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MASSMASS When astrophysicist Vera Ruban looked at When astrophysicist Vera Ruban looked at

Galaxies, she noticed a curious problem. She Galaxies, she noticed a curious problem. She expected that the outer parts of a galaxy expected that the outer parts of a galaxy would move slower than the inner parts. But would move slower than the inner parts. But she found that this is not the case. The she found that this is not the case. The rotation curves of galaxies (a graph of the rotation curves of galaxies (a graph of the radius of a galaxy versus rotational speed) is radius of a galaxy versus rotational speed) is flat, meaning that the outer parts move at the flat, meaning that the outer parts move at the same speed as the inner parts. Large amounts same speed as the inner parts. Large amounts of mass would account for the unexpected of mass would account for the unexpected speed, but we don't see the mass that should speed, but we don't see the mass that should be there.be there.

Dark mass-Dark mass-MASSMASS

To aid your understanding of this, think of how To aid your understanding of this, think of how planets revolve around the Sun in our solar planets revolve around the Sun in our solar system. Mercury (the closest planet to the system. Mercury (the closest planet to the Sun) zips around the Sun in 88 days, but it Sun) zips around the Sun in 88 days, but it takes the furthest planet, Pluto, 248 years to takes the furthest planet, Pluto, 248 years to orbit the Sun. If there were a solid sphere of orbit the Sun. If there were a solid sphere of mass between the Sun and Pluto, than Pluto's mass between the Sun and Pluto, than Pluto's orbital period would be the same as Mercury's. orbital period would be the same as Mercury's. No one is suggesting that galaxies are actually No one is suggesting that galaxies are actually solid spheres of matter, but there must be solid spheres of matter, but there must be more mass in these galaxies then we can see. more mass in these galaxies then we can see. Because we can't see it, the mass is called Because we can't see it, the mass is called dark matter.dark matter.

Baryonic matterBaryonic matter-MASS-MASS Dark matter may account for up to 90% of the Dark matter may account for up to 90% of the

Universe's total mass. Bernard Sadoulet, who Universe's total mass. Bernard Sadoulet, who leads a search for dark matter at theCenter leads a search for dark matter at theCenter for Particle Astrophysics has stated that "Not for Particle Astrophysics has stated that "Not only can we not see what most of the only can we not see what most of the Universe is made of, we aren't even made of Universe is made of, we aren't even made of what most of the Universe is made of!" What what most of the Universe is made of!" What did he mean by this? Scientists tend to did he mean by this? Scientists tend to categorize everything and matter is no categorize everything and matter is no different. The matter you are familiar with, different. The matter you are familiar with, matter composed of neutrons and protons, is matter composed of neutrons and protons, is called baryonic matter. called baryonic matter.

WIMPWIMP-MASS-MASS

Non-baryonic matter also exists, Non-baryonic matter also exists, but is generally difficult to detect. but is generally difficult to detect. Professor Sadoulet's experiment is Professor Sadoulet's experiment is looking for exotic, non-baryonic looking for exotic, non-baryonic particles called WIMPs. WIMP stands particles called WIMPs. WIMP stands for Weakly Interacting Massive for Weakly Interacting Massive Particle. There is a great deal of Particle. There is a great deal of theoretical work which suggests theoretical work which suggests that WIMPs exist and probably that WIMPs exist and probably account for a large fraction of dark account for a large fraction of dark matter.matter.

MAIN POINTMAIN POINT Large amounts of mass would account Large amounts of mass would account

for the unexpected speed, but we don't for the unexpected speed, but we don't see the mass that should be there.see the mass that should be there.

The outer parts of a galaxy would move The outer parts of a galaxy would move slower than the inner partsslower than the inner parts

No one is suggesting that galaxies are No one is suggesting that galaxies are actually solid spheres of matter, but actually solid spheres of matter, but there must be more mass in these there must be more mass in these galaxies then we can see. galaxies then we can see.

Dark matter may account for up to 90% Dark matter may account for up to 90% of the Universe's total mass.of the Universe's total mass.

MAIN POINTMAIN POINT The matter you are familiar with, matter The matter you are familiar with, matter

composed of neutrons and protons, is composed of neutrons and protons, is called baryonic matter. called baryonic matter.

Looking for exotic, non-baryonic particles Looking for exotic, non-baryonic particles called WIMPs. called WIMPs.

There is a great deal of theoretical work There is a great deal of theoretical work which suggests that WIMPs exist and which suggests that WIMPs exist and probably account for a large fraction of probably account for a large fraction of dark matter.dark matter.

There is a great deal of theoretical work There is a great deal of theoretical work which suggests that WIMPs exist and which suggests that WIMPs exist and probably account for a large fraction of probably account for a large fraction of dark matter.dark matter.

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