Recombination (cosmology)

See also: Recombination

In cosmology, the recombination is the time when the electron S combine with the atomic nuclei existing to form the first Atome S. This phenomenon occurs at the time when the Température of the Univers goes down below the threshold under which the average energy of the energy Photon S is below the energy of ionization of the atom considered.

In practice, at the time of the recombination, no star was already formed. The only atomic nuclei present in the universe are the Hydrogène, the Hélium and of the traces of Lithium. Lithium recombines before helium, itself recombining before hydrogen. In practice, when one speaks about recombination, one implicitly evokes that of the hydrogen only, which represents the great majority of the cores at that time existing (approximately 94%).

To note that the term of Re combination is somewhat unsuitable, since it represents the first time where electrons and cores combined.

Time of the recombination

Intuitively, one would be tempted to say that the recombination occurs when average energy photons is about the energy of ionization of the hydrogen atom, that is to say 13,6 electronvolt S (eV), or approximately 150  000 degrees. In practice, this estimate is very incorrect, because the universe is a system which has a considerable number of Photon S by atomic nucleus (about a billion for one). So what is essential is that the energy of the billionth of the energy photons is about the energy of ionization of the hydrogen atom. This occurs when the average energy of the unit of the photons is quite lower than 13,6 eV. Calculations show then (see Équation of Saha) that average energy must be approximately 0,3 eV, for a temperature of 3000 Kelvin S. the current temperature of the universe is that given by the cosmological diffuse Fond, that is to say 2,726 Kelvins. It is a factor 1  100 times weaker than the temperature of the recombination. That means that this one occurred when the distances in the universe were 1  100 times smaller than today, and thus that the Décalage towards the red is of 1  100. Calculations show (see Équations of Friedmann and standard Modèle of cosmology) that the universe was then old of 380  000 years since the Big Bang.

Recombination and decoupling of the radiation

Before the recombination, the totality of the electrons circulate freely in the universe. They then form the independent source of interaction between matter and light, via the Diffusion Thomson. The universe is then opaque with the radiation. With the recombination, the density of free electrons drops drastiquement, because the recombination is almost complete (only ten-thousandths of the electrons does not recombine). This moment is to correspond precisely at the time where this fall of opacity allows the universe to become transparent with the radiation. This coincidence is relatively fortuitous: the recombination could have occurred after the dilution of the free electrons by the Expansion of the universe made it sufficiently not very dense to be transparent, for example. So the recombination corresponds at the time known as of the Découplage of the radiation, and the two terms are often used in an interchangeable way, even if they do not correspond to the same thing in general.

Recombination and reionisation

The observations indicate that a broad fraction of the matter of the universe is today not in the shape of atoms, but in the shape of ions and free electrons. This is done via a test known as of Gunn-Peterson. As calculations indicate that the recombination is so to speak complete, one from of deduced that later on a process allowed réioniser the matter: it is the Réionisation. As the universe is as a whole increasingly cold during time, one from of deduced that in fact localized radiation sources were responsible for this reionisation. According to any probability, it is about the very first generation of star S having existed. These stars, known as of population III were exclusively made up of hydrogen and helium. The stellar evolution indicates that such stars were more massive and especially much hotter than current stars. That explains why their temperature of surface could be sufficient to dissociate the atoms formed at the time of the recombination.

See too

References

  • See Works specialized on cosmology

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