Astronomy midterm question: big bang expansion, not explosion?

Astronomy 210 Midterm 2, fall semester 2012
Cuesta College, San Luis Obispo, CA

[20 points.] An astronomy question on an online discussion board(*) was asked and answered:
strange questioner: Big bang: explosion or expansion? Most...sources say big bang as explosion, some others say it is wrong to say explosion and it's only expansion.
green meklar: It is more like an expansion [than an explosion].
Discuss why this answer is correct, and how you know this. Explain using observations and evidence related to the Hubble law.

*Adapted from: http://answers.yahoo.com/question/index?qid=20120820231957AAhZq7N.

Solution and grading rubric:
  • p = 20/20:
    Correct. Discusses (a) Hubble's law (recession velocity of galaxies is proportional to distance), (b) evidence for Hubble's law (greater redshift of absorption lines for distant galaxies compared to nearby galaxies), and (c) relates this to the expansion of space, as opposed to an explosion (where the velocity of particles is inversely proportional to the distance from the center of the explosion, and also has a unique center).
  • r = 16/20:
    Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Discusses two of the three (a)-(c) points in (p).
  • t = 12/20:
    Contains right ideas, but discussion is unclear/incomplete or contains major errors. Discussion of only one of the three points (a)-(c) in (p) is complete.
  • 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.
  • z = 0/20:
Grading distribution:
Section 70158
Exam code: midterm02s0Ur
p: 4 students
r: 11 students
t: 10 students
v: 6 students
x: 2 students
y: 0 students
z: 0 students

Section 70160
Exam code: midterm02NuF7
p: 7 students
r: 9 students
t: 9 students
v: 2 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 1105):

Another sample "p" response (from student 2344):

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