Astronomy midterm question: expansion, not explosion

Astronomy 210 Midterm 2, Spring Semester 2009
Cuesta College, San Luis Obispo, CA

[20 points.] Discuss why the expansion of the universe is not like an explosion, using observations and evidence related to the Hubble law in your explanation.

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. (Also there is no unique center to this expansion of space.) This is in opposition to an explosion, where the velocity of particles is inversely proportional to the distance from the center of the explosion.
  
• 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. Describes how an explosion is not like the actual expansion of the universe, 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 other aspects of explosions, 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
p: 9 students
r: 5 students
t: 10 students
v: 9 students
x: 0 students
y: 0 students
z: 0 students

A similar version of this question was asked as a multiple-choice question in Spring 2009.

A sample "p" response (from student 1959), positing that the galaxies in the universe have all drank the "Haterade":

Another sample "p" response (from student 2902), appealing to the "everyone smelt it, so everyone dealt it" theory:

A sample "p" response (from student 2947) explaining in pictures what word cannot do justice:

Another sample "p" response (from student 5398) using a "no-center" argument: