 OK, I'm here today to talk about what I do. And I'm an astronomer, so what do I do? Well, if you ask my friends, they usually come up with something like this, staying awake at night, watching the stars. If you ask my mom, well, she knows I'm working at the university, so she envisions me in a white coat with a microscope on my desk. My dad thinks I'm doing something like that. That's all cool stuff, but it's not really what I do. So to be a bit more precise, what did I do last week when I prepared for this talk? I was looking at the distribution of galaxies in the local universe. So what do you see here? Every white dot is a galaxy, so there are thousands of galaxies in this plot. There are some regions which you can't observe. These are these white regions, but still, we have thousands of galaxies, the Milky Way somewhere in the middle. And all these galaxies are very massive, and they interact with each other. And we can measure the interaction between these galaxies, and we can compare this to our theory of gravity. And our theory of gravity is Albert Einstein's general relativity. So this way we can test general relativity. So why do we want to do this? Well, the whole idea of our universe is based on general relativity. We believe that the universe started in a big bang and is expanding ever since. And we also believed that all the matter in the universe, so the Milky Way and all the other billions of galaxies, is gravitationally pulling on the universe and therefore slowing down the expansion of the universe. It's a bit like when you jump into the air, you expect that the gravity is pulling you down, so you slow down, at some point you turn around, you fall back to the ground. And we assumed that this should be true for the universe as well. It started with a big bang, but all the matter in the universe is pulling on the universe and therefore it should slow down. And this turned out to be wrong. And it gave the Nobel Prize last year for the discovery of the accelerated expansion of the universe. The universe is actually speeding up in the expansion. This is Sol Permeter, Adam Rees, and Brian Schmidt, and they got the Nobel Prize last year for the discovery of that. So we can explain this by introducing a new form of energy and we call it dark energy. And it turns out that most of the stuff in the universe is made out of dark energy. So dark energy is pushing the universe and is therefore contracting the gravitational pull. However, we have no idea what dark energy actually is. We just have to put it in there, otherwise we don't understand our measurements. So today, I would like to talk about an explanation for dark energy. And this is modifying the laws of gravity. So if you talk about modifying the laws of gravity in physics, usually physicists get very big eyes because the group of people who define the laws of gravity is quite in the lead group so far. So this is Isaac Newton. You know, the left, he wrote down F is equal MA, which you all learned in school. And in the middle, we have here Albert Einstein who came up with general relativity. And if we really can explain dark energy by modifying the laws of gravity, then your face could be here. So physicists are really excited about that, having your face beside the two greatest physicists of all time. So the specific idea I want to talk about here is introducing extra dimensions. So we live in a world which is three dimensional. However, there could be extra dimensions and we just can't see them because they're very large or very small. And therefore, we don't notice them. An easy example is we believe for a long time that we live on a two dimensional surface. It turned out to be wrong, we live on a sphere which is three dimensional surface. And the point why we didn't notice that is if you want to travel from A to B, usually it's okay to assume that the earth is flat. Only if you travel large distances, it really matters that we're living on a sphere. So it doesn't really affect our everyday life. And it's the same with these extra dimensions. They're there, but they don't affect our everyday life and therefore we don't notice them. So how can we explain dark energy by introducing extra dimensions? Well, imagine that the gravitational force is carried by little particles and we call these particles gravitons. And these gravitons can see these extra dimensions. And some of them will escape into these extra dimensions. And this means we end up with a smaller number of gravitons in our three dimensional world. Smaller number of gravitons means the weaker gravitational force. And the weaker gravitational force could be the reason why the universe is expanding faster than we would expect. It's a bit like, oh yeah, so this is it. So the exciting part about introducing extra dimensions is that this could be a first thing towards string theory. String theory is a theory which is based on 10 dimensions and string theory attempts to unify the four forces of nature, which is gravity, the electromagnetic force, the weak force, which is responsible for the radioactive decay, and the strong force which keeps the nucleus and an atom together. So all these three forces have their own set of equations and string theory attempts to unify them into one big grand unified theory. It's one of the biggest dreams in physics. So at this point, I have to say that so far, generativity passed all tests. So we don't have any hints that generativity is not true. However, all the tests we made so far are on very small scales. We made them on Earth or in the solar system. What we have to do is we have to test generativity on very large scales. And this brings me back to distribution of galaxies which I showed you earlier. The distance between these white dots, the galaxies, is millions of light years. And we can measure the gravitation action and the gravitation action on these very large scales allows us to distinguish between generativity and these new models. The data at the moment is not good enough to do this, but better data is coming up in the next years and it will be very exciting to see whether we can actually verify one of these new ideas. And if it turns out that we can solve the problem of dark energy by introducing extra dimensions, then this could just be the first step for grand unified theory. Thank you very much.