During the introductory lecture to a cosmology course I'm currently taking, there was a brief discussion of some of the "highlights" of the Big Bang model. One of these is, of course, recombination. This is the first moment in the history of the Universe that electrons bonded to nuclei, forming atoms: The Universe then became transparent, and this is when the cosmic microwave background radiation "last scattered"

Everybody in the room noted the strangeness of this name: The prefix re- is wholly unjustified! The wikipedia page offers no explanation and although I found some comments that seemed relevant in a book, it is still quite unclear to me how the term came into mainstream usage. Would anyone have the original reference, or a plausible explanation (possibly based on the above link)?


1 Answer 1


Recombination is a general term in chemistry for assembling molecules from atoms, ions and radicals, whether or not they were dissociated into them first. So the analogy holds. And the term is directly in line with usage in plasma physics, where it means "a process by which positive ions of a plasma capture a free (energetic) electron and combine with electrons", again regardless of whether ionization occurred prior to that. This is pointed out on the Wikipedia talk page that discusses the "misnomer". Zeldovich, Kurt and Syunyaev, for example, write in their 1968 paper:"In the hot model of the universe it is assumed that at an early stage of the expansion the fully ionized plasma is in equilibrium with radiation. Cooling as a result of expansion leads to recombination" with reference to Gamow's famous "hot Big Bang" paper from 1946. Cosmology and Controversy gives a detailed historical account of Gamow's discovery.

Also according to Wikipedia:

Before the late 1960s, many cosmologists thought the infinitely dense and physically paradoxical singularity at the starting time of Friedmann's cosmological model could be avoided by allowing for a universe which was contracting before entering the hot dense state, and starting to expand again. This was formalized as Richard Tolman's oscillating universe. In the sixties, Stephen Hawking and others demonstrated that this idea was unworkable, and the singularity is an essential feature of the physics described by Einstein's gravity.

So if they believed that the Big Bang followed the Big Crunch then electrons and nuclei literally recombined. Gamow for instance favored this picture, he stressed that "from the physical point of view we must forget entirely about the precollapse period". More recently, the oscillating universe has been revived under the name Big Bounce in some non-Einstein gravity theories.

  • $\begingroup$ I think that the plasma physics usage is what the book I linked referred to as well. However, I'd always imagined that plasma physicists wouldn't be the ones doing (early Universe) cosmology. Do you have any explanation for plasma physics terminology ending up in cosmology? Of course, the term should not have been carried over from there, because it is implicit in the basic assumptions of regular plasma physics that the electrons and ions had been bound at some earlier point in time. $\endgroup$
    – Danu
    Apr 16, 2015 at 20:23
  • $\begingroup$ Also, I'm not entirely convinced about the connection to cyclic Universe models. Do you have any reference to back this up? $\endgroup$
    – Danu
    Apr 16, 2015 at 20:26
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    $\begingroup$ Zeldovich was a physical chemist and a nuclear physicist before he became a cosmologist, I added a quote that mentions plasma from his recombination paper. Gamow started as a nuclear physicist, and consulted on high explosives during the war. I can't access his paper, but "recombination" may already be there. Gamow also favored cyclic models (abstractly), so if there ever was any discomfort over "re" they removed it for him at least. $\endgroup$
    – Conifold
    Apr 16, 2015 at 23:10

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