Did It All Start With A Bang?

Here's a really fundamental question, and yet one that I think that most people don't know the answer to:

How do we know that the Big Bang is the right theory of the origin of the Universe?

There are a bunch of alternative theories out there, after all, like Plasma Cosmology, the Steady-State Theory, and Godel's Universe. But the Big Bang Theory explains three things that none of the other model's I've seen do, and they are these three:

1. The Hubble Expansion of the Universe. Things that are close to us move away from us with a certain velocity, things that are twice as far move away twice as fast, 10x as far moves away 10x as fast... etc. The Big Bang gives space its initial expansion rate, and predicts that gravity slows it down at the observed rate.
2. The Abundance of Light Elements. Why is the Universe 75% Hydrogen and 25% Helium, and much less than 1% everything else? After all, stars typically produce many heavy elements. The Big Bang tells us that The Early Universe, hot and dense, will have a bunch of free neutrons, which can fuse with the protons into Helium nuclei, but very little else. What we observe is an incredible match of theory and observations.
3. The Cosmic Background Radiation. If the Universe started off hot and dense, all that energy, all that light, all those photons that were flying around, should still be flying around today. It should all be about the same temperature by today, and that temperature should be around 3 Kelvin. This one really sets the Big Bang apart from other theories; Fred Hoyle said, upon its discovery, "We live in a fog." But it isn't a fog at all, it's a leftover echo from the Big Bang, and it's really hard to predict without one! You can read about the initial discovery here.

And that's the story. And no other theory predicts all of those three things; it's that simple. Come back later this week for follow-ups on each of those three!