Cosmology
Modern cosmology is considered to have started in 1917 with the final paper of Albert Einstein's theory of general relativity. This made physicists realize that the universe changed. When a scientific discipline begins to change an idea that is believed by many people, it is known as a paradigm shift. During this paradigm shift, the Great Debate took place. Many scientists debated if there were other galaxies. The debate ended when Edwin Hubble found Cepheid Variables in the Andromeda Galaxy in 1926.