Big Bang Theory

Since ancient times, people have wondered about the origin of the Universe. Questions about how and when Earth and the heavens formed have been pondered by philosophers, theologians, and scientists. The modern, scientific study of the origin and structure of the Universe is known as the science of cosmology.

For many centuries cosmological thought was limited mostly to speculation. For example, it was not obvious to the ancients that the Sun and the stars are objects of the same sort. Today it is known that the Sun is a star much like the other stars, and is only brighter because it is closer. This simple fact took centuries to determine because it is difficult to determine the distance to most of the objects seen in the night sky. Early astronomers of the scientific era, although they knew that the stars are also suns, assumed that all stars have the same intrinsic brightness and thus, that only their distance from Earth determines their apparent brightness. This is now accepted as untrue—enormous variations in brightness among individual stars do exist. Examination of binary stars (paired stars that orbit each other) demonstrated these differences. When binary star systems in which the two stars did not have the same brightness were observed, it became clear that the amount of light received from any given star is dependent on more than just its distance. Until the elementary task of measuring the position of astronomical objects could be pursued systematically, larger questions about the structure and history of the Universe as a whole could not even begin to be answered.