Monday, April 16, 2018

Galaxies - spin and velocities



- 2062  -  Galaxies - spin and velocities. All disk galaxies rotate about once every billion years, no matter their size or mass.  The conclusion is that stars orbiting at the edges of galaxies have the same order of magnitude orbital periods.  The galaxies in the expanding universe are  separating from each other faster than expected.  There is still more to learn.
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-----------------------------  2062  -  Galaxies - spin and velocities
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-  In March 2018, some astronomers announced the discovery that all disk galaxies rotate about once every billion years, no matter their size or mass. This measurement is made on the extreme edge of the galaxy disk. To their surprise they did not find a dense galaxy rotating more quickly, while another with the same size but lower density rotating more slowly.
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-  The radial velocities of neutral hydrogen in the outer disks was measured for many galaxies. 
The galaxies ranged from small dwarf irregulars to massive spiral galaxies. These galaxies differed in both size and rotational velocity by up to a factor of 30. With these velocity measurements, the researchers were able to calculate the rotational period for each of the galaxies.  The conclusion was that the outer rims of all disk galaxies take roughly a billion years to complete one rotation.
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-   Based on theoretical models, the astronomers expected to find only sparse populations of young stars and interstellar gas on the outskirts of these galaxies. But instead, they discovered a significant population of much older stars mingling with the young stars and gas.
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-  . How does Dark Matter enter into or influence this phenomena?
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-   The conclusion is that stars orbiting at the edges of galaxies have the same order of magnitude orbital periods.   This is like Kepler finding that planets orbiting the Sun sweep out equal areas in equal time. A planet has to move faster when it is closest to the Sun and slower when further away.   Newton then showed that this was due to a "1 / radius squared"  gravitational force.
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-  Newton's laws of gravity explained the relationship that Kepler found when Kepler studied the data that Tycho found. Who can be the next Newton by coming up with a galactic dynamics model that shows that the outer edge of galaxies will have orbital periods of roughly the same magnitude, close to 1 billion years
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-  Our Sun's galactic rotation period is 240 million years. . Our Milky Way galaxy spiral pattern rotation period is 220–360 million years.  Not all disk galaxies rotate once every billion years, given this one example and this one measureement.  But , is this the outer edge being measured, and is the galaxy really rotating like a phonograph record?  Maybe the rotation is like a lawn sprinkler and the outer edges lag in the rotation and take longer to complete one rotation?
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-  This galaxy rotation is not the only mystery.  The galaxies in the expanding universe are  separating from each other faster than expected.
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-  The Hubble Space Telescope measures the distance to other galaxies by examining a type of star that varies in brightness. These stars, called Cepheid variables, brighten and dim in a predictable way that lets researchers judge the distance to them. This data is then used to measure the universe's expansion rate, known as the Hubble constant.  
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-  The new findings show that Cepheid variables in our Milky Way galaxy are up to 10 times farther away than any previously analyzed for a star of this kind. These Cepheids are more challenging to measure than others because they reside between 6,000 and 12,000 lightyears from Earth. To handle that distance, the researchers developed a new scanning technique that allowed the Hubble Space Telescope to periodically measure a star's position at a rate of 1,000 times per minute, thus increasing the accuracy of the stars' true brightness and distance.
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-  The Planck satellite mapped leftover radiation from the Big Bang,  known as the Cosmic Microwave Background. The Planck data revealed a Hubble constant between 67 and 69 kilometers per second per megaparsec. (A megaparsec is roughly 3 million light-years)  This is about 9 percent lower than that of the new Hubble measurements, which estimate that the universe is expanding at 73 kilometers per second per megaparsec, suggesting that galaxies are moving faster than expected.  ( In more familiar units, this is 49,306 miles per hour per million lightyears distance.)
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-  One possible explanation for the discrepancy is that dark energy is driving galaxies farther apart with greater intensity. In this case, the acceleration of the universe may not have a constant value but rather may change over time. It is also possible that elusive dark matter, which accounts for 80 percent of the matter in the universe, interacts more strongly with visible matter or radiation than we  thought.
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-   Another possible explanation includes a new kind of subatomic particle that travels close to the speed of light and would be affected only by gravity. Researchers named the superfast particles "sterile neutrinos".
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-   Any of these scenarios would change the contents of the early universe, leading to inconsistencies in current theoretical models.  These inconsistencies would result in an incorrect value for the Hubble constant of the expansion rate for the universe.  There is more to learn, stay tuned.
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