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I am a chemist with a passion for astrophysics and particle physics, and one of the most marvellous things I have learned in my life is the process of stellar nucleosynthesis. It saddens me how my colleagues point to the periodic table and draw compound structures so often, yet never seem to be curious about where the elements on which they rely came from, and completely miss the beauty underneath.

As far as I understand, there are several types of nucleosynthesis conditions (big bang, stellar, supernova, black hole accretion disk, artificial, pycnonuclear, and perhaps others), the first three of which are most important when discussing the composition of the Universe, but I wish to focus on an aspect of big bang nucleosynthesis.

The Universe became extremely hot after inflation (or so it is thought), and while cooling down there was a very brief period in which the temperatures and densities were adequate for protons and neutrons to exist and fuse into heavier elements. However, this period of nucleosynthesis was so brief that most of the nucleons formed in baryogenesis didn't even have time to fuse, ending up as hydrogen. Of the amount that did manage to fuse, almost all of it stopped at helium, helped by the exceptional stability of the $^4He$ nucleus relative to its neighbours. Only a tiny amount of lithium is said to have been made, and I've rarely heard anyone discussing elements heavier than that. It is often said that "metals" (in the astronomical sense) only really appeared after stellar nucleosynthesis began, but there was already a lot of matter around after baryogenesis ended (about the same amount as the $10^{80}$ baryons present now?), so even the tiniest atomic fraction of an element could correspond to galactic masses' worth of it.

It would be really neat if it turned out that carbon, the very stuff of life as we know it, didn't exist at all until over a hundred million years after the big bang, when the first generation of stars began to light up. However, I don't know if this is the case, as confirmation or refutation relies on precise quantitative modelling of the big bang nucleosynthesis. Is it possible to figure out when the first carbon nuclei came into existence? I tried using a search engine, but my specific query is completely drowned in links covering the more general aspects of nucleosynthesis.

Note: I suppose that if one were to consider the matter outside the observable Universe, then there probably were carbon nuclei even the tiniest timestep after nucleosynthesis began, because the sheer ridiculous amount of space expected to be out there could well overwhelm the nigh-impossible odds of carbon formation. Thus, the calculation is more interesting if limited to our observable volume.

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  • $\begingroup$ See physics.stackexchange.com/a/3837/37548 's first paragraph, it also answers your question. $\endgroup$
    – Kvothe
    Mar 28, 2014 at 2:08
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    $\begingroup$ That's the crux of my question. Everyone says that essentially all of the elements heavier than lithium come from stellar nucleosynthesis, but the problem is hidden in quantifying what "essentially all" means. Basically, have we performed any calculations which can resolve whether we should statistically expect at least one carbon atom from nucleosynthesis? $\endgroup$
    – Nicolau Saker Neto
    Mar 28, 2014 at 2:17
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    $\begingroup$ If you mean atom, stick with atom, but if you mean nucleus then edit. The two have different answers. $\endgroup$
    – ProfRob
    Jul 14, 2015 at 22:15
  • $\begingroup$ @RobJeffries Good point, it never occurred to me, but you are right. I'll edit the title to avoid confusion. $\endgroup$
    – Nicolau Saker Neto
    Jul 14, 2015 at 23:04

2 Answers 2

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The article The path to metallicity: synthesis of CNO elements in standard BBN attempts to quantify the amount of carbon produced during big bang nucleosynthesis.

It concludes that the ratio of carbon-12 formed to hydrogen was $~4 \times 10^{-16}$

with lesser amounts of carbon-13 and carbon-14.

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    $\begingroup$ Very interesting. The amount of carbon is actually far higher than I expected; I assumed the high energy barriers and limited intermediate abundances would have created a far sharper decline in production with increased nuclei mass. So even though the amount of carbon after big bang nucleosynthesis was approximately 10 orders of magnitude smaller than it is today, in absolute terms, there was still a hell of a lot of it in the observable Universe, some $10^{65}$ carbon nuclei. $\endgroup$
    – Nicolau Saker Neto
    Apr 21, 2014 at 21:00
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    $\begingroup$ The question asks when was the first carbon atom formed! These are the first carbon nuclei. I know, pedantry, but it means the answer is more like $10^5$ years after the BB. $\endgroup$
    – ProfRob
    Jul 14, 2015 at 11:44
  • $\begingroup$ @RobJeffries First we'd have to decide if "atom" means only neutral atoms or carbon with at least one electron. Then from density and temperature find the fraction of carbon in each ionization state like in table 4 here: adsabs.harvard.edu/full/1961ApJ...134..435R And finally consider the $10^{65}$ carbon nuclei and calculate the probability that one of these has been an atom. $\endgroup$
    – DavePhD
    Jul 14, 2015 at 12:30
  • $\begingroup$ Yes. I agree with your analysis - an atom would normally mean the full complement of electrons. $\endgroup$
    – ProfRob
    Jul 14, 2015 at 12:59
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    $\begingroup$ Problem solved! Now all you need to do is work out the when (in the first few minutes). $\endgroup$
    – ProfRob
    Jul 14, 2015 at 23:34
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Carbon has to be produced by the triple-alpha process because there is no stable nucleus with 8 or 5 nucleons. The probability of this is very low, because it requires three different particles to be in the same place at the same time.

You'll note that the Wikipedia article says:

One consequence of this is that no significant amount of carbon was produced in the Big Bang because within minutes after the Big Bang, the temperature fell below that necessary for nuclear fusion.

There would have been some carbon created in the Big Bang, if only because the universe is infinite (or at least very big) so even the small probability of the triple alpha reaction happening in a few minutes means the reaction must occur to some extent. I don't have figures for how much carbon was created, though with some head scratching and approximations about the conditions during nucleosynthesis it could be estimated. However it's clear that the vast majority of carbon has been created in stars.

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