Cosmology is the study of the structure and evolution of the universe as a whole. Prior to the twentieth century this was the domain of religion and speculative philosophy, but it is now a serious branch of physics, and one with spectacular implications. The transition from myth to science was brought about, in large measure, by two empirical findings: Hubble's observation (in 1929) that the Universe is expanding, and the discovery by Penzias and Wilson of Cosmic Microwave Background radiation (in 1965). Taken together, they indicate that the Universe as we know it began with a gigantic explosion 13.7 billion years ago – the Big Bang. In the next two sections we will explore these developments.

But first I would like to alert you to a fundamental assumption that informs almost all modern thinking about cosmology. Before Copernicus, most people assumed that the Earth is at the center of the Universe. The Sun, the Moon, the planets, and the stars orbit around us. Copernicus showed, to the contrary, that while the Moon orbits the Earth, the planets (including the Earth) orbit the Sun. And we now know that the Sun itself is just one of billions and billions of stars. The Earth is not at the center of the Universe – nor is the Sun. Modern cosmology takes the Copernican revolution to its logical conclusion: The Universe has no center, or any kind of “preferred” location – at a given time, it is the same in all places and in all directions. This is known as the Cosmological Principle.