Is the Universe Expanding?

The realization that our universe is expanding is one of the most transformative discoveries in cosmology. While once hotly debated, this observational reality now forms the bedrock for our modern conceptions of the cosmos.

The first evidence for an expanding universe came through Albert Einstein’s discovery of the general theory of relativity in 1917. Other scientists have since gathered more evidence, and expanded his ideas into more tangible models.
In an expanding universe, the space between galaxies continually grows, and this expansion causes light to stretch to longer wavelengths. The more space there is between us and a distant galaxy, the more stretch can happen as the light travels, and the greater the redshift.