Big Bang nucleosynthesis
As primordial nucleosynthesis is the process of the formation of the first composite nuclei shortly after the Big Bang cosmology. This results in deuterium, helium and traces of lithium. The observed today heavier elements come from mergers and other nuclear reactions in stars and thus a much later time.
The resulting within the first three minutes after the Big Bang members are distributed to 75% hydrogen, 24 % helium and 1% lithium isotopes. Later, temperature and density of the universe fell below the critical values , which are necessary for nuclear fusion. The short period of time declared a why heavier elements have not been formed during the Big Bang, and secondly, why reactive light elements such as deuterium could remain. The primordial nucleosynthesis found locally, but at the same place everywhere in the universe.
Emergence of the theory
The idea of primordial nucleosynthesis goes back to work of the American physicist George Gamow in 1946. 1950 described the Japanese Chushiro Hayashi, the neutron-proton equilibrium processes for the production of the light elements, and Ralph Alpher 1966 created a model of 4He synthesis.
As a result, there were further refinements of the model due to ever-improving knowledge of the nuclear reaction rates of the participating nucleons.
Timing
According to the currently accepted theory of the processes of formation of the first nuclei could start about a hundredth of a second after the Big Bang. The universe was now so far cooled that the previously present as a plasma quarks to form protons and neutrons in a 1:1 ratio condensed. The temperature at this time was still about 10 billion Kelvin, which corresponds to an average kinetic energy of about 1.3 MeV. An important parameter of the theory is the ratio of baryonic matter to photons, which is assumed in the order of 10-10. From this parameter, the date of deuterium synthesis is determined. In the course of the decreasing temperature shifted the neutron-proton balance more in favor of the protons.
About 1 second after the Big Bang neutrinos decoupled from matter. Electrons and positrons zerstrahlten. The ratio of neutrons to protons had decreased to about 1:6. The temperature at this time was about 600 million K, average kinetic energy of 0.8 MeV. Now, protons and neutrons can first connect to deuterons ( deuterium nuclei = ). However, this is split immediately by high-energy photons.
Only one minute after the Big Bang, the universe had cooled so far (60 million K or 80 keV) that effectively deuterium was formed. Since in this period more neutrons disintegrate ( the free neutron has a half-life of 10 minutes), the ratio of neutrons to protons is now only 1:7.
The remaining neutrons are now bound to 99.99% in 4He. Due to the high binding energy of the 4He nucleus and because no stable core exists with mass number 5 or 8, 4He is hardly degraded. Only the element lithium in the form of the isotope 7Li is still formed to a small extent in nuclear reactions.
The theory predicts a mass ratio of 75%-hydrogen ( proton ) to 25% helium. This value agrees very well with the observations of the oldest stars, which is one reason for the wide acceptance of this theory. Especially for 4He measurements were made outside our Milky Way, which confirm the result. The relative abundances of deuterium, 3He and lithium are very well explained by the theory.
5 minutes after the Big Bang, the particle density of the universe is dropped so far that the primordial nucleosynthesis is substantially completed. Traces of deuterium and tritium, and 3He are left. Furthermore, all those protons that do not have neutron found as a reactant. The still remaining free neutrons decay in the course of the next few minutes.
Connection to other cosmological models
The primordial nucleosynthesis is now one of the most important pillars of the standard model of cosmology. They provide the framework for the first time and the cosmic background radiation was predicted.
The primordial nucleosynthesis is also seen as important evidence for the existence of non - baryonic dark matter: on the one hand it limits the amount of baryons in the universe by their relationship to the photons; on the other hand it makes the uniform distribution of baryons during the primordial nucleosynthesis likely that the observed today granular structure of the universe, not by the baryons, but by the density fluctuations of a weakly interacting - could be pronounced heavy elementary particle - and that's not baryonic.