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This may sound like a chemistry problem, but I suspect this would have lot to do with primordial nucleosynthesis. After all physics underlies everything.

It is believed that several elements were formed during this period. Wikipedia says:

Primordial nucleosynthesis is believed by most cosmologists... to be responsible for the formation of most of the universe's helium as the isotope helium-4 (4He), along with small amounts of the hydrogen isotope deuterium (2H or D), the helium isotope helium-3 (3He), and a very small amount of the lithium isotope lithium-7 (7Li). In addition to these stable nuclei, two unstable or radioactive isotopes were also produced: the heavy hydrogen isotope tritium (3H or T); and the beryllium isotope beryllium-7 (7Be); but these unstable isotopes later decayed into 3He and 7Li, as above.

We see that there are several candidate elements that can make compounds. But given the extreme conditions prevailed right after the primordial nucleosynthesis, would any compound have formed before the birth of stars? If so, which would have been the very first compound have formed in the universe?

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    $\begingroup$ Chemical compounds can only form at low enough temperature. Lithium hydride, for instance, decomposes at 900-1000 degrees C, so it can't have existed before temperatures dropped below that threshold. It took the universe roughly 10 million years after the big bang too cool down that much. Beryllium hydride decomposes at 250 degrees Celsius and unlike LiH does not form spontaneously from elements. So my best guess is, that there may have been a tiny fraction (1e-11 to 1e-10) of LiH in the universe after approx. 10 million years. I might be wrong, though. $\endgroup$
    – CuriousOne
    Commented Oct 7, 2014 at 9:41
  • $\begingroup$ @CuriousOne: Hydrides were the first thing that came into my mind, but they might be just vanilla. I was wondering if there could be any exotic compounds that can form in higher temperatures. $\endgroup$
    – sampathsris
    Commented Oct 7, 2014 at 9:43
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    $\begingroup$ I have not found anything but Lithium and Beryllium in the usual nucleosynthesis simulations. So that leaves the hydrides, I guess. I am not familiar enough with the chemistry of Helium to tell if there is any chance of making a Helium compound. My guess is that there isn't. And the there are the really rare LiBe clusters, of course, and their hydrides... if that's "exotic" enough. All in all it seems that the early universe was a chemical wasteland. $\endgroup$
    – CuriousOne
    Commented Oct 7, 2014 at 9:48
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    $\begingroup$ see S. Lepp, P.C. Stancil, A. Dalgarno “Atomic and molecular processes in the early Universe” J. Phys. B.: At. Mol. Opt. Phys. 35 (2002) R57-R80, and this lecture: mpia-hd.mpg.de/homes/semenov/Lectures/Heidelberg_Uni_2012/… $\endgroup$
    – DavePhD
    Commented Oct 7, 2014 at 11:27

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The first compound/molecule formed was helium hydride. Why was helium hydride ion HeH$^+$, not hydrogen H$_2$, the first molecule formed in the early universe?

Helium hydride started to form at redshifts of $z \sim 2000$, after helium had recombined, but all the hydrogen was still ionised. $${\rm He} + {\rm H}^{+} \rightarrow {\rm He H^+}$$

This is about 260,000 years before before the epoch of recombination at $z\sim 1100$, after which H$_2$ became the dominant molecule, and way before when the first stars began to form at $z \sim 50$.

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After the first supernova, it could possibly have been water. Water is one of the most abundant compounds found in the universe. And hydrogen and oxygen are 2 of the most common elements.

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Probably the first compound would have been H2 as a consequence of the 2N^2 law which permits only that number of electrons in the N orbit where N=1,2,3......

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