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--------------------- 1636 - What does Dark Matter Look Like?
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- We don’t know it’s dark. Dark Matter is a discovery that needs more discovery. Astronomers see its gravitational effects, but, that is about as much as we know.
-. Dark Matter accounts for 5.4 times as much mass is ordinary manner in the universe. Ordinary matter is made of protons, neutrons, and electrons. We do not know what Dark Matter is made of, but, the math says it is out there made of something?
-. Ordinary matter in our galaxy is stars, gas, and dust in the structure of a giant disk. Our Milky Way is analogous to a warped vinyl record, 33 1/3 RPM.
-. The thin disk has a pin and wheel spiral pattern. In the center is a dense nucleus hosting a 4,500,000 Solar Mass blackhole. Through the center is in an elongated bulge known as a “bar“. Surrounding all of this is a “halo” of old stars.
-. Dark Matter’s structure is surmised from the gravitational effects on visible ordinary matter. Calculations tell us it is approximately spherical extending far beyond the halo. It's density falls off approximately as a square of the distance from the center the same as gravity does.
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- At distances of 50,000 light-years from the center of our galaxy it consist mostly of atomic hydrogen and a few stars. By the time you get out 75,000 light-years distance the disc is warped bending 7,500 light-years out of the plane. The gas density is oscillating above and below the plane as it rotates around the center. These oscillations are 100 billion years in their cycle, completing one orbit about the center
-. At one point astronomers thought the neighboring galaxy, the Megellanic Cloud, was gravitationally causing this distortion. However, better measurements have determine thar the Megellanic gravity to be far too weak to have any effect.
-. In addition to the Megellanic Cloud galaxy there are 16 known satellite galaxies orbiting the Milky Way galaxy. Some of these dwarfs contain only a few hundred stars.
-. Could there be Dark Matter galaxies orbiting our galaxy? Could galaxy collisions early in the evolution of our galaxy cause the ringing that warps the shape of our galaxies disk? Could the center of gravity of Dark Matter be offset from the center of gravity of ordinary matter causing the asymmetry to occur? The puzzle of Dark Matter has become one of the most vibrant research areas in both physics and astronomy.
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- Hydrogen gas emits a spectral line at the radio wavelength of 21 centimeters. Measuring the Doppler shift of this spectral line tells us relative velocities. With a little geometry a graph can be made showing the rotation velocity of stars, or galaxies, and their distances from the gravitational center.
-. Objects further from the center have further to travel and should be moving at faster speeds if the system is all held together. In contrast, the rotation curve for our solar system drops off with distance from the Sun, because inner planets orbit at faster speeds than the outer planets.
-. This is because the system’s mass is at the center, the Sun. The gravitational force holding a planet in orbit decreases as the inverse of the square of the distance from the Sun. A smaller force means lower orbital speed.
-. Our galaxy is different. Unlike the rotation curve for a solar system are galaxies orbital velocities remain constant beyond the inner thousand light-years which makes the rotation curve flat.
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- Unlike a solar system most of the mass of the galaxy must not be concentrated at the center. The orbits of progressively more distant objects must encircle more and more mass.
-. The Sun's orbit encompasses 100 billion Solar Mass. An orbit twice as large encompasses twice as much mass. Mathematical analysis implies that most of the mass is located in a spherical halo surrounding the disk of our galaxy. It must be 10 times the total mass in the galaxy disk. 90 percent of the total mass must be Dark Matter?
-. For example: a galaxy cluster has a radius of 6.2 million light-years. And an orbiting galaxy has a velocity of 1,350 kilometers per second, 3,019,864 miles per hour.
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---------------------- Kepler’s 3rd law about the radius and period of orbit:
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-------------------------- M = 4 * pi*a^3 / G * p^2
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-------------------------- v = 2 * pi* a / p
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-------------------------- M = r * v^2 / G
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-------------------------- M = 1.6*10^10 r * v^2
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- The mass is directly proportional to the radius times the velocity squared.
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-------------------------- M = 1.6 * 10^45 kilograms
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-------------------------- Solar Mass = 2 * 10^30 kilograms
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-------------------- M = 8*10^14 Solar Masses
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- The total mass is 800 trillion Solar Masses
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- One Milky Way mass is 1 trillion Solar Masses
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- The Galaxy Cluster is 800 Galaxy Masses
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-. Comparing the clusters mass to its luminosity determined the cluster's mass is made much, much, greater than the luminosity mass.
-. A second method to determine the clusters mass uses a temperature of X-ray hot gas. The gas temperature is related to the speed of the individual gas particles.
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------------------ velocity = 140 meters / second * square root of Temperature
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------------------ The hot gas temperature of the cluster = 90,000,000 Kelvin
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------------------- v = 140 * (9*10^7)^½
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------------------- v = 1.3 * 10^6 meters per second
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------------------- M = 1.5 * 10^45 kilograms
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- The Galaxy Cluster is 800 Galaxy Masses, same answer.
-No cluster of galaxy galaxies contains enough visible matter to stay bound together. Galaxies orbit at velocities that should be flying apart. So what is holding them together? The total mass needed to bind a typical cluster is 10 times greater than the mass of the material that shows up in the visible light images.
-. What is it and what effect does it have on us? An announcement will be made shortly stay tuned
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(1) www.ScientificAmerican.com/oct2011/blitz
(2) Some among us can feel the Moon. Those standing next to the sea shore and the cold water circles their feet as the tides come in. Gravity is the weakest force but it reaches us across the Cosmos.
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