Which principle explains the expanding universe?

The principle that explains the expanding universe is Hubble's Law. This law is derived from observations made by Edwin Hubble in the 1920s when he discovered that distant galaxies are moving away from us, and that their velocity is proportional to their distance. This observation leads to the conclusion that the universe itself is expanding.

Hubble's Law can be expressed mathematically as v = H₀d, where v is the recessional velocity of a galaxy, d is its distance from us, and H₀ is the Hubble constant, which represents the rate of expansion of the universe. As galaxies are observed to be moving away from each other, this suggests that the fabric of space itself is stretching, providing strong evidence for the Big Bang theory and the ongoing expansion of the universe.

In contrast, the other options are relevant to different aspects of physics and astronomy but do not specifically address the expansion of the universe. Newton's Laws describe motion and gravity, Einstein's Theory of Relativity fundamentally changed our understanding of space and time but does not specifically indicate that the universe is expanding, and Kepler's Laws describe the motion of planets around the sun, which pertains to planetary motion rather than cosmic scale phenomena.