Why didn't the strong nuclear force stop the Big Bang from happening?

Factual Questions
1

My understanding, admittedly weak, is that the strong nuclear force holds protons together that otherwise would repel each other for having like charges. (Neutrons too, but they don’t repel.) I’ve read that in order to force protons close enough together for the strong force to take hold, via the exchange of mesons, requires enormous pressures and/or temperatures. But I’ve also read that there have never been greater pressures and temperatures than in the earliest stages of the Big Bang.

So why didn’t the strong force hold particles together and keep them from expanding away?

2

As they say on television, “It’s complicated.”

First of all, what you refer to as the “strong force” is more appropriately called the residual strong force or nucleon interaction. The strong interaction or color force governs intranucleon connections, keeping quarks and gluons “stuck together”, or more explicitly, confined in a nucleon, hence why there are no free quarks under anything like normal conditions. The residual strong force–that which acts as a reservoir for the binding energies that keep atomic nuclei together–arises as a result of local imbalances in color charge, kind of like the polar moments of atoms and molecules from the distribution of electrons in valance orbits. Breaking (or making) those bonds in a stable nucleus is very difficult, and gets harder as nuclei size goes up, but many unstable or metastable nuclei need only a small push giving rise to nuclear fission, alpha decay, and other nuclear transformations.

At the time of the Big Bang, all force interactions that we experience (electromagnetic, strong, weak, and presumably gravity, although the lack of a useful quantitized field theory for that force means that the current Standard Model for particle physics can’t say anything worthwhile on that topic) were combined into one quantitized self-interacting field called the Higgs field. (Particles arrising therefrom are called “Higgs bosons” and are believed by some to be the truely fundamental field that controls interactions behind the scenes that define the other types of fundamental particles and their interactions.) This field has an enormous energy density, vastly beyond anything we can currently create by many orders of magnitude. However, being only a single force, it has nothing to couple with; that is, there is no opposing force as with the attractive color force and repulsive electrical force that keeps nuclei in stable tension. (The last statement is an oversimplification–the interactions are more complex than that–but it’ll do for the purposes of illustration.)

So the field has to either expand or contract; fortunately for us, the vector was pointing outward and the field expanded very rapidly, stretching space along with it. However, once it got below a certain energy density, the “symmetries” that permitted the force to be unified were “broken”, and other forces started falling out; first gravity, then the strong interactions, then weak interactions, and finally the electromagnetic force, resulting in our four existing forces. (There are some postulates about a fifth force, or multiple opposing components to gravity, but these don’t work with existing theories of unifed fundamental forces, as nascent as they are.) At the time of the Big Bang, there was no residual strong force because there wasn’t any separation of color charge, and indeed, no nucleons or other non-fundamental baryons to act upon; all matter was nothing but quarks in a sort of global confinement of a high pressure quark-gluon soup.

Stranger

3

The missing link in your reasoning is that no matter how strong the attractive force is between two nucleons, you can always overcome it by giving the nucleons even more energy. The interaction between two nucleons can be likened to rolling a ball up a hill that’s kind of volcano-shaped, with a big depression in the middle. If you give it a tiny bit of energy, it’ll roll partway up the hill and come back down. If you give it more energy, it’ll roll up the hill, fall into the depression at the top, and stick there. But if you give it even more energy than that, it’ll shoot up the hill, roll straight into and back out of the depression, and keep going out the other side.

So while it’s true that very high temperatures are required to get nucleons (protons and neutrons) to “stick together” via nuclear fusion, the temperatures at the earliest stages of the big bang were even higher. Given our current understanding of the Big Bang, we’re pretty sure that temperatures were up around the Planck Temperature (about 10[sup]32[/sup] Kelvin) shortly after the Big Bang, give or take a factor of 10. Nuclear fusion requires temperatures of only about 10[sup]8[/sup] Kelvin — cool and refreshing by comparison.

As the Universe cooled down, the nuclear energies in primordial plasma got low enough to “freeze out” into nucleons such as hydrogen, helium, and… well, that’s pretty much it. The trace amounts of lithium & beryllium that were created are important windows into the dynamics of the early Universe — by studying the relative amounts of these elements, we can better figure out what was going on back then. See Big Bang nucleosynthesis.