Cosmology

Using such instruments as the Hubble Space Telescope, modern cosmology is an attempt to describe the large scale structure and order of the universe. To that end, one does not expect cosmology to deal with the detailed structure such as planets, stars, or even galaxies. Rather it attempts to describe the structure of the universe on the largest of scales and to determine its past and future.

One of the earliest constraints on any description of the universe is generally attributed to Heinreich Olbers in 1826. However, more careful study traces the idea back to Thomas Digges in 1576 and it was thoroughly discussed by Edmond Halley at the time of Isaac Newton. The notion, still called Olbers' paradox, is concerned with why the night sky is dark. At the time of Newton it was understood that if the universe was finite then Newton's Law of Gravity would require that all the matter in the universe should pull itself together to that point equidistant from the boundary of the universe. Thus, the prevailing wisdom was that the universe was infinite in extent and therefore had no center. However, if this were true, then the extension of any line of sight should sooner or later encounter the surface of a star. The night sky should then appear to have the brightness of the average star. The sun is an average star, thus one would expect the sky to be everywhere as bright as the sun. It is not, so there must be a problem with the initial assumptions.

An alternative explanation, pointed out in 1964 by Ed Harrison, is that the universe had a finite beginning and the light from distant stars had not yet had time to arrive and that is why the night sky is dark.

At the turn of the twentieth century cosmology placed the sun and its solar system of planets near the center of the Milky Way Galaxy which comprised the full extent of the known Universe. However, early in the century information began to be compiled that would change the popular view that the universe was static and made of primarily stars. On April 26, 1920, there was a historic debate between H. D. Curtis and Harlow Shapley concerning the nature of some fuzzy clouds of light which were called nebulae. Shapley thought they were objects within the galaxy while Curtis believed them to be "island universes" lying outside the Milky Way.

Although at the time most agreed that Shapley had won the debate, science eventually proved that Curtis was right. Within a few years Edwin Hubble detected a type of star, whose distance could be independently determined, residing within several of these fuzzy clouds. These stars clearly placed the "clouds" beyond the limits of the Milky Way. While Hubble continued to use the term "island universe," more and more extragalactic nebulae were discovered, and they are now simply known as galaxies. In the span of a quarter of a century the scale for the universe had been grown dramatically.