This slide is presenting the history of the Universe as it cools down and becomes less and less dense. So that means that on the contrary, when one gets back in time, the Universe is becoming hotter and hotter and is also becoming more and more dense. That’s why one can imagine that there is a time when the Universe is infinitely hot and infinitely dense, and that is what one calls the Big Bang, which gives the name to the theory. Where does this expression Big Bang come from? Well, it was invented by the astrophysicist Fred Hoyle in 1948.
Fred Hoyle was very critical about the Big Bang theory– which was not called Big Bang at the time– or the theory of an expanding Universe. He was himself a proponent of a steady Universe. And so in a BBC radio show, which was called The Nature of Things, he criticised that theory of an expanding Universe by saying it all started in a big bang. And so the expression remained, and is used both for the initial moment and for the theory itself.
Now, what is this Big Bang? Is it the initial moment? Is it sort of a limit of our understanding? One has to realise that at this point, this is just when temperature and density are becoming infinite. So that just tells us that the Einstein’s equations, which are describing general relativity, are no longer valid. We still have to see whether we have to change the theory, and maybe there would be a “before Big Bang”, or if it is indeed the limit to our knowledge. Since I’m talking about the Big Bang, let me at least be a little more precise about the Universe at that time.
You certainly have the idea that at the Big Bang the whole Universe was in a very tiny portion of space. Now on the other hand, we will see next time that the Universe is probably today infinite. And so how is it possible that the Universe is today infinite and started in a very tiny volume? Well, maybe to your surprise, we think that the Universe was infinite at the time of the Big Bang. So in order to help you understand what I mean by that, let me take a very simple example, which is summarised in this simulation here. Let me take an infinite rope and paint every metre of the rope– a white mark.
And now let me suppose that this rope is expanding like our Universe. So if I run back in time, then I will see the rope contracting. And so that means that two marks will get closer and closer, because when I run back in time the rope is contracting. And so any two marks will become closer and closer as time is running back. And so you see that any portion of space-time, even in an infinite space– any portion will get closer and closer. And so this is exactly what is meant by an infinite density. So in other words, the Universe might be infinite.
It was infinite at the time of the Big Bang, but still that portion of the Universe that we observe today would be indeed enclosed in a very tiny volume.
Now, the very early Universe is very hot. So that means that there are lots of collisions due to the thermal agitation of molecules, of atoms, and that will destroy all these structures. And so for example, atoms will be destroyed into nuclei and electrons. Nuclei will be destroyed into protons and neutrons, and even protons and neutrons will be destroyed into quarks. And so the very early Universe is filled with elementary particles, is a soup of elementary particles, and thus provides a very nice laboratory for studying these very fundamental constituents of nature.
Let me take this opportunity to show you the table of elementary particles. They are all summarised on this slide, where you have a disc. On the upper part of the disc we have six slots, which correspond to the six fundamental quarks, in red. The lower part corresponds to the six leptons. The most famous of them is the electron. They are particles of the same type, so actually three particles similar to the electron, and three other particles which are called the neutrinos. And at the centre of the disc you have the particles which are responsible for the forces, for the interactions.
The gluon is responsible for the strong interaction– the interaction, the force that binds, for example, the quarks inside the proton or the proton and the neutron inside the nucleus. You have the photon, which is responsible for the electromagnetic interaction. And you have two remaining particles, the W and the Z, which are responsible for the weak force, the weak nuclear force, which is the source of radioactivity. And right at the centre, you have what was for a long time the remaining link, the Higgs particle, which is a particle that gives mass to all the other particles, and which was discovered at CERN a couple of years ago.
Of course there is one force which is missing in this description, and that’s gravity. And indeed, it is a challenge to try to have a single description of all fundamental forces. The first three forces which we have seen in the microscopic world– the weak, the strong, and the electromagnetic force, which are described by quantum physics; and gravity, which is described by general relativity. We’ll return to that challenge of unifying quantum theory and general relativity, but for the time being let me try to be a little more precise by introducing a scale of energy, which is the energy at which we should reach a quantum gravitational regime.
So in order to do that, let us use a trick which was proposed to us by Galileo himself, which is called dimensional analysis. We first identify the constants that should play a role, and then we build out of them a quantity which has the units of an energy. So these constants are– the gravitational constant, Newton’s constant, which describes gravity; the velocity of light, c, which is characteristic of relativity; and a constant which is characteristic of the quantum world, we will see next week, that this is what is called the Planck constant. Out of them, one can build an energy which is called the Planck energy, and its value is 10 to the 19 GeV.
So this is a very high energy, but that means that when the Universe is so hot that its average energy is 10 to the 19 GeV, then one has reached the epoch of quantum gravity. And the real theory should be a theory that unifies the quantum description to a description of gravity provided by general relativity.
To conclude, let me stress that there are two complementary points of view to describe the evolution of the Universe. In the first one, which is this familiar diagram, you are an observer outside the Universe. And you look at the time evolution of the Universe, on the left hand side from the Big Bang, and you see that as time evolves the Universe is expanding. The structures are forming, up to the very developed structures that are the brain, the man, the satellite. But there is another point of view, which is a point of view of the observer. This is the second slide, and you see here that the observer is looking at distances away from him or from her.
And the further he looks, the more ancient times he is observing, and so these different shells are shells of time, which is more and more ancient. And so you see that at very large distances, so the furthest shells would correspond to the Big Bang. So you see that in this picture, for example, it is much easier to understand why the Universe is infinite at the Big Bang, because in a sense, from the observer on Earth for example, then all directions point towards the Big Bang.
To sum up, we have seen that the Big Bang theory describes the Universe as initially very hot and very dense, and cooling down and becoming less and less dense. There is a time where the Universe seemed to be infinitely hot and dense. This is what is known as the Big Bang. Is it is the origin of time? Is it a frontier of our knowledge? Or is it just an artefact of a present theory? That remains to be seen. The primordial Universe is a laboratory for studying elementary particles.
And we have seen that there are two complementary points of view to describe the evolution of the Universe– one which is from an outsider’s standpoint, the other one from an observer who is observing the Universe from within.