1) A recent discovery in Astronomy was Vera Rubin's work which showed that about 90% of the mass of galaxies is dark matter.

a) (8 pts) Briefly explain the evidence Rubin used to make her case. Don't just name the evidence, but explain how it makes the case for the existence of dark matter.

b) (6 pts) One possible component of the dark matter is WIMPs (Weakly Interacting Massive Particles). Detectors to observe these particles are often located deep underground. Explain why, and also briefly describe the major source of "noise" in the detectors.

2) Einstein's theory of relativity has led to some remarkable observations and a revolution in thinking about the physical sciences.

a) (7 pts) Describe (a simple diagram will help) how we use relativity concepts to detect MACHOs, planet-sized objects that may exist in the galaxy.

b) (7 pts) Explain the "twin paradox" and what causes it.

3) When we detect MACHOs, it is often through the temporary brightening of stars.

a) (9 pts) Name two different ways in which MACHO-induced stellar brightness variations are different from ordinary variations in starlight. For one of these two differences, briefly explain why stars vary this way while MACHO-induced variations do not behave this way.

b) (5 pts) Explain how we use very precise observations of MACHO-star variations to deduce the existence of extrasolar planetary systems. How and why do these MACHO systems differ from normal MACHO systems?

4) Briefly describe how we use masers to deduce the distances to nearby galaxies. As part of your answer, explain why the maser technique is limited to only very nearby galaxies.

5) Cosmic rays often plague astronomers attempting to observe very faint objects in the sky. Two kinds of cosmic rays were covered in the thought questions for this part of the course.

a) (7 pts) What is a fastball, and why do we think fastballs originate relatively nearby in our own galaxy?

b) (7 pts) Ordinary cosmic rays seem to come from all directions, even though we think they probably originate in supernova explosions in specific directions. What makes the cosmic rays come from all directions like this?

6) Although the Hubble relation is typically presented as a linear relationship between radial velocity and distance as shown below, recent observations have shown that the Universe may in fact be accelerating due to some as yet unexplained force.

a) (4 pts) How would this cause the Hubble relation to deviate from a standard linear graph? Sketch your answer on the graph below.

b) (10 pts) Explain why the graph would deviate in the way youšve drawn in an accelerating Universe.

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7) Quasars are extremely luminous, very compact objects that existed when the Universe was much younger.

a) (7 pts) Describe the evidence that tells us quasars have absolute luminosities that are hundreds of times larger than ordinary galaxies.

b) (7 pts) Blazars are another class of exotic object that may have something in common with quasars. Explain how blazars are different from quasars, and explain why the appear different even though they originate from the same kind of object.

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8) On the graph below, draw in the observed abundance of Carbon in galaxies at different distances (redshifts) from Earth, then explain why the abundance of Carbon has the relationship with redshift that you drew on the graph.