Final answer:
Hubble's Law states that the velocity of recession of galaxies is proportional to their distance from Earth, confirming the universe's expansion.
Step-by-step explanation:
The scientific principle that all galaxies are moving away from each other, with the rate of movement being proportional to the distance each galaxy is from another, is described by Hubble's Law. Discovered by Edwin Hubble in 1929, this law states that the velocity of recession (v) of a galaxy is proportional to its distance (d) from Earth, and by extension, this implies the same for relative distances and velocities between any two galaxies. Consequently, Hubble's Law is fundamental in confirming that our universe is expanding, a concept that has been supported by the observation of the cosmological red shift in the hydrogen spectra of distant galaxies.