The funny thing is that the universe didn't need to switch from being mathematical, symmetry breaking, small-sized quantum factory to something containing large numbers of particles. The universe is still governed by quantum mechanics, to the fullest limit of our understanding. It never stopped being that.

What did occur is that we reached a point where the effects of this quantum behavior became well modeled as particles. There was enough separation between things that it became reasonable to treat them as particles or isolated wave packets, or whatever you choose to view them as.

The opposite behavior is perhaps easier to observe: going from discrete particles to flows. Put a ball bearing in a box. If you tip the box far enough, the ball bearing rolls to one side and out of the box. It's path is well modeled with particle physics. Now do the same, but put two ball bearings in the box. Still particle like. Then three, then four. We should be comfortable with treating this as a bunch of particles.

As they say on the playground, "One, two, skip a few. Ninety nine, one hundred!" Let's get a dump truck full of ball bearings. How do you think they will behave? It turns out that it's very effective to think of them as a fluid flow rather than a set of distinct particles. We can start talking about fluid drag terms and the kind of oscillations that occur in fluid systems. We can even measure the total energy of the system using continuous integration techniques. We can talk about the fluid density.

So at what point did it switch from being particles to fluids? The answer is "never." It always was simply what it was. However, it was more convenient to think of it in terms of particles on one scale and more convenient to think in terms of fluids on the other. In the middle is a frustrating region where you see behaviors that are really hard to explain with particles, and really hard to explain with fluids.

So from that perspective, according to current theory, at roughly $10^{-37}$ seconds after the start of the universe we start to see behaviors that we are comfortable with calling particle/anti-particle creation and destruction events. Or at least, that's what we think. During that time, we predict the universe to have a temperature much higher than anything we've ever actually attained, so it's purely speculative. By $10^{-11}$, the universe had cooled down enough that the energies reached a range we can achieve in a particle accelerator, and we're very comfortable with thinking of particles at those energies.

The funny thing is that the universe didn't need to switch from being mathematical, symmetry breaking, small-sized quantum factory to something containing large numbers of particles. The universe is still governed by quantum mechanics, to the fullest limit of our understanding. It never stopped being that.

What did occur is that we reached a point where the effects of this quantum behavior became well modeled as particles. There was enough separation between things that it became reasonable to treat them as particles or isolated wave packets, or whatever you choose to view them as.

The opposite behavior is perhaps easier to observe: going from discrete particles to flows. Put a ball bearing in a box. If you tip the box far enough, the ball bearing rolls to one side and out of the box. It's path is well modeled with particle physics. Now do the same, but put two ball bearings in the box. Still particle like. Then three, then four. We should be comfortable with treating this as a bunch of particles.

As they say on the playground, "One, two, skip a few. Ninety nine, one hundred!" Let's get a dump truck full of ball bearings. How do you think they will behave? It turns out that it's very effective to think of them as a fluid flow rather than a set of distinct particles. We can start talking about fluid drag terms and the kind of oscillations that occur in fluid systems. We can even measure the total energy of the system using continuous integration techniques. We can talk about the fluid density.

So at what point did it switch from being particles to fluids? The answer is "never." It always was simply what it was. However, it was more convenient to think of it in terms of particles on one scale and more convenient to think in terms of fluids on the other. In the middle is a frustrating region where you see behaviors that are really hard to explain with particles, and really hard to explain with fluids.

So from that perspective, according to current theory, at roughly $10^{-37}$ seconds after the start of the universe we start to see behaviors that we are comfortable with calling particle/anti-particle creation and destruction events. Or at least, that's what we think. During that time, we predict the universe to have a temperature much higher than anything we've ever actually attained, so it's purely speculative. By $10^{-11}$, the universe had cooled down enough that the energies reached a range we can achieve in a particle accelerator, and we're very comfortable with thinking of particles at those energies. During that fuzzy region in between, you have a strange mix which sometimes exhibits particle like properties, and other times does not.

Another real-life example of these phase transitions acting interesting is the superfluid state. We know that if you add heat to a liquid, you can make it evaporate and turn into a gas. We also know that if you pressurize a gas, you can turn it back into a liquid. We can draw a line between these, and draw a phase diagram:

Note the "critical point." When you reach a high enough pressure in a gas or a high enough temperature in a liquid, it actually stops acting like either. There's too much energy for it to act like a liquid, but its too dense to have the free-space between collisions associated with a gas. It actually starts acting like "something else" which is neither liquid nor gas. We simply stop drawing the line between liquid and gas at that point because it ceases to be meaningful.

So what defines this critical point? Well, we do. The critical point is defined to be the pressure/temperature at which we simply cannot see the distinction between the phases. It ceases to be meaningful to model the material as a liquid or a gas.