Primordial nucleosynthesis

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In cosmology, primordial nucleosynthesis is the process of forming the first composite atomic nuclei shortly after the Big Bang . According to the theory, deuterium , helium and traces of lithium are formed first . The heavier elements that can be observed today come from fusion and other nuclear reactions in stars and thus from much later times.

About 75% of the elements created within the first three minutes after the Big Bang are made up of hydrogen 1 H and about 25% helium 4 He. The small proportions of D = 2 H , 3 He, 3 H and free neutrons (10 −4 to 10 −7 each ), as well as the much rarer beryllium and lithium isotopes, are not significant. Later the temperature and density of the universe fell below the critical values required for nuclear fusion . The short period of time explains, on the one hand, why heavier elements did not form during the Big Bang and, on the other hand, why reactive light elements such as deuterium could remain. Primordial nucleosynthesis took place locally, but simultaneously everywhere in the entire universe .

Origin of the theory

The idea of ​​primordial nucleosynthesis goes back to the work of the American physicist George Gamow in 1946. In 1950, the Japanese described Chushiro Hayashi the neutron - proton - equilibrium processes for the production of the light elements, and 1966, created Ralph Alpher a model of the 4 He-synthesis.

As a result, the model was further refined due to increasing knowledge of the nuclear reaction rates of the nucleons involved .


According to the theory accepted today, the processes for the formation of the first atomic nuclei could begin about a hundredth of a second after the Big Bang. At this point in time, the universe had cooled down so much that the quarks previously present as plasma condensed into protons and neutrons in a ratio of 1: 1. The temperature at this time was still about 10 billion Kelvin , corresponding to an average kinetic energy of about 1.3 MeV. In the further course of nucleosynthesis, the decreasing temperature shifted the neutron-proton balance more and more in favor of the protons.

About 1 second after the Big Bang, the neutrinos decoupled from the matter . Electrons and positrons annihilated. The ratio of neutrons to protons had dropped to about 1: 6. The temperature at this time was approx. 600 million Kelvin, the mean kinetic energy almost 80 keV, so that for the first time protons and neutrons were able to combine to form deuterons (= deuterium nuclei). However, this was immediately split up again by high-energy photons. An important parameter of the theory is therefore the ratio of baryonic matter to photons , on which the beginning of the effective deuteron synthesis depends. The standard model of cosmology assumes this in the order of 10 −10 .

It was only one minute after the Big Bang that the universe had cooled down enough (60 million Kelvin or just under 8 keV) that deuterons could effectively be formed. Since further neutrons decayed during this period (the free neutron has a half-life of 10 minutes), the ratio of neutrons to protons was now only 1: 7.

99.99 percent of the remaining neutrons were bound in 4 He. Due to the high binding energy of the 4 He nucleus and because there is no stable nucleus with a mass number of  5 or 8, 4 He is hardly broken down. Only the element lithium in the form of the isotope 7 Li was still formed to a small extent in nuclear reactions .

5 minutes after the Big Bang, the particle density of the universe had dropped so far that the primordial nucleosynthesis was essentially ended. The result of the nucleosynthesis was, in addition to 4 He, traces of deuterons, tritons (= tritium nuclei) and helions ( 3 He nuclei) as intermediate products of the helium-4 synthesis as well as the protons that had not found any neutrons as reaction partners . The remaining free neutrons decayed over the next few minutes, the tritons over the course of decades.

The theory predicts a mass ratio of 75 percent hydrogen (protons) to 25 percent helium. This value agrees extremely well with the observations of the oldest stars, which is one reason for the wide acceptance of this theory. Measurements outside of our Milky Way were made especially for 4 He , which confirm the result. The theory also explains the relative abundances of deuterium and 3 He very well. For lithium, however, there is a discrepancy between the measured value and the theoretically calculated value, which is almost three times larger. This is known as the primordial lithium problem .

Connection to other cosmological models

Primordial nucleosynthesis is now one of the most important pillars of the standard model of cosmology . In this context, the cosmic background radiation was also predicted for the first time .

Primordial nucleosynthesis is also seen as an important indicator of the existence of non-baryonic dark matter : on the one hand, it limits the number of baryons in the universe through their relationship to photons; On the other hand, the even distribution of the baryons during primordial nucleosynthesis makes it probable that the granular structure of the universe observed today could not be characterized by the baryons, but by the density fluctuations of an only weakly interacting - and thus not baryonic - heavy elementary particle .


See also

Web links

Individual evidence

  1. ^ Edward L. Wright: Big Bang Nucleosynthesis. UCLA website
  2. ^ SQ Hou, JJ He, A. Parikh, D. Kahl, CA Bertulani: Non-extensive statistics on the cosmological lithium problem . In: The Astrophysical Journal . tape 834 , no. 2 , January 11, 2017, ISSN  1538-4357 , p. 165 , doi : 10.3847 / 1538-4357 / 834/2/165 ( [accessed July 23, 2019]).