Which law is foundational for understanding the universe's expansion?

Hubble's law is foundational for understanding the universe's expansion because it describes the relationship between the distance of galaxies from Earth and their recessional velocity. Formulated by Edwin Hubble in the 1920s, the law states that the farther away a galaxy is, the faster it appears to be moving away from us. This observation supports the theory that the universe is expanding, as it indicates that galaxies are moving away from each other over time.

The significance of Hubble's law lies in its empirical foundation; it provides observational evidence for the Big Bang theory, which suggests that the universe has been expanding since its inception. This expansion implies that the fabric of space itself is stretching, making it a cornerstone concept in cosmology and the study of the universe's evolution.