© Matt Strassler [March 15, 2014]

The inflationary epoch was a possibly very brief but certainly spectacular period when the space inside a region of the universe which includes our own observable patch (i.e., the part of the universe that we can observe today) expanded with blistering speed — completely unbelievable speed. The expansion rate was so big that it sounds totally insane. And the only thing that keeps it from being insane is that the theory of inflation makes predictions which, so far, agree with our measurements of the cosmos. (Including those by BICEP2.) That doesn’t mean it’s right, but it does mean that

there are good reasons to think it might be right, and

no one can currently show that it’s wrong.

Let me say again: the space expanded. Things didn’t rush into the space: the space simply became much larger. It’s not one bit like an explosion. Click here to read more about the difference between an explosion of something into space and an expansion of space itself.

How insane is this rate of expansion? A patch of the universe no larger than your computer screen expanded to the size of the observable patch of the universe, or larger, in less than the time it takes for a quark to cross from one side of proton to the other. I won’t even bother to tell you the numbers, partly because we don’t actually know how long inflation lasted, but also the numbers are too big in size and too small in time for humans to think about them. Basically, a giant chunk of universe was created from a tiny one almost instantaneously.

What was the universe like during the period of this expansion? Empty. Extremely empty. Much, much, much emptier than space is now. Extremely cold. Extremely dark. Anything which might have been there before inflation started would have been pulled apart and dragged to great distances in an instant. [Small Caution: There is a moderately important and very subtle caveat to the empty/dark/cold statement, and I haven’t figured out how to write a comprehensible article on it yet. Rather than “extremely”, it would have been more accurate to say that the universe was “maximally'” empty, dark and cold — empty of everything except quantum fluctuations.]

What happened before inflation, and how inflation got started, we don’t know. There are a number of reasonable scientifically-grounded theoretical ideas, but they’re all speculation until someone thinks of a way to test them by making measurements. There may not even have been a “before inflation”, either because inflation is always going on somewhere in the universe, or because time doesn’t really make any sense if you go back too far, or for some other reason. But in many contexts it almost doesn’t matter, as I’ll now explain through a set of figures, answering some frequently asked questions along the way.

What caused this insane rate of expansion?

The cause was a large amount of what is often called

“dark energy” (but it’s not energy, it’s energy and negative pressure in the right combination) or

the “cosmological constant” (Einstein’s [non]-blunder: but fortunately it wasn’t constant, or the universe would have inflated forever) or

“dark smooth tension” (which is correct but it’s kind of clunky-sounding and not any clearer.)

Anyway, the universe has some of this stuff now, which is why the universe’s expansion rate has started to increase in the past few billion years. But (we suspect!) at some point, for some reason, it had a lot, lot more. And this caused the region containing our part of the universe to expand with a rate that accelerated enormously… i.e., caused it to “inflate”. See Figures 1,2,3, which contain a wild and surely wrong guess as to how inflation started, but by Figure 4, the details of the guess have become completely irrelevant.

Where did this huge amount of “dark energy” come from?



We don’t know. There are various suggestions … some of which have been ruled out by recent data. We hope to learn more about this question in the coming decade.

Why doesn’t the rate of expansion slow down as the dark energy becomes diluted by the expansion?



Curiously and surprisingly, as the universe inflates and its volume grows, the amount of dark energy per unit volume stays the same. That means it will inflate and inflate and inflate, without slowing down, until something makes the dark energy go away.

Note how the red and green dots are receding from each other with enormous speed.

Doesn’t that incredible expansion mean that things moved apart faster than the speed of light … the universal speed limit?



Yes it does.

And doesn’t that violate Einstein’s theory of relativity?



No it doesn’t. Einstein’s theory says that if two objects pass each other at the same point, an observer moving with one of them will measure the other to be traveling below or at the universal speed limit, and never faster. But two objects at two different points can move apart faster than the speed of light if space itself expands… which is what happens in the expanding universe. Read more here about the expansion of space, and how and why it is completely different from an explosion.

That’s right. [Meh. Kinda right. As cold as it could possibly get; but there’s those quantum fluctuations around that make this statement subtle.] The universe became hot after inflation; see below for more on this. Whether it was also hot at some period before inflation is completely speculative; there’s no evidence one way or another. But during inflation, the temperature dropped to a tiny fraction of a degree above absolute zero [but… but… this needs an article…].

Why did inflation stop?

We don’t know. But again, there are a number of scientifically grounded suggestions, ones with equations and predictions and ways to test them, at least in part. We may learn more soon from ongoing studies of the cosmos.

What happened when inflation stopped?

The best guess as to what happened (and our equations show this is possible, but don’t tell us the details) is that all that dark energy got turned into particles — including particles we’re made from, and lots of other types of particles we know about, and perhaps lots of particles we don’t know about. And when this happened, the universe became very hot, and very dense — and it continued expanding, though much more slowly.

This was what is the origin of the Hot Big Bang. Some people (including me) simply say: “This moment is the start of the Big Bang”. Others say that the Big Bang includes the Hot Big Bang and inflation, though this is odd, since inflation is more of a Whoosh than a Bang. Some say that inflation is what put the “Bang” into “Big Bang”, by first making the universe large and expanding, and then making it hot. Still others say that it includes the Hot Big Bang, inflation, and everything that came before it… but this is risky, because before inflation there might have been something that does not in any sense deserve the term “Bang” (which implies a very energetic, intense and sudden event.)

Since this terminology hasn’t settled yet, what you decide to call “The Big Bang” is kind of up to you. It’s just important to know that you have different options, and that different scientists and websites may use different meanings for “Big Bang”.

Read more about the period before inflation or after inflation.