Hello everyone, in this week we're going to talk about search for dark matter. We're living at really fascinating times, you see in the map, this is a map of the world at time of Ptolemy. So, it has been around thousands years ago. This is pretty much what people like in Greece knew about the rest of the world. So, you see there is no Australia, no America, no Antarctic and the situation at those times is pretty similar to the situation that now because well, we don't know much about most of the matter that floats around us. I mean dark matter. So the scientists, looking at the rotation of galaxies have came to conclusion that there are five times more mass than mass that we are used to encounter with on daily basis. Namely, if you look at how fast peripherical parts of galaxies rotate, you will see that the speed of this rotation is more or less the same as the speed at the center of those galaxies. So, it means that there is something more mass that is rotating along the stars in the galaxies, but this mass doesn't emit any kind of radiation, any kind of light, but it is there. Another clue about existence of this mass, of this dark mass is so-called gravitational lensing. So, if we look at the tiny patch of the sky we can see a galaxy there, because there are billions of galaxies out there. Sometimes, if we look a little bit different angle we can see exactly same galaxy, a little bit distorted. It is depicted at the slide, so you have two identical galaxy, is a little bit distorted run from another, but those are the same objects. This happens due to the effect that is called gravitational lensing. So, here is a schematic representation what happens. So, if we have a massive objects, it shapes their space around it, change the curvature, and the straight lines in the presence of this object are not exactly straight, if we look from a different frame. So, a real object that we observe actually emits two different beams of light and those two beams get curved a little bit and we'll look at those two different lights, and if we project the trajectory of the light for their own, we can think of the same real object as two different images. All right. This is something that has been noticed many years ago, and the magnitude of this effect sometimes can be explained by the mass that sits between us and the real object, but sometimes and we can measure this mass if it is a black hole or star or a galaxy, another galaxy. But sometimes the mass of this object is not enough to explain the curvature or the bending of the light trajectories. So, this is another clue that gives us hint about the presence of something else. So, why do we bother, because no presence of dark matter has been found around us at the acceleration experiments or other experiments yet, but still it's quite fascinating topic because it means that there are forces that we have no clue about, and there are particles that also we have no clue about. The goal and the challenge is how can we design an experiment that would allow us to see those objects and allow us to operate with those particles in our daily basis and eventually maybe we can come to design the technology that would allow us to use this in other source of energy maybe. Actually, there are two big challenges related to discovery of dark matter. The first one is that we don't know what we're looking for, because there are plenty of theories that tries to compete with each other and try to predict presence of dark matter, and there is no clear explanation what kind of effect we should be looking for. The second big challenge is that there is no totally clean, observable that we can look at, measure and say that, okay, we have a hint of a dark matter. So, there are two strategies that scientists follow to discover dark matter. The first one is a direct search. So, the direct means that we design an experiment and we hope that the particle of a dark matter comes into the sensitive volume of this experiment and somehow excites anything, any particle, any atom or a subatomic thing inside this volume, and then we will have some kind of recoil energy measured by light charge or whatever inside this experiment. There are several detectors that are working in this mode. Well actually, there are plenty of them, but no hint so far. The second approach is a little bit more complicated because it implies that we can see a result of interaction of dark matter with something else, maybe with dark matter or something else and we can discriminate the first thing, we can detect the particle that comes out of this interaction and then we can discriminate such interaction from interaction of a regular mass or regular particles with each other. So, this is something that is a bit complicated because we don't know what kind of processes are happening and the star or distant galaxies, but this approach may sound much more applicable to the accelerator-based search of dark matter
