The Big Bang is what you get when you back-extrapolate the today visible expansion of the universe into the past. One gets to the point where there is the so called “reheating” after inflation. The result of reheating is the Big Bang, a hot and dense state for sure, but it is not thought to be the beginning anymore.
The Big Bang is a set of conditions of an extremely hot, dense, expanding Universe that exists after the end of inflation.
As you see, before the yellow Big Bang, now inflation is the mysterious event. And wow does it get “blown up”: It is an incredibly fast expansion, from the size smaller than an atom to the size of gazillions of universes in a tiny fraction of a second, exponential, faster than light, or at the speed of light, anyways mind bogglingly rapid, overwhelmingly fantastic. Not just media hype inflates inflation like this. Prominent physicists sell their field in this manner.
Actually, inflation is painfully slow and boring. How so? The smoothness of the cosmic microwave background (CMB) implies an expansion of the length of the universe by a factor of more than 1026. This is a large number, but 1026 roughly equals 287. Thus, the length of the universe has to only double 87 times. Twice the length results in eight times the volume. Hence, volume doubled only 260 times. If we were to imagine fundamental volumes (microscopic boxes of space) multiplying like bacteria, they would have to only go through 260 generations! Not such large numbers anymore, are they?
Straw-man: “But you forgot the incredibly small time of inflation.”
Yes, Δt, the time interval during which the expansion occurs, just a blink of the eye. Let me get back to this in the next post, because I want to consider the most incredibly fast inflation ever. You will see that it is still at least 100 million times slower than light.
For those who like equations: Exponential inflation means that length grows proportional to exp(HΔt). H is the Hubble constant. A factor of about 1026 implies that (HΔt) is about 60. The rate r at which the universe doubles its volume is simply r = 3 H/ln2. The factor 3 comes from space being 3 dimensional. The doubling yields the factor ln2 = 0.693… . The volume doubles N = r Δt = 3 HΔt /ln2 = 3 * 60/0.693 = 260 times.