How do scientists know that the universe is expanding? A. Galaxies are moving away from one another. B. Hundreds of new galaxies form every year. C. No more stars are forming from nebulae. D. Stars are getting closer and closer together.

Scientists know the universe is expanding primarily because galaxies are moving away from one another. This phenomenon was observed by Edwin Hubble in the 1920s when he discovered that distant galaxies are redshifted, meaning their light is stretched to longer wavelengths due to their rapid movement away from us. This redshift provides critical evidence supporting the idea that the universe is expanding. Additionally, the concept of an expanding universe revolutionized our understanding of cosmic history! It led to the formulation of the Big Bang theory, which suggests that the universe began as a singularity and has been expanding ever since. This not only explains the current observations of distant galaxies but also provides insight into the early conditions of the universe and its eventual fate.