We can say that the universe is expanding based on the observation of redshift in light from distant galaxies. When we observe these galaxies, their light appears redshifted, meaning the wavelengths are stretched as they move away from us. This phenomenon indicates that these galaxies are receding, providing strong evidence that the universe is expanding. The farther away a galaxy is, the greater the redshift, suggesting that the universe has been growing larger over time.

Hubble's Law further supports the idea of an expanding universe. Edwin Hubble discovered a direct relationship between the distance of galaxies and their recessional speed: the farther away a galaxy is, the faster it appears to be moving away from us. This observation implies that the universe is expanding uniformly in all directions, as more distant galaxies are moving away at greater speeds. Together, the redshift of light and Hubble's Law form a compelling case for the ongoing expansion of the universe.