String Cosmology Group Leader at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute), Potsdam, Germany.
Breaking the Wall of the Beginning of Time. How Cosmology Will Tell Us What Happened Before the Big Bang
Good morning. It is a great honour to be here and to be able to give this talk. In my talk, I am going to take an even longer view by considering the evolution of not just species, but the evolution of the universe as a whole. I will tell you about the Big Bang and what might have come before. To start, it is useful to remember, to just briefly recall, the biggest discovery in cosmology in last century, which was the expansion of the universe. As early as 1917, Einstein discovered his equations only allow for universes that either expand or contract. But he was himself so shocked by this that he didn’t believe it and he tried to modify his equations. Later on, he regretted this. Meanwhile, Georges Lemaitre, who was a priest and a cosmologist at the same time, studied these solutions of expanding universes and developed the idea of a Big Bang – what he called a primeval atom – as the starting point of the expansion of the universe. Edwin Hubble, of course, confirmed the expansion with astronomical observations in 1929. If the universe is expanding, then if we go back in time, it was smaller and in fact also hotter and denser. This leads to the idea that there was a beginning to this whole expansion. There was some evidence that was found in 1965 for why the universe really was hot and dense at early times. This is the best evidence we have that the Big Bang took place. I will briefly describe it to you.
Shortly after the Big Bang, the universe was very hot, very dense, and all particles were very energetic and constantly bounced off of each other. There were no atoms at the time – only really elementary particles. Even particles of light couldn’t travel freely for very long; they just bounced off something else all the time. So, the universe was opaque like a soup. Then, 300,000 years after the Big Bang, for the first time it was cold enough in the universe that the first atoms formed. Now, light interacts less with atoms and so suddenly, in one moment, the universe became transparent. All the light that was emitted then is still flying through the cosmos andcan be measured. This is called the cosmic background radiation. It gives us an image of how the universe was shortly after the Big Bang. So, it is like a baby image of the universe. That is what this picture is showing. The picture shows that the universe was at almost the same temperature everywhere, but there were tiny fluctuations in temperature: these are shown by the different colours in the picture. The radiation is now at the wavelength, which our eyes cannot see. That is why one has to use artificial colours. This picture, this baby picture of the universe, was the blueprint for how galaxies were distributed later on.
Now after that, most people started believing in the theory of the Big Bang, and the idea that the Big Bang was the beginning of the universe and also the beginning of time became widespread. However, as I am going to tell you now, this is very likely to be the wrong idea. I will illustrate to you why. I will ask you to please look underneath your seats; some of you are equipped with a musical instrument, a cymbal, please take it out. There should also be a little stick as well. I think there are about ten people in the audience who have one. If you have one, please hold it up – just for a second. Ok, great. Could one of you please – you sir over there – could you please just strike it once? Great. That is the idea of the Big Bang. At one point, it goes “bang”, and from there the whole universe expands.
Now, this is not really how it happened. In fact, if we go back in time, we don’t know exactly how big the universe is, but we know “at least” how big it is, because we can see quite far into the universe. If we take that region and move it back in time, evolve it back in time, according to Einstein’s equations, one finds that actually when you get to the time of the Big Bang, you don’t get to a point, but you get to a whole surface, to lots of regions. So the Big Bang didn’t happen at a point but at lots of different places. So, lets try that: all of you who have the cymbals, I will count “three, two, one, now”. When I say “now”, could you please all try to strike it at the same time? Ready? Three, two, one, now! Ok, so that is what it was like.
Now, if you think about it, trying to think that this was the beginning of time makes no sense at all, because we know from this baby picture of the universe that these Big Bangs were all almost identical – in all these different places. So, you have to imagine that you have all these Big Bangs going off at the same time, in the same way, without there being time before. So, how do you synchronise that? The only way we could do it here is because we could communicate before. We could organise this whole thing – that it went off at the same time.
It is much more reasonable to think that there was some evolution prior to the Big Bang, which resulted in triggering the Big Bang in all these regions at the same time. So, of course, you are going to ask next: well, what came before? There are currently two different theories, only two, that really make sense. Surprisingly they are very different. One is called the Theory of Cosmic Inflation. The idea is that there was a very small region of space, which got blown up hugely in a very short interval of time. This small region of space then got stretched out. By being stretched out enormously, it made the universe very smooth and regular over very large regions. This could then trigger the Big Bang over large regions simultaneously.
The second idea is very different, namely that the universe goes through cycles of evolution, that the universe expands and contracts alternatively. Then the phase of contraction can also manage to synchronise the universe, and the reversal from contraction to expansion corresponds to the Big Bang in that case.
How can we know which one of these is true – if any? Well, it turns out that both these theories predict or explain this baby picture of the universe only in the rough details. They give roughly the same baby picture of the universe, but they predict very different patterns for the small details in this picture. According to current measurements, both theories fit the data equally well, but there is currently a European satellite called “Planck”, which is in orbit, which is taking data, and which is making a more precise picture. Next year the results should be out, and they may very well be precise enough that we can tell which one of the two makes more sense.
Of course you can ask: is this all? We have already gone beyond the Big Bang, so why not go further? Is this all there is, or is there more? If we want to answer that, then we should see what our best theory says. Our best theory currently is String Theory. String theory, I should say, has not been tested yet. So, everything I am saying is still completely speculative. But I am going tell you just what the situation is according to the way the theory is currently known. According to string theory, both these types of universes are possible: inflationary universes and cyclic universes. In fact, many other universes are possible. Some of which are very different from ours, some of which are just a little bit different from ours, but according to the theory they are possible solutions of the theory.
Moreover, if the theory is right, these are not just theoretical possibilities, but all these universes are real, because there is a mechanism in a theory in which inside one universe a new universe can form. This only happens very rarely and requires a very large quantum jump over a whole region of space; so, it is very rare, but there is plenty of time in the universe. So it will happen, necessarily; in fact, over an infinite amount of time, it will happen an infinite number of times. What is important is that this new universe can be different from the parent universe. With this process, all different universes come into existence inside each other.
So, what the string theory leads to is the following picture: we don’t have a universe, but we have a multiverse. These are all different universes, which come into existence inside each other. Some have cycles, some have inflation in them, some are green, some can have life on them, some probably cannot. The question becomes first of all: is this true? Can we test this? The most obvious way would be, in fact, if we could see the remnants of a collision of two universes. So, it happens in this picture, for example, up on the left see this small yellow triangle. If two of these universes form close together, they can actually collide, because they are both expanding; they can collide with each other. So there is a possibility that in the past history of all universes are universes that collided with another universe. This would leave an imprint on this baby picture of the universe.
People have analysed the data so far and haven’t found anything yet. That doesn’t mean that it won’t be there once we have better data. The problem is, on a theoretical level, it is very hard to calculate what this probability should be. So, it is difficult to know at the moment whether this is something that we should expect to see or not expect to see. So, currently we are stuck with the situation that we have to analyse this multiverse on a theoretical level. We have to try and see: how do you describe something like this mathematically? How do you make predictions? How can you find out where we should be, which universe we should be in? How do we know once we know that, what predictions we get for future observations?
That is the situation we are faced with. Nobody has any good answers to this at the moment. There about as many answers to these questions as there are cosmologists working on these problems. But, we are talking about the falling of walls here, so I just wanted to give you an impression of what people are currently thinking about in cosmology.
I am almost done, but I will just conclude with one remark: these – they are called “bubble universes”, because they arise as a bubble and then expand – there are more and more of them, as you go into the future. So if you go back in time, there were fewer and fewer of these. If you go back far enough, there necessarily was a point when there was just a single universe. You can ask: “But where did that universe come from?” And that – nobody has any idea about! So, thank you for your attention.