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The Hunt for Dark Matter

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The Hunt for Dark Matter Phantom of the Universe The Hunt for Dark Matter Educator’s Guide PhantomOfTheUniverse.com Big Questions About Discovery What does it mean to discover? How do we discover new things? Discovery can mean different things to different peo- Engineers and scientists have built many tools that help ple and doesn’t just mean traveling to unknown places us study objects and how the world works. In space or finding new things. In astronomy and physics, it is sciences, this includes satellites, space- and ground- about understanding different objects and processes in based telescopes and robots. In physics this includes our universe through the use of tools and measurement. laboratory experiments like sensitive detectors and For a world traveler, it’s about rediscovering places for huge particle colliders. We also have conceptual tools themselves. For a small child, it’s about finding new like mathematics, models and simulations. However, sights, sounds, and textures. What does it mean to you? one of the biggest and best tools we have is us. We work together on all of these problems, come up with the ideas and help inspire each other to not only answer What have humans discovered? questions, but to ask new questions so we can continue A lot! We have discovered the fundamental laws that to discover new things. control the behavior of matter in the Universe. We have figured out what atoms are made of and how they Why is it important to discover? interact together. We have also discovered a lot about things we cannot physically touch or reach. Our tech- Humans have an insatiable curiosity. Discovery, in any nology creates tools to help further our understanding form, is about finding answers and new knowledge that of things otherwise beyond our reach. By studying helps us understand the world around us. The answers light emitted by celestial objects with telescopes we to our questions can lead to great new breakthroughs have learned what other stars are made of, that gal- that help humans make our lives better or help us make axies are formed from stars and that the universe is sure we do not cause too much harm to our surround- filled with a strange and mysterious substance that we ings. Discovery and exploration also allows us to cannot see feel or touch. Likewise, our huge particle know what our place is in the universe. It is part of accelerators allow us to peer deep into the microscopic a long human quest to figure out how we fit in, and world of the extremely small. where we come from. What is left to discover? While there aren’t any new continents left to discover on Earth, there is still a lot to learn about our Universe. For example, we don’t know what most of the universe is made of or why the universe accelerating in its ex- pansion. We only very recently discovered how matter in the universe gets its mass! In order to ask questions we need to figure out what we do know and, more im- portantly, what we don’t know. There’s plenty we still have questions about. Show Overview “Phantom of the Universe: The Hunt for Dark Matter” takes the visitor on a journey of discovery, following the efforts of scientists around the world as they try to unlock the mystery of Dark Matter. Starting with its cre- ation during the Big Bang, the role of dark matter in the formation of galaxies and thus ourselves is ex- plored, highlighting its discovery from its effects on the motions of stars and galaxies. The visitor is then brought back thousands of feet below the earth’s surface where teams of scientists are building extremely sen- sitive experiments to detect the very rare direct interactions of dark matter particles with normal matter. Finally, the visitor is taken on a whirlwind tour of the gigantic CERN laboratory where beams of protons are hurled to- gether in head on collisions in an attempt to create new dark matter particles for study. Background Information What is Dark Matter? Hunting for Dark Matter with Gravity In the 1950’s the study of galaxies was an extremely If Dark Matter is dark and hardly interacts with nor- exciting topic. It was only quite recently (in the early mal matter, then how does one detect it? Although you 1900’s that astronomers had come to realize that gal- can’t see it directly, Dark Matter does have gravity and axies were not part of our own Milky Way, but actu- so one can see the effects of Dark Matter. One example ally distant stellar “cities” of their own. An American is the excess motion of galaxies in clusters found by astronomer, Fritz Zwicky, was studying the motions Zwicky. More recently, Vera Rubin was studying spiral of galaxies within large clusters of galaxies and real- galaxies, flat disks whose stars all revolve around the ized that these motions were far faster than could be center in the same direction. According to theory, the explained by what he saw. The gravity of the visible farther out a star is from the center the more slowly it galaxies was simply not enough to hold the clusters to- should move, much like the slower motion of planets gether against their tremendous velocities. There must farther from the Sun. But Rubin measured something be extra, and unseen, matter holding the clusters to- surprising: farther out stars move just as quickly as gether. He called this Dark Matter. those close in. Again, it is as if there were extra, un- seen, matter holding the stars in their orbits. While Zwicky’s conclusion was mostly ignored at the time, evidence began to pile up from studies of other In the past couple decades another powerful, and very galaxies, the expansion of the Universe as a whole, and beautiful evidence for dark matter has emerged: The the distribution of the galaxies in space. By the 1980s amount and distribution of galaxies in the Universe. and ‘90s the evidence was irrefutable: most of the mat- When the Universe first formed it was extremely uni- ter in the Universe was dark and unseen. What’s more, form with only tiny ripples in the distribution of mat- this Dark Matter did not seem to interact with “nor- ter. It is gravity that enhanced these ripples to the point mal” matter except through the force of gravity. where some areas of the universe are very dense (stars, planets, galaxies). But if the only matter in the Uni- verse were what we see directly there would not have The Big Bang been enough gravity to have pulled together the galax- To really understand dark matter one has to go back to ies. Without dark matter we wouldn’t even exist! the very beginning, the origin of the Universe in the Big Bang. The Big Bang is simply the name we give Hunting for Dark Matter in the Lab to the state of the Universe when it was very young, very hot and very dense. It was not an explosion within But studying Dark Matter by looking for its gravita- the Universe, but rather it is the rapid expansion of the tional effects is unsatisfying. While we know it exists, whole Universe at very early times. and can even tell a bit about its properties, these stud- ies will not tell us what it *is*. In the past few de- The farther back one looks in the history of the Uni- cades particle physicists, scientists who study the fun- verse the hotter and denser it was. Since then it has damental structure of matter, have begun to develop a been expanding and cooling off. At these late times the powerful understanding of what Dark Matter *might* Universe is a frigid 2.7K, but in the first seconds the be. All normal matter is made up of a very limited temperatures were trillions of degrees kelvin. At these number of types of “elementary” particles. There are temperatures new matter can be born and destroyed electrons and “up and “down” quarks. Up and Down in a continuous cycle. Dark Matter was created at this quarks in different combinations make up protons and early time and came to dominate the mass of the Uni- neutrons. Protons and neutrons make up the nuclei of verse. atoms. Electrons surround the nuclei to form neutral atoms. That’s it. Everything we see in the Universe is fundamentally made up of only these three particles arranged in different ways. But what we don’t see, Dark Matter, seems to be made of a another type of particle entirely. These particles are far more massive than electrons and quarks and very hard to create. But in the very energetic collisions be- tween particles in the early universe they were created in abundance. To study them in the lab today one has to create similarly energetic collisions. This is done in the huge particle collider called the Large Hadronic Col- lider (LHC). Protons are send whirling around a ring at tremendous speeds and slammed head on into protons whirling around the other way. The resulting collisions produce many particles and occasionally, it is hoped, a particle of Dark Matter. This Dark Matter particle, called a neutralino, will escape without being detected, but its signature in the shape of the collision, will re- main. From the details of the dark matter production we will finally learn what Dark matter really is. Teaching with the Show Before the Visit • Discuss with your students what matter is.
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