Not only is the Big Bang Theory a popular TV sitcom, but it’s perhaps one of the most well-known scientific theories of our age. Except, how much do you really understand about the Big Bang? I find that most people have a completely incorrect understanding of what the theory says, so I thought it might be useful to explore some of these misconceptions here.
The Big Bang theory is not an explanation of how the universe was created
It’s an explanation of how we find the universe to be today. I noted in my earlier article about the astronomer Edwin Hubble, that at the beginning of the 20th century, astronomers imagined that the universe was an essentially static object, with stars fixed in space. Hubble showed that galaxies are moving away from each other, and that the more distant the galaxies are, the faster they are moving.
Clearly this is a description of how the universe is today. But if we extrapolate the data back in time, we find that almost 14 billion years ago, the galaxies that we see today must have been compressed into a tiny volume – so small that normal matter like atoms couldn’t have existed in their current state. Once we get back that far in time we can’t really say what happened, because we don’t understand the physics of such hot, dense states. Yet.
Of course, scientists speculate about what that early universe was like, but the Big Bang theory itself doesn’t really tell us. That’s one of the reasons why the Large Hadron Collider at CERN is so important. It allows us to explore much higher energies than we find today – the kind of energies that were around during the first second the universe existed.
There’s a lot of other data that supports the Big Bang Theory too, like observations of the cosmic microwave background.
The Big Bang didn’t happen in one place
This is perhaps the hardest part of the theory to grasp. People imagine the whole universe being compressed into a tiny sphere and then exploding outwards into space, but this is entirely wrong.
First of all, there was no explosion. There was, disappointingly, no Bang. Instead there was an expansion, which is still continuing today (and in fact accelerating, thanks to something called Dark Energy).
Secondly, the Big Bang didn’t happen at some centre point of the universe. It happened everywhere simultaneously. There is no centre point of the universe. You can’t point at a cosmological map and say, “The Big Bang happened there.” So, 14 billion years ago, when the universe was very, very dense and hot, the whole thing expanded in every direction, at every point, at the same time. What happened before that? The theory doesn’t tell us.
Thirdly, the universe isn’t expanding into space. Space itself is expanding. Yeah, that’s weird, blame Einstein. What’s happening is that the distance between any two stationary objects is increasing with time, as explained by Einstein’s General Theory of Relativity. But don’t rush out and buy a new ruler – the rate of expansion is only 46 miles per second per megaparsec. Now in case your smartphone doesn’t have a handy conversion tool for this unit of measurement, a megaparsec is roughly 3 million light years, so this works out at about 2.6 E-18 miles per second per mile. Still baffled? It means that your ruler will still be accurate for at least several billion years, which will come as a relief for all “Back to School” parents.
So forget explosion and think expansion. Yeah, I know that explosion is more fun, but there just wasn’t one, OK?
Space doesn’t have an edge
Cosmologists don’t know how big the universe is, but the best data indicates that it’s infinite. This isn’t a guess.
We can’t see anything beyond a certain distance in the universe. That’s because the light travelling from the most distant stars has been travelling for nearly 14 billion years, and that puts a limit on how far we can see. As the universe gets older, light from ever distant objects will reach us and we will be able to see further. But we’ll have to wait a long time before we can see a lot further.
So how do we know that there’s anything beyond the distance we can see now? It’s because wherever we look in the universe it all looks the same – not in detail, but in broad terms. Stars are stars, gravity is gravity, whatever direction you look and however far you look.
That tells us that the observable universe is nowhere near the edge of the entire universe. We can’t see any “edge effects.” So, the current data suggests that the universe is unbounded – or at least that the universe is much bigger than the part that we can see.
By the way, the diameter of the observable universe is 93 billion light years, or 546,000,000,000,000,000,000,000 miles. That’s why cosmologists don’t use miles to measure things.
Question – how can the observable universe be 93 billion light years in diameter if the light from the furthest stars has only been travelling for 14 billion years? Surely that would make the observable universe 28 billion light years across?
Answer – it’s because of the expansion of space. In the time that the light from those most distant stars has been travelling, space itself has expanded, and so those distant stars are now much further away from us than when they first shone light in our direction. You see – the theory all hangs together, even if it’s a bit mind blowing to grasp.