Ever since I was a child I have had this instinctive urge for expansion and growth. To me, the function and duty of a quality human being is the sincere and honest development of one’s potential.

-Bruce Lee

It isn’t only our heros that expand and grow; the Universe does that, too! In the first three parts of our series, we talked about inflation, its end, and the hot big bang. But during all of this time, the Universe has been expanding.

This is probably the most confusing aspect of cosmology, so tread slowly through this. First off, *it is space itself* that is expanding. It doesn’t make sense to ask “what’s it expanding into?” Because space is like the surface of an infinitely stretchy balloon: it can get bigger or smaller, but it doesn’t require anything external to itself.

Now, one of the biggest thing that we learn from General Relativity when we apply it to cosmology is *how* space expands, and what happens to the Universe as a result. In fact, it turns out that the **only** thing that governs the expansion of a flat Universe (like ours) is **the Universe’s energy density**, and that’s *it*.

Well, during inflation, the energy density of the Universe **is a constant**, which means the expansion rate (which is *fast*) is also constant. This means the Universe expands **exponentially**, like so.

The way this works is that after a short amount of time (for inflation, the time is something like 10^{-20} or 10^{-30} seconds), the size of the Universe doubles. After another one of those “doubling times”, the Universe is double that new size, or 4 times the original size. If you can go for 100 doubling times (10^{-18} or 10^{-28} seconds in our two examples), the Universe will now be **2 ^{100}** times its original size. It is in this fashion that we can get an arbitrarily large Universe in a fraction of a second, and that’s what inflation gives us.

Come now to the end of inflation, where the entire Universe may be the size of your thumb, or it may be a bajillion (roughly) times the size of our Universe right now. The actual, physical size of our Universe at that time **doesn’t really matter**. Why not? Because the part of it that becomes the entire observable Universe — about 100 billion light-years across today — was only about the size of your thumb at the moment the big bang began.

But when inflation ends and (what I call) the Big Bang began, everything changes. The expansion rate was still the same, and the energy density is still incredibly large, but now the Universe’s energy is in the form of matter and radiation. When the Universe continues expanding now, the **energy density drops**, because you’ve got the same amount of stuff, but the volume it occupies increases! This means the energy density goes down, and hence, so does the expansion rate!

So the Universe inflates, and the expansion is very fast and constant. When inflation ends, matter and radiation fills a small region of this space (the region that becomes our observable Universe), and the expansion — although still very fast — is now slowing down. The Universe is therefore **cooling** as it’s expanding, and despite being filled with all the possible fundamental particles that can exist at these incredible energies, will not stay that way for long.

The first thing the Universe needs to do? Figure out a way to create more matter than anti-matter, because at the very beginning, there’s about a billion times more of both matter and antimatter than there is of either one today. That’s the next chapter in our story, so come back for it!