Decoupling of the radiation

In Cosmology, the decoupling of the radiation represents the time when the universe became transparent with the electromagnetic Rayonnement, this one not interacting more, or not being more coupled with the matter (from where the name).

If one goes up in time, no Photon could have travelled freely for a time former to the decoupling of the radiation: before this time, the Libre range of the photons was by very weak definition. The time of the decoupling of the radiation thus represents the most remote time for which we have been able to receive an electromagnetic radiation. In practice, the time of the decoupling of the radiation corresponds to that where the density of free electrons brutally decreased, at the time when those bound to the atomic nuclei to form the first atoms, which one calls the recombination. So the terms of recombination and decoupling of the radiation are used in an interchangeable way. The recombination and the decoupling of the radiation occurred approximately 380  000 years after the Big Bang, when the distances were approximately 1  100 times smaller and the Temperature 1  100 times higher than today.

Decoupling of the radiation and cosmological diffuse Fund

At the time of the Big Bang, the universe is an extremely hot medium: the radiation in which it bathes has a very high Température. This radiation remains with the Expansion of the universe, but cools during time, in the same way that a gas which one slackens cools. This radiation thus exists still today, there is called cosmological diffuse Fond. It is to some extent the luminous echo of Big Bang. Its temperature is from now on extremely low: 2,726 Kelvin S only. If this radiation existed as of the very first moments of the universe, it is propagated freely only since the time of decoupling. Thus the observation of this radiation gives us a kind of photography of the universe at the time of decoupling, in the same way that the observation of a star lets to us see his surface (the area which emits the radiation) and not its interior (the area which produce the radiation). This photography is thus the oldest image which one can have of the universe (at least in the electromagnetic field). The area which emitted this radiation received today on Ground is called, for obvious reasons, Surface of last diffusion. It is not possible to receive a more remote electromagnetic radiation, because this one should have been emitted and should have travelled freely for a time former to decoupling.

See too

References

  • See Works specialized on cosmology

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