Astronomy midterm question: exploding big bang?

Astronomy 210 Midterm 2, spring semester 2011
Cuesta College, San Luis Obispo, CA

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

nema: What is [the] big bang theory...?
Wobzter: First all matter and energy was stored [in] a ball the size of a fist...and exploded. Then all matter and energy spread out, and moved away from the place of the fist...

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

[*] Source: http://answers.yahoo.com/question/index?qid=20100117025456AAzBY3X.

Solution and grading rubric:

• p = 20/20:
  Correct. Discusses Hubble's law (recession velocity of galaxies is proportional to distance) and evidence (greater redshift of absorption lines for distant galaxies compared to nearby galaxies), and uses this to contradict one or both: (a) no unique center, (b) expansion is not like an explosion (where speeds would decrease with increasing distance).
  
• 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. Hubble's law discussion is problematic or
  incomplete, but least understands that as a consequence there must be no unique center, and/or expansion cannot be like an explosion.
  
• v = 8/20:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Discussion based on other aspects 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:
  Blank.
  

Grading distribution:
Section 30674
Exam code: midterm02N3n4
p: 8 students
r: 7 students
t: 15 students
v: 15 students
x: 1 student
y: 0 students
z: 0 students

Section 30676
Exam code: midterm02Sys7
p: 4 students
r: 5 students
t: 14 students
v: 12 students
x: 15 students
y: 0 students
z: 0 students

A sample "p" response (from student 4200):
Another sample "p" response (from student 2364):