Why do we think the Big Bang happened?

…and you may ask yourself, “Well, how did I get here?”
– Talking Heads

Everyone has at least heard about the Big Bang theory (the scientific theory, not the television show). It’s the best theory to explain the creation of the universe. But what’s less apparent to nonscientists is why we believe the Big Bang theory is correct as opposed to, say, the theory that the Universe was sneezed out of the nose of a being called the Great Green Arkleseizure?

It turns out that Big Bang theory makes predictions that can be tested observationally. If the observations confirm the predictions, the theory is upheld. If they don’t, the theory has to be either substantially modified or discarded altogether.

Cosmologist Mario Livio has an elegant writeup on why we believe the Big Bang theory is the best explanation we have for the beginning of the Universe. I’ll offer a summary here, but Mario’s post is well worth a read. First, a handy-dandy info-graphic:

Illustration of the origin of the universe

Like I said, the Big Bang theory makes some predictions that can be tested observationally. Here are the top 3:

Expansion of the Universe

Every galaxy we look at is moving away from every other galaxy*. But it isn’t because all galaxies are moving through space away from one another, but that space itself is expanding, carrying the galaxies along with it.

But how long ago was this? It turns out the expansion velocity is proportional to the distance. So for example, a galaxy that is 20 million light years from earth recedes twice as fast as a galaxy that is only 10 million light-years from earth.

Not only does this mean that the universe is expanding (and therefore was much smaller in the past than it is today) but we can also work out just how long the expansion has been going on – about 13.7 billion years.

Cosmic Microwave “Afterglow”

If there was a Bang, there should be an afterglow somewhere, right? There certainly should, but because the Universe has had time to cool down quite a bit, the afterglow should be very cool – about 2.7 kelvin cool. Sure enough, that very afterglow was discovered by accident in 1965. We know this today as the Cosmic Microwave Background and it’s temperature has been measured very precisely to 2.73 kelvin.

The Abundance of Helium

Hydrogen is the simplest element in the universe, helium is the next-simplest, formed by the fusion of two hydrogen atoms. If helium were only formed in the cores of stars, there wouldn’t be very much of it around – only about 1-2% of the elements in the universe. That’s because in such a scenario, helium could only be formed in the cores of stars, and then further fused into heavier elements such as carbon, neon, oxygen, and so on. In such a universe, helium would be fairly rare.

But our universe is about a quarter helium, so where did it all come from? It turns out the Big Bang theory makes a prediction about the conditions in the early Universe. In the first few minutes after the Big Bang, the universe would have been small enough and hot enough to fuse at least 23% of the available hydrogen into helium. In other words, the entire universe was a nuclear furnace at the time!

Other theories?

Of course, there are other Big Bang-less cosmologies, but they have a pretty high bar to clear given how well the observations match the predictions made by the Big Bang theory, and so many cosmologists remain skeptical until sufficient evidence can be produced.

Anyway, my “summary” of Mario’s post is probably a bit longer than his actual post, but I highly recommend reading it anyway. And given that we think the Universe began in a Big Bang, how might it end? Will the Universe expand forever? Will it contract in on itself in a Big Crunch?

Or will it be wiped away in the Coming of the Great White Handkerchief?


* Obviously, there are plenty of examples of galaxies that are clearly moving toward and even colliding with each other, but this is because those galaxies are so close their mutual gravitational pull overcomes the universe’s local expansion, allowing the galaxies to collide.

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