Dark Matter

In: Science

Submitted By BlackKitten
Words 2578
Pages 11
Dark Matter

Matt Cannon (928)
Physics 222
2/15/02
Final Draft

Dark Matter
The ultimate destiny of our expanding universe depends on how much matter it contains and whether or not the amount of matter will be enough to one day stop the expansion. When astronomers add up all the visible matter (matter that emits or reflects light) in the universe, the result is that there is clearly not enough to stop the expansion of the universe. However, they have discovered over the past several decades that there may be additional matter hidden from view.1
This raises some important questions. What is this dark matter made of? How can it be detected?

Astronomers first found substantial observational evidence of unseen matter in the early twentieth century when studying the galaxy M31.2 They discovered that the velocity curve for the outer limbs of the galaxy, which should drop off with increasing radius as mandated by Kepler's
Laws and the Virial Theorem, leveled off at approximately 200 km/s. This observation was confirmed in the early 1970’s when radio astronomers found that hydrogen gas at the edge of galaxies moved with roughly the same velocity as hydrogen gas at the center of galaxies. If the visible matter seen in galaxies is the only source of mass then the observed uniform velocity of hydrogen gas is a direct violation of Kepler’s Laws and the conservation of angular momentum.
Thus, the simplest explanation (which is usually the correct one) is that the visible matter in galaxies doesn’t account for all of the mass! This claim suggests that most of the mass in the universe, as much as 90%2, is contributed by unseen or “dark” matter. So, what is dark matter made of? There is no reason why dark matter has to be made of one single material. It is likely that there are different types of dark matter.

Dark matter candidates are generally…...

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