# Why did the Big Bang produce hydrogen?

I know that first generation stars' main fuel was Hydrogen. I know the Big Bang happened at some point in time. Now if the strong force exists, then why aren't different, higher mass, number elements produced? why was there single proton nucleus like hydrogen?

• I'm not familiar enough with all the details to give a full answer, but I would recommend you search for "Big Bang nucleosynthesis" on this or other sites until someone provides a more complete answer here. – Michael Seifert Nov 20 '19 at 12:40
• Worth noting that small amounts of helium and even smaller amounts of lithium were produced... just not in significant proporitons compared to hydrogen – Alex Robinson Nov 20 '19 at 13:10
• The Big Bang happened at a point in time, but it most definitely did not happen at a point in space. See physics.stackexchange.com/q/136860/123208 – PM 2Ring Nov 20 '19 at 14:13
• @AlexRobinson Well, the "small amount" of helium runs to about 25%. (Though I can't recall if that figure is by number or by mass. If by mass it comes to about 6% by number.) Perhaps "not significant" is too strong to be applied to helium sysnthesis in the big bang. – dmckee --- ex-moderator kitten Nov 20 '19 at 23:49
• @dmckee at the start of BBN the ratio of protons to neutrons was ~7:1. Of a given 14 protons and 2 neutrons, 2 of each end up as one helium atom, leaving 12 protons. The He/H ratio is therefore 1:12 (7.7%) by number of atoms, or 4:12 (approx) (~25%) by mass. [As an aside: it's quite exciting to do this maths with a junior school student, then get them to look up on the internet the ratio of elements we observe in the current Universe, and watch the reaction: genuine amazement that they themselves were able to calculate this, armed only with an N:P ratio!] – Chappo Hasn't Forgotten Monica Nov 21 '19 at 22:44

The key idea is the following: In order to form metals (anything heavier than hydrogen and helium), you need deuterium nuclei. But deuterium nuclei are only formed significantly when the temperature of the primordial plasma falls far below the binding energy of deuterium (T ~ 2.2 MeV). Why? Well, the formation of deuterium has to compete with the enormous amount of high-energy photons in the universe at the time, which split up the deuterium nuclei. So the photon bath has to be cool enough, so that most of the photons don't have enough energy to split the nuclei apart again. The relevant number is the baryon-to-photon ratio $$\eta\sim10^{-9}$$, i.e. for each baryon we have $$10^9$$ photons.