Two modern cosmologies are outlined: the Big Bang Theory and the Expanding Universe Theory. The Big Bang Theory proposes that approximately 18 billion years ago, the universe exploded into existence from a single point. This is supported by the Second Law of Thermodynamics, which states that the universe is constantly increasing in disorder, and by gravity, as described by Newton. The Expanding Universe Theory, developed in the 1920s by Edwin Hubble, found that galaxies are moving away from each other, indicating the universe is expanding from an initial smaller state, supporting the idea of an initial explosion that set the universe in motion.