I keep wondering about how can we be so sure about what happened before the CMB, given that it is the oldest thing we can actually see.
It seems like we are very confident about what really happened from $10^{-12}\ \mathrm s$ after the Big Bang until $380\,000$ years after the Big Bang, but how?
How do we know the universe was expanding before the CMB? We are sure it is expanding since after the CMB until now, but are we really sure it was expanding before the CMB, and if yes, how can we be so sure?
How do we know the temperature of the universe was approximately $10^{12}\ \mathrm K$ at $10^{-12}\ \mathrm s$ after the Big Bang?
I have read that we were able to recreate temperatures as high as $10^{12}\ \mathrm K$ in the Large Hadron Collider and to observe it directly. OK, but how would that be an evidence about what happened $10^{-12}\ \mathrm s$ after the Big Bang?
Best Answer
You are quite correct that we can't see what happened before the CMB (this time is known as recombination) but this is not unusual in Physics. For example we can't see what happens at collisions in the Large Hadron Collider. All we can see is the debris that comes flying out of the collisions. But we understand the physics involved so by measuring the properties of the debris we can calculate what happened in the collision. That's how the Higgs boson was discovered. It wasn't directly observed but its existence was shown by precise measurements of the particles that we can detect.
And the same applies to the universe. The CMB is the debris that came flying out of the Big Bang, so by measuring the properties of the CMB we can calculate what happened at times before recombination.
The obvious question is how we know our calculations are correct. The way we approach this is to try calculating the same thing in different ways. For example Higgs bosons can be detected in several different ways, and if those different measurements gave different masses for the Higgs boson we'd know at least some of our calculations must be wrong. This is harder for the universe since we only have the one universe and the creation of the universe isn't an experiment we can repeat. But we can still cross check various different calculations and at least make sure they are consistent, which is exactly what is done.
Recombination happened about 370000 years after the Big Bang, and in fact the physical properties of the universe at this time are easy to understand. The density and temperature are in the range that we can recreate in the lab so we can directly probe the properties of plasma under these conditions. Indeed even as far back as nucleosynthesis, which happened only a few minutes after the Big Bang we still understand the physics well from experiment.
For example you mention a time $10^{-12}$ seconds after the Big Bang, and this time is normally taken to be the end of the electroweak epoch. From this time on the interactions between particles in the universe occur at energies that can be probed in colliders so we can experimentally determine what would be happening from this time onwards. Incidentally the temperature at this time was more like $10^{15}$ K than $10^{12}$ K.
But it is certainly true that as we go back towards the Big Bang there comes a point where the density and temperature exceed anything we can study experimentally, and we can be less sure what happened then. This is still an active area of research.