Recently, a discussion started in one of my comment threads about whether the Big Bang was necessarily valid or not, and whether there were any reasonable alternatives. The answer is that not only is the Big Bang the best theory to explain the start and evolution of the Universe, it’s the only one that doesn’t make incorrect predictions. Let’s see this in action.
This all started in the late 1920s, when we realized that spiral nebulae were not just galaxies, but that these other galaxies were nearly all moving away from us. Moreover, the ones that were farther away from us were moving away faster than the ones that were nearer to us. This lead us to the picture that the Universe was expanding, like so:
After a little bit of math to go along with this, we quickly realized that the Universe is acting much like the Earth would if you had detonated a huge bomb at the center:
In this example, pieces of the Earth would be flying apart from each other, but Gravity would be struggling to pull the pieces back together. So it is with the Universe: galaxies are flying apart from one another, but Gravity is trying to pull them back together. This means that the expansion is slowing down as the Universe gets older, a hugely important discovery.
The reason I say this is important is because if the Universe is expanding and the rate of expansion is slower in the future, that means the expansion was faster in the past! It means that, billions of years ago, galaxies were much closer together to one another than they are today. It means that, over 13 billion years ago, all the matter in the Universe was bunched together at higher and higher densities. If you extrapolate all the way back to the entire Universe being in a single location with a nearly infinite density, before it expanded into what it is now, what do you get?
Well, folks, that’s the idea of the Big Bang. Now, there are other possible implications for the Hubble expansion of the Universe, not all of which result in a Big Bang. But let’s imagine the Big Bang were correct; what else would happen? Well, if we look at the Universe when it was early enough, it would also be denser and hotter. In fact, we can go back so far, when the Universe was only a few hundred thousand years old, to when it was too hot for neutral Hydrogen! We can recreate this in labs, and it’s known as plasma:
Well, when this plasma finally cools enough (due to the Universe’s expansion), it ought to emit light, and allow all the light left over from the big bang to simply move through the space of the expanding Universe until it reaches our eyes. But, that light wouldn’t be UV light or visible light anymore. After 13 billion years of expansion, that light ought to only be a weak signal peaked at a temperature of only a few Kelvin. And it ought to be everywhere on the entire sky, nearly exactly the same in all directions, and it ought to fluctuate on timescales so slow that we’ll never live to see it. The data from the result of this search?
Oh, was it momentous when we discovered this! The graphical depiction of the temperature of this leftover radiation on the sky looks like this:
But there’s more. You see, we can extrapolate all the way back — not just before neutral atoms — to when it was so hot and dense that individual nuclei couldn’t exist! Well, when the Universe finally cools enough so that they can exist, there should exist a certain number of neutrons, protons, and photons. We can predict how old the Universe is when this critical cooling occurs, and we find that the abundances of Deuterium (one proton and one neutron), Helium-3 (two protons and one neutron), Helium-4 (two protons and two neutrons), and Lithium-7 (three protons and four neutrons) are all highly dependent on one parameter: the ratio of photons to nucleons. In other words, we get four separate predictions for elemental abundances from the Big Bang theory. Then, we go out and measure these abundances, and see if they match up. The results?
They all — to within reasonable errors — match up as well. And the combination of these three observations, which are in strong agreement with what the Big Bang predicts, are why we subscribe to this theory so strongly.
Like I said, there are plenty of other explanation for just the Hubble expansion of the Universe. The problem is, none of them also explain the Cosmic Microwave Background and the abundance of the lightest elements. If you’d like to know about the many alternative explanations, I recommend reading Chapter 7 of this book, which goes through each possibility and why it’s discredited. But the Big Bang isn’t the final theory of the Universe, it’s just a step along the way, as it’s already been expanded upon. But, for what it is, it isn’t just the best theory we’ve got. It’s a great one.
Enjoy your weekend, everyone!