I have read this question:
Over the next 380,000 years, the universe gradually cooled down enough for the sub-atomic particles to condense and form the first Hydrogen atoms
As far as I understand, heavier elements are formed in the core of super-heavy stars, where there is extreme pressure.
https://en.wikipedia.org/wiki/Stellar_nucleosynthesis
But the answer says that the early universe was too hot (and I guess this means too dense and too much pressure) for atoms to form.
So basically what I am asking is, why was the pressure in the early universe conceptually different (hindering atoms from forming) from the extreme pressure required at the core of the stars to form atoms (which on the other hand was the reason for heavier atoms to form)?
You need pressure to form atoms, and more pressure to form heavier atoms, but too much pressure means no atom can form. Why? What happens above certain energy (pressure) levels that hinders atoms from forming, when the very driving force of atom formation is pressure itself?
Question:
- If for heavier atoms to form, you need a lot of pressure, then why do we say that the early universe was too hot for atoms to form?
Best Answer
TL;DR: Your question should really be asking why Big Bang Nucleosynthesis (BBN) does not produce heavier nuclei (not about the formation of atoms). There's no fundamental reason BBN couldn't produce nuclei up to Iron, if the Universe stayed at a sufficiently high temperature for long enough; but since BBN only lasted for 3 minutes, it could only get so far up the periodic table. Stars get up to Iron because they have so much more time; also, metallicity increases over the history of the Universe so later generations of stars get a "head start" over earlier ones). To get beyond Iron, you need an extremely violent non-equilibrium process, which you find in supernovae and neutron-star mergers.
There are quite a few things to unpack here.
First, let's take pressure out of the game, since I think it is just confusing things, and focus on temperature.
Second, there is a difference between the formation of atoms and the formation of nuclei. The quote you give is about the formation of atoms, but the image you show is about the formation of nuclei.
Before going further, let's define some terms. The formation of atoms (in cosmology) is called recombination. I think the reasoning behind this name is that electrons and protons are "combining" to form atoms. This is a bad name since atoms had not previously formed in the Universe, so nothing is recombining, but we are stuck with the terminology. Meanwhile, the formation of nuclei is called nucleosynthesis. Nucleosynthesis can be further broken down into the region of spacetime where it occurs: primordial nucleosynthesis or Big Bang Nucleosynthesis (BBN), which happened everywhere in the very early Universe (about 10 seconds to three minutes after the Big Bang); stellar nucleosynthesis occurs in the cores of stars when the Universe is sufficiently old that stars are able to form; and then what I'm going to call nuclear capture nucleosynthesis as a grab bag of other processes; the $s$-process ("slow" process) which occurs in supernovae, and the $r$-process ("rapid" process) which is now believed to occur in the collisions of neutron stars -- these processes happen as some stars reach the end of their lifetime.
The reason that recombination cannot happen until the Universe has cooled to a sufficiently low temperature, is that Hydrogen has a binding energy of $13.6\ {\rm eV}$. If a typical photon in the primordial plasma has enough energy to ionize Hydrogen, then Hydrogen won't form (I'm focusing on Hydrogen since the majority of the nuclei were Hydrogen at this stage). You might think that recombination would happen at a temperature of $kT=13.6\ {\rm eV}$, but in fact the recombination was around $3000\ {\rm K}$, for which $kT=0.26\ {\rm eV}$! The reason is that the number of photons in the Universe vastly exceeds the number of baryons, so for recombination to occur the temperature needs to cool down enough that the ionization of a Hydrogen atom is extremely rare, accounting for the enormous number of photons per each Hydrogen nucleus.
I think your question is also asking about nucleosynthesis. This story is quite involved; here I will just try to summarize a few of the main points.
First, BBN took place over the course of about 3 minutes (hence Weinberg's famous book, "The First Three Minutes"); it begins when free neutrons and protons can form, and ends when the rate of nuclear reactions becomes negligibly small. This timescale is set by the Universe's expansion rate; as the Universe expands, it cools, and over the course of three minutes it cools enough that nuclear fusion processes are no longer thermodynamically favorable. The time scale is crucial; BBN reaches several "bottleneck" processes which are very rare, and so have a very small rate, that essentially stop BBN from producing nuclei heavier than Lithium. On the other hand a main sequence star's lifetime is about 10 billion years. So there is much more time for nucleosynthesis to occur, and generate heavier elements, than in BBN.
Second, the conditions of BBN and stellar nucleosynthesis are quite different. At the start of BBN, the Universe consisted of a gas of protons and neutrons that had to fuse into Hydrogen, and work up to heavier elements one step at a time. Meanwhile, there have been a few (2-3) generations of stars; each generation undergoes nuclear fusion and populates the interstellar medium with heavy elements. These heavy elements in turn fuse future generations of stars, which are not "starting from scratch".
Finally, even in stellar nucleosynthesis, there is an upper limit on what fusion can do. Nuclei heaver than iron have a lower binding energy than iron, and so in thermal equilibrium will tend to disassociate and become iron. Therefore, non-equilibrium processes need to come into play to generate nuclei heavier than iron. This is the role of the $s$-process and $r$-process I mentioned earlier. In a violent event such as a supernova or a neutron star merger, nuclei that are ejected with very high energies will fuse, form stable heavy nuclei, and leave the cataclysm from whence they came to be distributed safely in the cosmos. These processes are responsible for the elements colored purple, green, and yellow on your chart.
To summarize: