Astronomy midterm question: Doppler redshifts and the expanding universe

Astronomy 210 Midterm 2, Spring Semester 2010
Cuesta College, San Luis Obispo, CA

[20 points.] Doppler shifts of galaxies is evidence that the universe is expanding. Explain what these Doppler shifts are, and how they are used to support the theory of an expanding universe.

Solution and grading rubric:

• p = 20/20:
  Correct. Explains Doppler shifts show that light from galaxies are redshifted (stretched out to longer wavelength values) proportional to their distances, and thus indicate the universe is expanding (Hubble's law).
  
• r = 16/20:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors.
  
• t = 12/20:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Description of Doppler shifts missing/problematic, but adequate discussion of Hubble's law (or at least the general implication of an expanding universe based on galaxies receding), or vice versa.
  
• v = 8/20:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner.
  
• x = 4/20:
  Implementation/application of ideas, but credit given for effort rather than merit.
  
• y = 2/20:
  Irrelevant discussion/effectively blank.
  
• z = 0/20:
  Blank.
  

Grading distribution:
Section 30674
p: 14 students
r: 4 students
t: 23 students
v: 1 student
x: 0 students
y: 2 students
z: 0 students

Section 30676
p: 16 students
r: 7 students
t: 32 students
v: 19 students
x: 1 student
y: 0 students
z: 1 student

A sample "p" response (from student 4747):

Another sample "p" response (from student 4733):

A refreshingly honest "y" response (from student 7272):