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Dark Puzzles

Pierre Binétruy explains the role of dark matter in the formation of galaxies, and stresses remaining questions about the nature of dark matter.
We have seen that dark matter is the main form of matter in the Universe. And so it is natural that dark matter will play a significant role on the evolution of this Universe and especially on the evolution of the structures in the Universe. Now we have seen that quantum fluctuations during the phase of inflation are leading to fluctuations in the CMB, the Cosmic Microwave Background light, which lead to the fluctuations that we observe today, the anisotropies that we observe today. We have also seen that before hydrogen recombination, matter and light are tightly coupled.
So these fluctuations that I was just describing will lead to fluctuations in the matter distribution and these fluctuations in matter in the very early Universe will start attracting dark matter. They will be seen as seeds of gravitational attraction. And so dark matter will be falling onto those original seeds. And you see behind me precisely a simulation of this phenomenon. In red, you have dark matter. And you will form, in this way, galaxies and clusters of galaxies of dark matter. And then luminous matter, which is less abundant, will be falling into these galaxies and will form the luminous galaxies that we observe today.
We can draw maps of dark matter so we know where dark matter is in the Universe. But we don’t know its exact nature, especially its microscopic nature. If dark matter is made with particles, then those particles are different from the protons and the neutrons of a Standard Model. In particular, they do not emit light, contrary to protons and neutrons. And also we have seen that they interact weakly. Remember the Bullet Cluster picture, where the two halos of dark matter were colliding without even interacting. So that means the particles that would make those halos would be very weakly interacting particles. Now such particles are not found in the Standard Model of particle physics.
But they are quite often found in extensions of the Standard Model, what we call theories beyond the Standard Model. But we still have to identify their exact nature.
How does one detect these dark matter particles? Well, just imagine we are in this room. There are dark matter particles going through the room, because they interact very weakly. So they go through the walls. And we put a detector, which is made of protons and neutrons. So let me consider one proton of a detector and a dark matter particle that is coming from the cosmos and which hits the proton. So it will give a kick to the proton inside the detector. And so we get in the end, that the proton is kicked out and, of course, the dark matter particle leaves.
And so you see that the signal of detection will be the kicking of the proton by an unknown particle, which will be the dark matter particle. Now there are similar events which we would like to get rid of, which consist of a cosmic ray, which is also a particle, hitting a proton and doing the same thing. And so this is why these detectors are not placed on ground but are placed underground, something like a mile underneath in a mine or in a mountain tunnel. Precisely because the cosmic rays are stopped by the mile of rock. Whereas the dark matter particles, because they interact very weakly, will go through and will get to the underground lab like this one, for example.
And will be detected. So this is to prevent what we call the noise from the cosmic rays that we go underground.
There is also the possibility that there is no dark matter particles. Remember that we have seen that the sign for dark matter, the evidence for dark matter, is all gravitational. So it means that there is another way of explaining all these facts, which has nothing to do with introducing new particles, but which is rather to change the theory of relativity, to change the theory of gravity. Now the difficulty with that possibility, which has been tried by physicists, is that we had signs, evidence of dark matter or modification of gravity at the level of galaxies, clusters of galaxies, and at the level of the whole Universe, the whole observable Universe.
And so, if you want to modify the theory of Einstein, if you want to modify the theory of gravity, then you have to find modifications at the level of galaxies, at the level at the distance scale of clusters of galaxies, and at the distance scale of 14 billion light years, with the size of the observable Universe. And that turns out to be a very difficult task. Moreover, Einstein’s theory is unique in the sense that it is difficult to modify it slightly. There are, of course, other theories of gravitation.
But if you want to keep all of the successes of Einstein’s theory and account by modification of that theory of all the facts that we have seen and which are believed to be a proof of dark matter, then you have an extremely difficult task. And, frankly speaking, nobody has succeeded so far. But that remains a logical possibility to explain all these gravitational facts with the modification of Einstein’s theory.
Before concluding, let me return to one of the key predictions of the scenario of inflation. The fact that space is flat. As you will see, this has some importance on issues that we are discussing, the issues of the dark components of the Universe. Now before I do that, let me be a little more precise about what I meant by flat space. Of course, we have seen that locally matter is curving space. So why suddenly should I be talking about flat space? Well, the reason is the following. Let me give you two examples. Let’s just imagine for a second that the Earth is flat, like in a plain. And, of course, it has mountains.
But, of course, we know that the Earth is spherical more or less. And the Earth has also mountains.
Now, in this case, this is a Universe which is basically flat. It’s a plain. But, of course, locally it has got mountains, it has got curvature. So it’s locally curved but globally flat. Now the Earth is different. The Earth is, of course, locally curved because of the mountains. But it’s also globally curved, because it is a sphere. So you see that even when we take into account the curvature, local curvature due to mass, which, for example curves the light rays, still, there is the question of the overall Universe, whether it is globally flat or globally curved.
And what the inflationary scenario is telling us is that at the end of inflation, the Universe has been basically flattened by the very rapid expansion. And the Universe is globally flat. Now we have seen that there is a connection between the energy in the Universe, or the energy locally, and that curvature. And it turns out that when one looks at Einstein’s equations, one realises that the Universe is globally flat, like in this case, for a very precise value of the energy density in the Universe, which is today 10 to the minus 26 kilogramme per cubic metre. Just to give you an idea, imagine the order of magnitude of this density. Imagine a cube of side, length, one light year.
So this energy density correspond to the mass of 1/7 of the moon. So that means this is a very tiny energy density in the Universe. And the energy density in the Universe has to have precisely this value in order to account for the flatness, the global flatness of the Universe. Now some 20 years ago, people looked at what they have in the Universe, not locally, of course, because locally there is much more matter. It’s much more dense but when you average the whole Universe. So they took into account all matter, all radiation in the Universe. And they concluded that the Universe was far from having this energy density.
Basically, the energy density, in average in the Universe, was 1/10 of this. Now we have seen that people realised that there was dark matter in large quantities. And so, the conclusion at the end of these studies of the galaxy rotation curves, and so on, was that we have not– this number, 10 to the minus 26, but 30% of that. So, basically 1/3 of that. And there were long debates within theorists, who were saying that the theory of inflation explained so many things that it had to be right. And so, the energy density of the Universe was precisely this number. And observers, who were saying, OK, we have been looking everywhere.
In every corner of the Universe, every form of matter, and we come to a number which is just 1/3 of this one. Well, what we’re going to see in the next two sequences of this week is that one found, finally, one has found another dark component of the Universe, which account for the missing 70% percent. And so now we think that the energy density of the Universe is this number, which is absolutely compatible with the predictions of the inflation theory.
Recapitulation. The most common form of matter in the Universe is dark matter, which is present at the level of galaxies, clusters of galaxies, and even the whole observable Universe. This matter plays a leading role in the formation of large structures such as galaxies and clusters. It is probably formed by particles of a new type, particles which are actively searched for. And luminous and dark matter, as well as radiation, are not enough to insure an energy density in the Universe, which is sufficient for it to be specially flat, which is a prediction of the scenario of inflation.
Dark matter plays a significant role in the formation of galaxies. But many mysteries remain: what is the nature of dark matter? why is dark/luminous matter insufficient to account for the flatness of space, as predicted by inflation? (13:02)
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