Astronomy final exam question: dark energy properties and evidence

Astronomy 10 Final Exam, Spring Semester 2008
Cuesta College, San Luis Obispo, CA

Astronomy 10 learning goal 12.3

[15 points.] Describe the properties of dark energy, and explain the observational evidence for its existence.

Solution and grading rubric:

• p = 15/15:
  Correct. Distant type Ia supernovae are dimmer than expected from their redshifts, which means that the expansion of the universe today is different (faster) now than back then. Dark energy is the proposed agent of this acceleration, and produces a "pressure" that accelerates the expansion rate of the universe, and is itself produced by space itself, resulting in more dark energy as space expands.
  
• r = 12/15:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Only correctly describes properties of dark energy, or only correctly describes the observational evidence for the existence of dark energy.
  
• t = 9/15:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Typically claims dark energy causes the expansion of the universe (it is causing the recent acceleration of the expansion of the universe), or had caused the inflation which happened just after recombination.
  
• v = 6/15:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Confuses dark energy with dark matter, scalar field energy, etc.
  
• x = 3/15:
  Implementation/application of ideas, but credit given for effort rather than merit.
  
• y = 1.5/15:
  Irrelevant discussion/effectively blank.
  
• z = 0/15:
  Blank.
  

Grading distribution:
Section 5166
p: 1 student
r: 5 students
t: 4 students
v: 26 students
x: 3 students
y: 1 student
z: 0 students

A sample "p" response (from student 2431):
Student 1886 wanders off-topic from dark energy by including details about dark matter, for an "r" response:
Student 1426 at least remembers the story behind dark energy being a "sexy" placeholder name for whatever it is that is accelerating the expansion of space, for an "x" response:
Student 2743 has another example of an "x" response that has a good start, but ends abruptly before the magic can begin:
An "x" response (from student 7499) attempts to appeal to the obvious:
Student 3635 is a race to the bottom, earning a "y" response: