Another big mystery is the origin of the Universe. Early theories held that the Universe has always existed and always will exist, but not too many scientists hold that theory today. This is because of the overwhelming evidence that the Universe is expanding.

     Isaac Newton made extensive studies of light. He shined sunlight through a prism to discover what causes color. He discovered that white sunlight is actually made up of all the different colors of light. Light bends when shined through a glass made in a prism shape. Different colors bend at different angles. Newton called this "refrangibility." It happens because different colors of light are different wavelengths, and the different wavelengths bend at different angles causing the white light to separate into it’s constituent colors.

     Two centuries later, many advances had occurred in the fields of astronomy, physics, and optics. Scientists could shine the light from an individual star through a prism and record the pattern made by the separation of colors. This is called "spectrography." A lot of information about a light emitting object can be derived from it’s spectra-analysis, such as what it’s made of, or how fast it is moving in relation to the Earth.

     If a star is moving away from the Earth, the light pattern in it’s spectrograph will have a measurable shift towards longer wavelengths which are red in color. This is believed to be the result of the Doppler effect. Each wave begins a little farther away than the previous wave, causing them to appear stretched, or of a redder color. A shift towards the blue or shorter wavelengths indicates a star that is moving towards the Earth.

     Three hundred years after Galileo began using a telescope, astronomers noticed fuzzy looking objects that did not look like stars. They called them "spiral nebulas." Edwin P. Hubble was the first astronomer to declare that these nebulas were actually galaxies like our own Milky-way galaxy, each containing hundreds of billions of stars. From 1919 until his death in 1953, Hubble worked at the Mount Wilson Observatory in California. Mount Wilson had the 100 inch Hooker telescope, which was the world’s largest until 1948.

     To find the distance to these galaxies, a method was developed involving a type of star known as a "cepheid variable." A cepheid star varies in it’s brightness on a regular cycle. By comparing the apparent brightness and cycle time to cepheid stars who’s distances are known by other methods, the distance to a galaxy can be accurately estimated.

     In the 1910’s and 1920’s other astronomers had measured the red-shift from many spiral nebulas before they were classified as galaxies. Hubble also made his own red-shift measurements to find the speed of these galaxies. In 1929 Hubble compared the distances of the galaxies to the speed at which they were moving away from Earth, and he found a direct and very consistent correlation: The farther a galaxy was from Earth, the faster it was receding. This relationship is so consistent throughout the sky, that it is now known as Hubble’s Law. Hubble concluded that the relationship between speed and distance must mean that the Universe is expanding. Hubble’s measurements were the first observed evidence of an expanding universe, which was first proposed in 1927 by Belgian scientist Georges Lemaitre, who incorporated Einstein’s theory of relativity in his model.

     The evidence of an expanding universe led cosmologists to begin tracking the history of the Universe. If the Universe is expanding outward and cooling now, than it must have been denser and hotter farther back in time. In the 1940’s Russian American physicist George Gamow developed the idea of a hot explosion of matter and energy at the time of the origin of the Universe. This theory was nicknamed "The Big Bang Theory." Gamow predicted that there would be a uniform cosmic background radiation leftover from this primordial explosion. In 1965 faint isotropic radio waves were discovered by physicists working at Bell Telephone Laboratories. This cosmic background radiation is now considered proof that the big bang theory is probably true.