“It is said that there’s no such thing as a free lunch. But the Universe is the ultimate free lunch.” –Alan Guth
We’ve talked about the Big Bang too many times to count, and why it’s easily the leading theory regarding the origin of the Universe. It’s the only cosmic idea we have that can explain all three of the following observations:
1.) The Hubble Expansion of the Universe. When we look out at distant galaxies, we find that the farther a galaxy is from us, the faster it’s receding from us! It was realized pretty early on that this is because space itself is expanding!
This means that the farther away a galaxy gets from us, the faster it will speed away in the future. Weird!
So logically, you reason, in the past, things were closer together!
Well, what are the consequences of that? If things were denser and space was smaller, than things were hotter in the past, which means that we can look at what happens if things get hot enough.
Well, if you heat normal matter, the electrons get ionized, and so there should be a “leftover glow” from when your atoms went from being an ionized plasma to being a neutral gas. And that energy should have redshifted into the infrared, or even farther, into the microwave.
2.) That’s the Cosmic Microwave Background! And its discovery in 1964 (and amazingly precise measurement, above, by COBE in the 1990s) confirmed that not only does it exist, but it’s the exact temperature and spectrum as predicted by the Big Bang!
In fact, if we go back farther, what would we expect? Perhaps it was so hot and dense that the plasma could even destroy individual nuclei, so that only protons, neutrons, electrons, and radiation were left?
Well, you’d expect very precise predictions for the amount and type of nuclear fusion you get when the Universe becomes cool enough to form stable nuclei that aren’t blasted apart by the radiation. So we come to #3…
3.) The abundances of the light elements due to Big-Bang nucleosynthesis.
And that’s exactly what we get! So the Big Bang looks great all around!
But it’s a 65-year-old idea; there are a bunch of things it doesn’t say anything about. Such as…
Why is there all this structure — stars, galaxies, clusters of galaxies and more — if the Universe was so smooth and even at the start?
It’s a good question, and one that the Big Bang, on its own, doesn’t answer. What else doesn’t the Big Bang do a good job of answering?
Why is the Universe spatially flat? It could, in principle, be any shape or curvature, and what we find, experimentally, is that it’s really, ridiculously flat. In fact even looking at the temperature fluctuations as we measure them today…
…we find that the curvature of the Universe is less than 2%, out of our observable Universe that’s 93 billion light years across! That’s pretty flat.
And then there’s the nagging problem of temperature.
The Universe — and in particular the Microwave Background — is the same exact temperature in all directions in space. But why should that be? Stuff on one side of the Universe is completely disconnected, and always has been, from stuff on the other side. Yet they’re the same temperature as surely as though they had always been touching one another. How’s that? The Big Bang doesn’t say.
Where’s all the weird stuff? If our high-energy theories are correct, the Universe shouldn’t just have a microwave background and the observed abundances of the light elements, but should also have a bunch of other weird relics, like magnetic monopoles, cosmic strings, and other topological defects. Yet not a single one has ever been found.
So, Big Bang, what are we gonna do with you? Well, if you were Alan Guth, you’d find one theoretical solution to all of these problems, and it would do what any good scientific theory would do: make a new prediction, too!
And that’s what inflation is. It says that, in order to set up the Big Bang, just prior to this hot, dense, expanding state, the Universe was expanding exponentially fast, doing the following things:
- Stretching space — whatever shape it was in before — to be so large that it appears flat.
- Taking a teeny-tiny region that was in equilibrium — where it all had a chance to reach the same temperature — and stretches it across the Universe to fill our entire, observable Universe with what was once that small, uniform-temperature region.
- Taking whatever existed before — magnetic monopoles, cosmic strings, etc. — and inflating them away, so that at most, one of them exists in our Universe.
- And finally — that new prediction — quantum fluctuations that exist during inflation give a prediction for the type and spectrum of fluctuations that started off in our Universe.
This last one is particularly important, because these fluctuations will not only be measurable in the cosmic microwave background (but weren’t in 1979, when Guth first wrote about it), but also in the large-scale structure of our Universe!
So when we made these measurements of the microwave background…
…and of large-scale structure…
And that’s why, unlike for Supersymmetry at the LHC, I think that there’s about a 98% chance that inflation is correct!
This also means that every single image you see that places the Big Bang before inflation in the cosmic timeline:
is not only wrong, but dishonest. Why? Because we have no idea what happened prior to inflation, and by the very nature of inflation, we can’t. That’s as far back as physics allows us to go, at least, right now.
So to recap, inflation is the thing that happens that sets up the Big Bang. Based on what we’ve seen, it stretches the Universe flat, makes it the same temperature in all directions, gets rid of any junk that was lying around beforehand, and creates the proper fluctuations on all scales — small and large — to grow into the stars, galaxies, and clusters that we have at present. If I were offering odds, I’d bet you 50-to-1 that inflation has it right. And that’s all I’ve got for today!