PrepTest 159, Section 2, Question 21

What we’re looking for

Working the answers

• A
  Correct

  The simplest dark-energy account uses something like Einstein's cosmological constant. If that kind of quantity is needed to explain other astronomical phenomena too, there is an independent reason to take it seriously. That supports the proposed ingredient of the first explanation, even though it doesn't by itself solve the energy discrepancy or prove the whole theory.

• B

  Canceling the vacuum energy's effects would undermine its proposed role in driving the acceleration. A canceled effect doesn't provide the explanatory contribution the theory needs.

• C

  This gives a limit on interactions between two kinds of particles. It doesn't establish that virtual particles generate the right vacuum energy to explain accelerated expansion. Nothing in the answer supplies that missing link.

• D

  This describes a relationship between recession speed and distance, but doesn't identify dark energy as its cause. A pattern of movement isn't, by itself, support for this particular explanation of acceleration.

• E

  Doubt about whether Einstein's gravitational theory applies universally points toward the second explanation. It doesn't directly support the first theory's proposed dark energy.