Astronomy midterm question: no center for universe?

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

[20 points.] An astronomy question on an online discussion board (http://answers.yahoo.com/question/index?qid=20100905145749AAyQLJr) was asked and answered:

Bell: What is at the center of the universe?

Stimpy: The universe is not defined to have [a] center. Wherever you are in the universe you will always appear to be in the center.

Discuss why this answer is correct, and how you know this. Explain using observations and evidence related to the Hubble law.

Solution and grading rubric:

• p = 20/20:
  Correct. Hubble's law is that the recession velocity of galaxies is proportional to distance, evidence is that there is a greater redshift of absorption lines for distant galaxies compared to nearby galaxies. This corresponds to the expansion of space between galaxies, such that each galaxy seems to be center of expansion as observed from their position. May use analogies as not-like-an-explosion, raisin bread, enlargement of between-spaces, etc.
  
• r = 16/20:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Discusses how Hubble's law expansion will be observed identically for all viewpoints, but does not explicitly discuss absorption line redshift evidence for Hubble's law.
  
• t = 12/20:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Describes how Hubble's law expansion will be observed identically for all viewpoints, but Hubble's law discussion is problematic or incomplete.
  
• v = 8/20:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Discussion based on evidence of the earlier stages in the history of the universe, with little or no substantive discussion of Hubble's law.
  
• x = 4/20:
  Implementation/application of ideas, but credit given for effort rather than merit.
  y = 2/20: Irrelevant discussion/effectively blank.
  
• y = 2/20:
  Irrelevant discussion/effectively blank.
  
• z = 0/20:
  Blank.
  

Grading distribution:
Section 70160
p: 11 students
r: 10 students
t: 11 students
v: 5 students
x: 3 students
y: 1 student
z: 0 students

A sample "r" response (from student 2421):