Question: Was the inflation, like a second big bang?

Laurence Perreault Levasseur answered on 16 Jun 2014:

That is a tricky question.
If we track the expansion of the Universe backward in time, we can get an idea of what the Universe looked like very far in the past. If you go back to when the Universe was just 400 000 years old (it’s almost 14 billion years now!), the Universe was a lot denser and hotter, so much that all the stuff that makes up galaxies formed a sort of uniform soup. If you go even further back, the Universe was even denser and hotter, and if you go all the way back, everything – all space and all matter – was contracted into a single point of zero volume. That is the time of the Big Bang.

Often, in popular science, the Big Bang is pictured as some sort of explosion. But this is not technically correct. The Big Bang did not occur at a single point in space as an ‘explosion’. It is better thought of as the simultaneous appearance of space everywhere in the universe, from literally nothing. (I know, this is a really hard idea to image, at least for me!!)

Inflation is something different. It’s a period of time just after the Big Bang where the expansion of the Universe was crazy insanely fast (exponential, to be precise), for a crazy insanely short period of time (10^-33 seconds, that’s 0.000000000000000000000000000000001 seconds). So it was an extremely violent even, but still it’s not quite like an explosion, because (just like is the raisin bread analogy on my profile) it’s space itself that was inflating and becoming bigger, not the particles of matter moving away from each other through space really fast.

So, to summarize: The Big Bang is the instant in time when the Universe was born and all space simultaneously appeared from nothing, and inflation is a (very short) period of time when the expansion of space was extremely rapid.

(This bit of the answer is a bit more technical, so don’t worry too much about it if it seems more complicated: Actually, at some point when we go backward in the history of the Universe, somewhat before inflation and just just before we hit the time when everything was contracted in a single point, matter becomes so hot and dense that the current laws of physics we understand here on Earth seem to break down. That means we theorists have no idea how to go further back in time, what happens, or why. That point where all our physical theories break down is as far as we can go back in time, and ‘Big Bang’ is the name we give to the time when we don’t know what’s going on. To know what happens, we need a better, or more complete, theory of nature, something called quantum gravity. Some theorists think string theory can be such a theory, but all this an active area of research.)