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Linné on line arrow Physics and the Cosmos arrow Macrocosmos arrow Big Bang arrow Formation of atoms

Formation of atoms

Neutral atoms will be formed when the energy of the photons has decreased so much that it is lower than the typical energy needed to ionise an atom.

Since the photons do not have enough energy to ionise (excite) neutral atoms they will no longer interact with them. Instead the photons are decoupled from the other particles and their evolution will only depend on the expansion of the Universe. This radiation is called the cosmic microwave background radiation and its spectrum corresponds to an ideal black body with the temperature 2.726 K. As the photons have not interacted with other matter after being decoupled, they kee their properties (except that the energy and density is lower). Therefore, by measuring these photons we measure what happened when they were decoupled. This is actually the only way to look so far back into the early Universe.

The inhomogeneities and irregularities that exist in the Universe at this time can be amplified by the gravitation leading to the accumulation of gas forming early stages of galaxies.