Rick's Cosmology Tutorial: Chapter 8 Abstract

The Time of Recombination: The Formation of Atoms

The period of frenetic activity after the Big Bang in which the positrons disappear and nucleosynthesis occurs is over within an hour or so. Thereafter things occur at a more leisurely pace. When the universe has cooled sufficiently, the positive nuclei capture electrons to form neutral atoms for the first time. This event is known (inappropriately) as recombination. In Chapter 8 we show how to calculate the relative proportions of ions and neutral atoms, and how these change with temperature and hence with time.

Helium and lithium nuclei may capture their first electrons in the order of thousands of years. However no significant numbers of neutral atoms are formed before about 57,000 years - at which time about 1% of helium-4 is in the form of neutral atoms. Virtually all helium-4 is neutral atoms by 94,000 years. However it is not until 185,000 years that neutral hydrogen starts to form, and this is not complete until about 363,000 years. The last nuclei to capture a full complement of electrons to become neutral atoms are lithium-7. This takes roughly 4 million years, though there are very few lithium-7 nuclei.

Read Chapter 8 (pdf): The Time of Recombination: The Formation of Atoms

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Crab Nebula (NGC 1952, Taurus A), an 11 light-year wide expanding remnant of a supernova explosion. It is known to have occured in 1054 because the event was recorded by Chinese astronomers