Cuesta College, San Luis Obispo, CA
An astronomy question on an online discussion board[*] was asked and answered:
?: Is there an edge to all of the galaxies in the universe?Discuss why this answer is correct, and how you know this. Explain using the properties of the speed of light, stellar evolution, and galaxies.
Nick: The galaxies could go on forever, but we wouldn't ever be able to see the rest of them. We've basically seen as far as we'll ever see, because we can't see any galaxies further than about 14 billion light years away.
[*] Adapted from http://answers.yahoo.com/question/index?qid=20110218090525AAf19m6.
Solution and grading rubric:
Correct. Recognizes how (1) light travels at a finite speed, so looking further out will see further back in time (look-back time); and why not seeing any galaxies further than approximately 14 billion light years out is an indication that the universe has a finite age of approximately 14 billion years (Olbers' paradox).
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: 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.
Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner.
Implementation/application of ideas, but credit given for effort rather than merit. Discusses factors other than relevant to the speed of light, and Olbers' paradox.
Irrelevant discussion/effectively blank.
Exam code: finals0oB
p: 18 students
r: 5 students
t: 8 students
v: 1 student
x: 4 students
y: 3 students
z: 0 students
Exam code: finaln4A6
p: 8 students
r: 3 students
t: 5 students
v: 4 students
x: 4 students
y: 0 students
z: 1 student
A sample "p" response (from student 0615):
A sample "y" response (from student 0221):