What does Hubble's Law refer to?

Hubble's Law describes a fundamental relationship in cosmology that connects the distance of galaxies from Earth to their velocity as indicated by redshift. Specifically, it states that the farther away a galaxy is, the faster it appears to be moving away from us. This relationship is a key piece of evidence supporting the expansion of the universe.

When observing light from distant galaxies, astronomers notice that the light is shifted toward the red end of the spectrum, a phenomenon known as redshift. The degree of this redshift correlates with the distance to the galaxy, which supports the idea that the universe is expanding. This correlation suggested by Hubble's Law was crucial in leading to our understanding of the expanding universe and the Big Bang theory.

The other choices focus on different aspects of cosmic phenomena and do not describe Hubble's Law directly. The expansion of the universe being accelerated relates to observations made by probing supernovae and cosmic microwave background radiation, but this is a separate concept from Hubble's Law itself. Similarly, the collapse of galactic structures pertains to gravitational interactions rather than the relationship between distance and redshift, and the rotation speeds of spiral galaxies involve dynamics within galaxies rather than their relation to the overall cosmic scale defined by Hubble's Law.