The Absurd Search For Dark Matter

I'm in a gold mine a couple of hours from Melbourne because, a kilometer underground, they are installing a detector to look for

dark

matter

. Come on. (epic music) It will take 30 minutes to go one kilometer underground. Dark

matter

is believed to make up 85% of all existing matter. It could form a shadow universe five times more massive than anything we can see. Over the past few decades, more than 50 experiments have been performed to directly detect

dark

matter, but none of them have found anything except one. Under a mountain in the Italian Alps there is a dark matter detector called DAMA/LIBRA.

He's been collecting data for about 20 years, and every year he sees the same peculiar results. The detection rate increases to a maximum in June and then decreases to a minimum in November. Some scientists believe this could be the first direct evidence of dark matter. But why would dark matter create a periodic annual signal? Well, that's what our galaxy looks like, or at least what it looks like in visible light. Astronomers suspect that it is surrounded and permeated by a huge sphere of dark matter, invisible particles that spin in random directions. According to most theories, dark matter does not interact with anything, including itself, except through gravity.

We think there should be five times more dark matter than ordinary matter. Currently, our solar system is moving around the galaxy at 220 kilometers per second. That means we're also moving through dark matter at this rate, except the Earth orbits the Sun at 30 kilometers per second. So, for half the year, we move with the Sun, moving faster through dark matter. And, the other half of the year, we move in the opposite direction, that is, slower through dark matter. And the idea is that we find more dark matter when we move through it faster, which happens in June, and less when we move slower, which happens in November.

The actual geometry is a little more complicated. The solar system is tilted 60 degrees to the plane of the galaxy, but the idea still works. So the signal observed in DAMA/LIBRA may be due to this motion through dark matter or it may not be due to dark matter at all. It could just be something mundane, like the temperature, humidity, soil moisture, snow in the mountains, or the number of tourists in Italy. All of these things fluctuate over a period of one year. And that's why they're going to build an almost identical experiment in the southern hemisphere, at the bottom of this gold mine outside of Melbourne, because there the seasons are reversed, but our movement through dark matter is still the same. same.

So if we see the same signal, it's pretty strong evidence for the existence of dark matter. One of the big problems that LADY/LIBRA has is that there are other very similar experiments that don't see anything. And this has created a lot of uncertainty about whether the DAMA/LIBRA signal is really dark matter. So yeah, we don't know, right? Science's favorite thing... But why do we think dark matter exists? In 1933, Swiss astronomer Fritz Zwicky was studying the Coma Cluster, a collection of more than a thousand galaxies. These galaxies are gravitationally bound. So they all orbit around their collective center of mass.

Zwicky measured the orbital velocities of these galaxies and discovered that somewhere he was moving much faster than he expected, it was as if there was a lot more matter in the cluster than he could see, pushing everything inward. Thus, he proposed the existence of invisible matter, which he called dunkle matter, origin of the term dark matter. No one took this idea seriously, but 40 years later, dark matter appeared again. Vera Ruben and Kent Ford observed the movement of stars in the Andromeda galaxy. And they expected that the further they got from the center, the slower the stars would orbit, but this is not what they found.

The rotation speed remains almost constant with increasing distance from the center. Without additional mass in the galaxy to attract those stars, they would have to be flung into space. The result was the same in other galaxies. Using radio telescopes, Albert Bosma and others measured hydrogen gas even further from the center of a galaxy, but the rotation speed still remained constant. One way to explain this is to postulate the existence of matter that we cannot see, dark matter, which holds all these galaxies together. So let's say you have a star and this represents the mass of everything in the center of the galaxy that attracts the star.

The star can maintain a stable orbit if its centripetal force is equal to the gravitational pull toward all mass in the galaxy. rest of the galaxy. And as you can see, at about a distance of one meter, this is the speed of the orbit. Okay, but what happens if we add some dark matter? So this water bottle represents the matter we can't see, there is now more mass pushing this star towards the middle, which means that, in the same orbit, it can now go much faster, and in fact, it must go faster. fast. to maintain that orbit.

And this explains the observation. This is what we see. (metal clang) (Derek laughs) By looking at the rotation speeds of stars, scientists estimate that about 85% of a galaxy's mass is dark matter. But there is another way to explain these observations without invoking dark matter. And that is modifying our theory of gravity. What is the evidence to support the idea that the particle idea is totally wrong and that we should actually consider a revised theory of gravity? We can invoke something we can't see or just say, well, the universe is what we can see and we need a way to explain what's going on out there.

And the only way to do this is by modifying the laws of physics. So when you look at the outskirts of galaxies, they have a lot of centripetal acceleration. Dark matter says that centripetal acceleration is due to the gravitational effect of dark matter. While people who like MOND will say, no, that's centripetal acceleration, that's just the fact that it's now reached this floor and can't go any lower. So, they say there is no additional force due to dark matter, but there is a limit as to how low the acceleration could go. I think the consensus is huge in favor of it being a physical substance, in the sense that it seems reasonable that there could be other particles that we haven't seen yet.

And there is more evidence. This is the bullet cluster, a site where two galaxy clusters collided. Most of the ordinary mass of these clusters is found in interstellar gas. And, when the collision occurred, the interstellar gas interacted, heated up, and slowed down. So you would expect most of the mass of the bullet cluster to be in the middle, where all this gas is. But, if you use gravitational lensing, the way gravity bends light, you can measure where most of the mass is in this image. And it's not in the middle. It's actually on both sides. So the best way to explain this is that when the clusters collided, all that gas was trapped in the middle, but the dark matter passed through, creating the more gravitational lens where we can see the less ordinary matter.

Even more evidence of dark matter comes from the oldest light in the universe. 380,000 years after the Big Bang, light was finally able to travel through the universe unhindered. And this is what we see as the cosmic microwave background or CMB. The red dots show where the early universe was a little hotter and the blue dots show where it was a little colder, but these temperature differences were small. Only 0.01%, but there they are. And you can turn this image into a graph by counting how many spots there are of different sizes. So there is the most common size mass, which results in this peak, but there are also other common size masses.

And thus these other peaks of decreasing size are obtained. Now, the height of these peaks depends on how much dark matter there is. In a universe without dark matter, the graph looks like this, but as dark matter increases, the amplitudes of the even peaks decrease. To match the CMB measurements, we need about five times more dark matter than ordinary matter. This figure also agrees with the amount of dark matter needed to explain the motion of stars in galaxies and the motion of galaxies in clusters. So the dark matter hypothesis explains many different observations with a simple theoretical framework, that there exists some type of particle that only interacts through gravity.

But what exactly is this particle? Well, since we don't know, scientists have proposed a lot of different things it could be. And now we have to try to go out and find them. The approach differs depending on what you are trying to find. Lady Libra and the detector at the bottom of the gold mine look for WIMPs, weakly interacting massive particles. These particles are expected to weigh about as much as a proton, but they interact with ordinary matter extremely weakly. At the heart of the detector are our seven seven-kilogram crystals of pure sodium iodide. So what's actually there is sodium iodide.

Yes. I didn't expect it to be so clear. The idea is that very, very rarely, a dark matter particle can hit a nucleus in the crystal and transfer its energy. This creates a flash of light called scintillation, which is detected by photomultiplier tubes, very sensitive light detectors, which are placed above and below each crystal. But there's a problem: Even the purest sodium iodide crystal contains radioactive potassium. And, when a potassium atom decays, it emits an electron and a gamma ray. Now, the electron can cause a twinkle in the crystal just like the supposed dark matter particle.

So, to eliminate these events, the sodium iodide crystals are immersed in a tank filled with 12 tons of linear alkylbenzene. It is a liquid scintillator that emits light when exposed to a gamma ray and that light can then be detected by photomultiplier tubes in the tank. So if there is a simultaneous detection in the crystal and in the tank, it was most likely a potassium decay, not a dark matter event, but there is another problem: cosmic rays. Energetic particles from the sun and other galaxies hit the top of Earth's atmosphere creating muons, essentially heavy electrons, that flow toward Earth at near the speed of light.

Muons can also create flashes of light in glass. This is a muon detector and it has three plastic vanes separated by some steel pieces. If we see a flash of light on all three, basically at the same time, then we know that Muon has passed through them. So if I press reset, we can see, counting the muons that are seen. So, it's at least a few seconds. That is why all sensitive particle detectors are located deep underground. Here we have the muon detector, now a kilometer underground, and it's been running for about 15 minutes, and there have been no muon counts.

Yeah, I think we'll have to let this go for a long time, even if we wanted to get just one hit. We expect the number of muons down here to be about a million fewer. And we didn't see a million at the top, so we probably won't see any down here. And this is the goal of placing a dark matter detector underground. You want to get rid of all the background that would create noise in the detector. But even this shielding is not enough. We all have muon detectors immediately above the tank. So if a flash is seen in a crystal at the same time a muon is detected, it can be ruled out.

Being underground brings its own challenges. The walls of the mine contain traces of radioactive elements such as uranium and thorium, which decay into radon gas. The requirements here are quite serious for dark matter experiments. We have to completely control the environment, particularly the radon level. To counteract this, the walls of the cavern are coated with a special paint that contains radioactive particles. The crystals are immersed in a continuous stream of pure nitrogen gas. And the entire detector is protected by 120 tons of steel and plastic. Wow, look at the size of that cavern. There is a lot at stake in this experiment.

It will validate or refute one of the most controversial results in physics. So if we don't see anything, well, this is the death of LADY/LIBRA but if we see something, well, we are all happy. In fact, I like the idea that because 80% of the mass of the universe is dark matter or dark matter, maybe there is more than one particle that dark matter is made of. It could be a completely dark standard model if you want. A dark version of everything we can see or perhaps something more complex, because there is much more. I really hope that's it. - Do you think dark matter interacts with ordinary matter? -If we want to find out what this is, we better hope that there is some level of interaction that we can at least test when it comes to doing experiments.

If God gave me the big book of physics andIf there were two sections, section A and section B, one for light matter and one for dark matter, and they did not communicate with each other, I would say it was a very peculiar situation. universe. But, in science, we have to live with the possibility that, at some level, we will never find the answer. We may miss it, but at least we tried. The sponsor of this video, Brilliant, is the opposite of dark matter. You see it everywhere and it's very interactive. Brilliant is an innovative stem learning platform that guides you through engaging and practical courses in math, science, and computer science.

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