One of the most fundamental discoveries about the Universe has been that it is expanding. "The universe is expanding" means
A) our estimate for the size of the universe has increased over the last century.
B) galaxies are growing with time.
C) the cosmological horizon is growing with time.
D) distances between most galaxies are getting larger.

Question

Quizplus · Accepted Answer

The expansion of the universe means that the distances between most galaxies are getting larger. This has been observed through measurements of the redshift of light emitted by galaxies, which indicates that they are moving away from us (and from each other) at increasing speeds. The expansion of the universe does not mean that galaxies themselves are growing or that our estimate for the size of the universe has increased, although our understanding of its size has been refined over time. The cosmological horizon refers to the boundary between observable and unobservable regions of the universe, and is not directly related to the expansion of the universe.

One of the Most Fundamental Discoveries About the Universe Has