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In the late 60s and early 70s Vera Rubin (and others) were recording the doppler shifts of galaxies to measure their rotation, ultimately to find their mass.
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Astronomical laws state that for a given orbit the farther away from the center an object is the slower it will go. If it were going faster, it would be flung out of its orbit.
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Rubin found that the stars at the outer edge of a galaxy were orbiting at the same rate as stars closer in, but were not being flung away.
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These higher-than-normal velocities while maintaining orbit meant gravity is holding the outer stars in place.
This additional gravity had to come from additional mass where there didn’t appear to be any.
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According to Rubin’s calculations (confirmed by others) there had to be an additional 5 times more mass than is observed directly.
No matter which galaxy these measurements were applied to, the results were the same. All galaxies appear to have 5 times more mass than is observed.
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Several possible explanations were given:
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The most successful explanation to date is dark matter.
This theory states that there actually is some exotic form of matter to account for the observed discrepancy in gravitational force.
The dark matter forms a halo around the entire galaxy out to an as yet undetermined distance.
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This matter interacts gravitationally, but not physically (or nearly so) and does not emit or interact with light. To many this sounded like a fantasy, but other branches of science offered some help.
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Particle physics has been extremely successful in explaining and accurately predicting the behavior of the universe and parts of it.
It also predicts many particles that have not been observed (yet?).
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One such predicted particle is the Axion. This particle is predicted to have mass and therefor gravity. It does not interact with other matter well. It does not give off much light.
These properties would make it an excellent candidate for dark matter.
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If clouds of axions were surrounding galaxies, it might explain the observations.
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In science, theories need to make predictions that can be shown as true in order to gain acceptance.
Dark Matter is no exception.
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The Bullet Galaxy Cluster
When galaxy clusters collide we observe 2 things:
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The Bullet Galaxy Cluster
When galaxy clusters collide we observe 2 things:
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Dark matter theory predicts that the amount of heat produced when 2 clusters collide will be much less than expected from the amount of mass because most of the mass (the dark matter) will pass right through each other.
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This has been tested on many colliding galaxy clusters and it turns out to be true.
This doesn’t prove dark matter theory, but it does give it a great deal of credibility.
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While MOND potentially explains the galaxy rotation problem, it does not address colliding galactic clusters at all.
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Currently, attempts are underway to directly detect axions specifically and dark matter more generally.
There has been no success yet.
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If correct, dark matter theory also helps explain apparent discrepancies in data regarding the formation of the earliest galaxies in the universe.
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Observation strongly suggests that the first galaxies formed roughly 200 million years after the Big Bang.
Observation also strongly suggests that at this time matter was too energetic to condense gravitationally to form galaxies.
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If we take dark matter into account, the additional gravity would have been strong enough to form the earliest galaxies even under the more energetic conditions.