Measurement of the sizes of hot spots in the temperature distribution of the cosmic microwave background suggests that the universe is flat. Yet the combined density of all the normal matter seen, plus the dark matter inferred, is only about one-quarter of the critical density. What conclusion have cosmologists drawn from this discrepancy?
A) The universe is not flat after all.
B) There must be a large amount of luminous matter and radiation in the universe, which cosmologists have not yet found.
C) The universe is smaller than cosmologists have been assuming and thus the measured density is larger.
D) There is an entity to which cosmologists have given the name "dark energy" that contributes to the total density but that cannot be detected by the usual methods.

Question

Quizplus · Accepted Answer

Cosmologists have concluded that "dark energy" contributes to the total density of the universe, making up for the discrepancy between the observed density from matter (normal and dark) and the critical density required for a flat universe. This dark energy cannot be detected by traditional methods used to detect matter.

Measurement of the Sizes of Hot Spots in the Temperature