Week of February 5th

Now that the first part of our hybrid meson project is complete, Dr. Bellis is having me help him with the dark matter research he's working on. Unfortunately, his son was sick today, so I wasn't able to come to Siena to work. However, I still worked from school, and answered a few questions about dark matter that he prepared me. (Source is Wikipedia for all of the answers)

Why do we believe dark matter exists?

Scientists first hypothesized the existence of dark matter to account for the discrepancy between the mass of large astronomical objects determined from their gravitational effects and the mass calculated from the "luminous matter" (stars, gas, etc.) they contain. We believe that the reason for this difference is that there is another type of matter, dark matter, which isn't reactant to light. There is no direct evidence that it exists, so we can't be entirely sure that it is the reason for the discrepancy, but we can infer its existence from its gravitational effects on visible matter and radiation.

Who was Vera Rubin?

Vera Rubin is an American astronomer who, in the 1970s, discovered convincing evidence for the existence of dark matter. She observed that stars in spinning galaxies were all rotating at roughly the same velocity (with the distance from the galactic center having no visible effect on the velocity). The stars were not rotating around the visible center of the galaxy but around many unknown centers, all providing additional gravitational attraction. This contradicted Kepler's law of planetary motion.

Rubin's observation of the velocity of stars in spinning galaxy provided convincing evidence for dark matter, because the velocity curves that she observed could only occur if huge amounts of invisible matter were causing additional gravitational attraction.

What does the "weak" in Weakly Interacting Massive Particle (WIMP) refer to?

The weak in WIMP refers to the weak nuclear force, one of the four fundamental interactions of nature. It's responsible for the radioactive decay and nuclear fusion of subatomic particles. It is caused by the emission and absorption of W and Z bosons. The weak interaction is also capable of changing the flavor of quarks.

How "massive" are these WIMPs believed to be? How big is that relative to the mass of a proton?

10-100 GeV/c^2

proton mass ~ 1 GeV/c^2

WIMPS are predicted to be between 1-100x mass of a proton.