Astronomy final exam question: dark matter properties and evidence

Astronomy 10 Final Exam, Spring Semester 2008
Cuesta College, San Luis Obispo, CA

Astronomy 10 learning goal 11.3

[15 points.] Describe the properties of dark matter, and explain the observational evidence for its existence.

Solution and grading rubric:

• p = 15/15:
  Correct. The orbital speeds of stars are nearly the same for all distances from the center of the Milky Way, instead of decreasing with increasing distance. This is due to the mass of the Milky Way not being concentrated at the very center (despite a supermassive black hole there), but being diffusely distributed above and below the disk of the Milky Way. Because this amount of mass is not visible in a manner such as luminous stars and gas, this unseen mass is termed "dark matter." May also discuss properties of WIMPs and MaCHOs, the latter of which has been observed due to gravitational lensing.
  
• r = 12/15:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Only correctly describes properties of dark matter, or only correctly describes the observational evidence for the existence of dark matter.
  
• t = 9/15:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. At least identifies dark matter as being in the halo of the Milky Way.
  
• v = 6/15:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Confuses dark matter with dark nebulae, dark energy, etc.
  
• x = 3/15:
  Implementation/application of ideas, but credit given for effort rather than merit.
  
• y = 1.5/15:
  Irrelevant discussion/effectively blank.
  
• z = 0/15:
  Blank.
  

Grading distribution:
Section 4160
p: 7 students
r: 6 students
t: 6 students
v: 13 students
x: 2 students
y: 1 student
z: 0 students

A sample "p" response (from student 1721) ponders the MaCHos versus WIMPs debate:
Another "p" response, from student 6221 discusses the gravitational lensing evidence for dark matter:
Student 7045 gives a "p" response discussing how Kepler's third law is apparently violated by the gravitational properties of dark matter:
An "x" response from student 0501 appeals to a perceived duality of dark matter and luminous matter:
Student 0223 goes for the whimsical "y" response: