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 matter and energy we see,plus the dark matter we infer,is only about a quarter of the critical density.What conclusion have we 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 that we have not yet found.
C) The universe is smaller than we have been assuming and thus the measured density is larger.
D) There is an entity to which we have given the name "dark energy" that contributes to the total density but that we cannot detect by the usual methods.

Question

Quizplus · Accepted Answer

The discrepancy between the observed density of matter and energy and the critical density suggests the presence of an unknown entity called "dark energy" that contributes to the total density of the universe. This entity cannot be detected by the usual methods and may account for the observed flatness of the universe despite the low measured density.

Measurement of the Sizes of Hot Spots in the Temperature