Plus Advent Calendar Door #13: What happened before the Big Bang?

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On the thirteenth day of advent we pose another puzzling question: If the Universe started with the Big Bang, then what came before it? We are not the only ones to ponder this question; cosmologists have already considered it at length. Here is what the late John D. Barrow (who also happened to be our boss) had to say on the subject.

A Hubble image

An image taken by the Hubble Space Telescope. It shows stars in the Globular Star cluster NGC 6397, which is one of the closest to us, about 8500 light years from the Earth. Image courtesy NASA.

The term Big Bang has several layers of meaning. What most astronomers mean when they refer to a Big Bang theory of the Universe is the idea that the Universe is expanding, as Edwin Hubble first discovered in the 1920s — distant galaxy clusters are moving away from one another with ever increasing speed. This implies that in the past things in the Universe were closer together, the Universe was more compressed, hotter and denser, and that in the future it will become even less hot and dense. [Puzzled by the idea of an expanding Universe? See the box below for Barrow's explanation.]

But within this picture there are all sorts of options. One is that as you go back into the past, following the Universe to earlier and earlier times when it was hotter and denser, you reach some special time when, if you believe your equations, density and temperature were infinite. Einstein's theory of gravity gives you a way of calculating when this infinite state occurred: only 13.8 billion years ago. This is very striking, because you can walk around places in Scandinavia and Scotland and pick up rocks that are 3 billion years old. We believe that the whole solar system is only about 4.6 billion years old. So we're apparently very close to what seems to be the beginning of everything.

The expanding Universe

Many people think of the expansion of the Universe like an explosion, so it must have a centre and there must be an edge. If we see everything expanding around us in the Universe, doesn't this mean that we're at its centre? But there is no centre of the Universe and there is no edge of expansion. The easy case to visualise is that of an infinite Universe. Let's think of a two-dimensional Universe, a rubber sheet that goes on infinitely in each direction. If this infinite sheet was being stretched, then no matter where you stood on it, you'd see everything expanding away from you. You could draw a circle around yourself which describes the edge of your observable universe — the radius of that circle would be the distance that light has been able to travel since the sheet started expanding. We call that our horizon. It's not that there is no sheet beyond the boundary of that circle, it's just that we can't see it yet.

But what if the Universe is finite? If you pick up a flat piece of paper, it seems obvious that there has to be a centre and an edge. But the Universe is all there is. It's not an explosion and it's not expanding into something. So a two-dimensional Universe can't be flat like a piece of paper. But it can be like the surface of a sphere. It's finite — if you wanted to paint that surface you'd need only a finite amount of paint — but if you were an ant walking around on it, you'd never run into an edge. So a curved surface can be finite but have no edge.

This is how we should think of a finite expanding Universe. If we inflate a balloon marked with crosses, all the crosses move away from each other as the balloon expands. If you were sitting on the balloon, you'd see all the crosses moving away from you. The centre of the expansion does not lie on the balloon.

And this is where scientists start to worry. A prediction of something infinite is often a sign that the theory you are using to make that prediction has reached the limits of its applicability. For example, imagine you are an aerodynamicist wanting to predict the speed of an air flow. If your model is very simple, for example if it ignores the friction of the air, then it might predict that something changes infinitely quickly in a finite time. But no aerodynamicist would believe that this is what really happens. They would take that prediction as an indication that you have to go back to square one and make your model a little bit better, for example by introducing the friction of air. When you then solve the equations you will find that things change very, very quickly, but not infinitely quickly.

So what cosmologists are working very keenly on today is a possible extension of Einstein's theory of gravity, one which includes quantum theory, which can give a more accurate description of the apparent beginning of the Universe. Nobody agrees on exactly how to do this: it's right on the edge of current research. Some theories predict that the Universe doesn't have a beginning at all, but that if you follow it backward in time, it eventually bounces, almost like a ball, into a previous state in which it was contracting. The Universe may behave cyclically — contracting, expanding and contracting again — or it may be that it bounced into expansion only once and will keep on expanding forever. Another possibility is that the Universe began in some rather uninteresting stationary state, and then started to expand due to the effect of quantum fluctuations. In that scenario, the expansion has a beginning, but the Universe itself doesn't necessarily have one.

There are also much more exotic possibilities and they are usually associated with the idea of a multiverse. To find out more, read the full version of this article.

This year's advent calendar was inspired by our work on the documentary series, Universe Unravelled, which explores the work done by researchers at the Stephen Hawking Centre for Theoretical Cosmology and is available on discovery+. Return to the 2020 Plus Advent Calendar.