium-4 means that helium-4 atoms account for 25% of the mass, but less than 8% of the nuclei would be helium-4 nuclei.
Other (trace) nuclei are usually expressed as number ratios to hydrogen.
One feature of BBN is that the physical laws and constants that govern the behavior of matter at these energies are very well understood, and hence BBN lacks some of the speculative uncertainties that characterize earlier periods in the life of the universe.
Another feature is that the process of nucleosynthesis is determined by conditions at the start of this phase of the life of the universe, and proceeds independently of what happened before. Free neutrons are less stable than helium nuclei, and the protons and neutrons have a strong tendency to form helium-4.
However, free neutrons are unstable with a mean life of 880 sec; some neutrons decayed in the next few minutes before fusing into any nucleus, so the ratio of total neutrons to protons after nucleosynthesis ends is about 1/7.
Almost all neutrons that fused instead of decaying ended up combined into helium-4, due to the fact that helium-4 has the highest binding energy per nucleon among light elements.
Hence, the formation of helium-4 is delayed until the universe became cool enough for deuterium to survive (at about T = 0.1 Me V); after which there was a sudden burst of element formation.
However, very shortly thereafter, around twenty minutes after the Big Bang, the temperature and density became too low for any significant fusion to occur.