Astronomy final exam question: why universe has no center?

Astronomy 210 Final Exam, spring semester 2014
Cuesta College, San Luis Obispo, CA

An astronomy question on an online discussion board[*] was asked and answered:

?: Why does the universe have no center?
im: According to the big bang theory everything in the universe is moving away from each other, therefore there is no one "center."
cr: If the big bang theory is correct, the universe does have a center. We just don't know where it is, and it may not be within the observable universe.

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

[*] Adapted from https://answers.yahoo.com/question/index?qid=1006041908256.

Solution and grading rubric:

• p = 20/20:
  Correct. Hubble's law is (1) 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 (2) 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. One of the two points (1)-(2) correct, other is problematic/incomplete.
• t = 12/20:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Both points (1)-(2) problematic/incomplete, or one point correct while other is missing.
• 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. Discusses factors other than relevant to Hubble's law, redshifts/recession velocities, expansion of space, big bang, etc.
• y = 2/20:
  Irrelevant discussion/effectively blank.
• z = 0/20:
  Blank.

Grading distribution:
Section 30674
Exam code: finaln0oN
p: 3 students
r: 4 students
t: 15 students
v: 1 student
x: 1 student
y: 0 students
z: 0 students

Section 30676
Exam code: finals0N6
p: 6 students
r: 4 students
t: 10 students
v: 6 students
x: 2 students
y: 3 students
z: 3 students

A sample "p" response (from student 4380):