Modern cosmology is considered to have started in 1917 with the final paper of Albert Einstein's theory of general relativity. This paper was known as "Cosmological Considerations of the General Theory of Relativity" which made physicists start to change the assumption that the Universe never 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.