Astronomy quiz question: dark energy evidence

Astronomy 10 Quiz 12, Fall Semester 2007
Cuesta College, San Luis Obispo, CA

Astronomy 10 learning goal Q12.3

[3.0 points.] Which one of the following choices best describes the evidence for the existence of dark energy?
(A) Stars in the inner and outer parts of the disk orbit the center of the Milky Way with the same speed.
(B) Each and every galaxy sees all other galaxies moving away from its location.
(C) Nearly equal amounts of cosmic background radiation are detected from all directions.
(D) Distant galaxies appear to be much younger than nearby galaxies.
(E) Type Ia supernovae in distant galaxies appear to be dimmer than expected from their redshifts.

Correct answer: (E)

Since type Ia supernovae in distant galaxies appear to be dimmer than expected from their redshifts, this shifts the slope of the Hubble plot for the long-ago (distant) universe, compared to the recent (nearby) universe. Thus the expansion rate of the universe has been accelerating recently, and this is the evidence for dark energy.

Response (A) describes the evidence for dark matter in the halo of the Milky Way. Response (B) is the consequence of an expanding universe. Response (C) describes the "horizon problem" of the cosmic microwave background, and is the evidence for the inflation era of expansion. Response (D) is the consequence of the finite speed of light.

Student responses
Section 0135
(A) : 4 students
(B) : 3 students
(C) : 13 students
(D) : 2 students
(E) : 7 students