- 3661 - DARK MATTER - what is it? - We know more about what dark matter is not than what it is. Increasingly sensitive detectors are lowering the possible rate at which dark mark matter particles can interact with normal matter.
--------------------- 3661 - DARK MATTER - what is it?
- We can observe the effects of Dark Matter. While we can't see dark matter, we can observe and measure its gravitational effects. Galaxies have been observed to spin much faster than expected based on their visible matter, and galaxies move faster in clusters than expected. Scientists can calculate the "missing mass" responsible for this motion.
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- Dark Matter is abundant. It makes up about 85 percent of the total mass of the universe, and about 27 percent of the universe's total mass plus energy.
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- We don’t know if dark matter is made up of one particle or many particles? Dark matter could be composed of an entire family of particles
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- Are there forces beyond gravity and other known forces that act on dark matter but not on ordinary matter, and can dark matter interact with itself?
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- Various experiments are searching for signs of theoretical dark matter particles known as WIMPs (weakly interacting massive particles).
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- Could dark matter have an antimatter counterpart, as does normal matter, and is there a similar imbalance that favored dark matter over "dark antimatter" as with normal matter-antimatter? Still dark matter is the mysterious material that holds the Universe together.
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- A laboratory 1,000 meters below the ground in the Stawell gold mine halfway between Melbourne and Adelaide, Australia, could be the epicenter of the discovery of dark matter.
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- First there was the Higg's Boson, confirmed by experiments at the Large Hadron Collider in 2012, nearly 50 years after it existence was first proposed. Then in 2015 the LIGO detectors found gravitational waves, a century after Einstein predicted them.
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- If all goes to plan, in the next few years dark matter will move from the realm of the hypothetical to the observable, opening a new era in experimental physics, and a major step towards a fundamental theory of nature.
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- How do we know what we are looking for? Cosmic rays are absorbed by rock so if you go deep enough in the Stawell mine you can reduce these to almost zero.
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- This search started with some simple calculations based on Newton's Law of Gravity. If I look at the solar system, there's the sun and all the planets, and if I know the velocity of a planet and its distance from the sun, you can calculate the mass of the sun. You can do the same with our galaxy. You can take the motion of a star, and, knowing the distance of the star from the center of the galaxy, we can calculate how much material there is in the galaxy.
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- When we do this calculation, the amount of material that we see and the amount of material that should be there by our calculations don't match. Our calculations say that there is much more material in the galaxy than what we can see. And this is “dark matter“.
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- Physicists think dark matter is made up from an as-yet-unknown fundamental particle that has no electric charge, doesn't produce light, and doesn't interact very much with anything. It’s dark.
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- We know that there is a lot of dark matter in the galaxy, between 70 and 80 percent of the material in the galaxy is dark matter, and so every second our bodies are traversed by billions of these dark matter particles that don't do anything to us. We are transparent to these particles.
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- We think that very, very rarely, a dark matter particle interacts with the nucleus of an atom in a material that we can see, what we call normal matter. The problem is that other things also interact with the atomic nuclei; notably cosmic rays and radioactivity.
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- The probability that dark matter interacts with a nucleus is one event per 10 kilograms of material per day. The probability that radioactivity or cosmic rays interact with one nucleus is 10 billion times per 10 kg of material per day. In other words, 1 in every 10 billion of these interactions is dark matter, making it impossible to distinguish any signal from the noise.
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- To improve these odds we need to reduce the number of other interactions occurring. The first step is to head underground, “way” underground. Cosmic rays are absorbed by rock so if you go deep enough you can reduce these to almost zero.
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- We also need to eliminate all the residual radioactivity in the material we build our detector from, because even if we take a banana into our lab, it will have so much radioactivity that there will be a few counts, a few interactions, per day, with my nuclei, and it will shade dark matter.
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- We choose the right material, or build a material with very low radioactivity, and then we construct our detector underground and we start waiting, and count how many times there is this interaction.
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- Several underground dark matter detectors exist, and one even claims to see dark matter. The Gran Sasso Laboratory, built inside a mountain in Italy, tracks what they say is the annual modulation of dark matter arriving on Earth.
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- As the Earth goes around the sun, its velocity with respect to the center of the galaxy changes. In June the Earth is traveling around the galaxy at about 260 km per second, whereas in December it is more like 200 km per second. Therefore we would expect the Earth to be hit by more dark matter in June than in December. And this is what the Gran Sasso team observes in their data.
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- Seasonal changes, from summer to winter, could also lead to a cycle in the rate of interactions on the dark matter detectors. And this is where Australia comes in. To remove this potential error, we need to repeat the experiments in the Southern Hemisphere, where the seasons are flipped.
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- If we see a seasonal pattern, with a peak in summer and a minimum in winter, then the data from Italy needs to be rethought. If we see the same annual pattern, however, with a peak in June and a minimum in December, this could be the evidence that finally confirms the existence of dark matter.
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- The Stawell gold mine in western Victoria is one of the few places in Australia that can house a dark matter detector. The Underground Physics Laboratory will be up and running this year, 2022.
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- This will be the first dark matter detector in the southern hemisphere, and, when combined with data from other detectors around the world, could finally confirm the existence of dark matter.
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- It takes around half an hour to get to the Stawell Underground Physics Laboratory. Thirty minutes may not sound like a long time, but that's half an hour spent plunging downwards in a truck as it twists and turns its way deeper into the pitch-black tunnels of the Stawell Gold Mine. It gets hotter and more humid as the truck's headlights light up another sheer wall of rock curving off into the darkness, but the lab has air conditioning.
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- We can only really observe about five percent of the whole universe; the rest is partly made of dark matter, invisible fundamental particles that make up the majority of matter, have no electric charge, don't produce light and don't interact very much with anything we can see.
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- The major experiment going into the underground lab is known as the Sodium Iodide with Active Background Rejection Experiment South , or SABRE South for short. The device used to detect dark matter will take up almost a third of the completely sterile lab which measures 33 meters long and 10 meters wide with a 14-meter-high ceiling. A small box.
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- It will use seven ultra-pure sodium iodide crystals housed in cylinders and wrapped in copper, with two very sensitive instruments, called photomultipliers, at either end. These seven crystals are then housed in a radiation-shielded tank filled with about 12 metric tons of a liquid, benzene.
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- If the dark matter particles interact with the crystal, it produces a flash of light that will be picked up by the photomultipliers.
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- Dark matter interacts with the nucleus of the crystal, so the mass of the nucleus is important. Depending on the mass of dark matter, different materials will have a different sensitivity. So, if the dark matter is a large mass, a nucleus with a large mass will be more sensitive.
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- There are now five other experiments trying to verify the results of the Italian research in Spain, Korea, Japan, Austria and the US. Not just this century, it will be one of the biggest discoveries ever, finding out what the universe is made of. More news is still to come, later.
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August 23, 2022 DARK MATTER - what is it? 3660
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