Physical cosmology is the branch of physics and astrophysics that deals with the study of the physical origins of the Universe and the nature of the Universe on its very largest scales. In its earliest form it was what is now known as [[celestial mechanics]], the study of the [[celestial sphere|heavens]]. The Greek philosophers [[Aristarchus of Samos]], [[Aristotle]] and [[Ptolemy]] proposed different cosmological theories. In particular, the [[geocentric]] [[Ptolemaic system]] was the accepted theory to explain the motion of the heavens until [[Nicolaus Copernicus]], and subsequently [[Johannes Kepler]] and [[Galileo Galilei]] proposed a [[heliocentric]] system in the [[16th century]]. This is known as one of the most famous examples of [[epistemological rupture]] in physical cosmology.
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With [[Isaac Newton]] and the 1687 publication of ''[[Philosophiae Naturalis Principia Mathematica|Principia Mathematica]]'', the problem of the motion of the heavens was finally solved. Newton provided a physical mechanism for [[Kepler's laws]] and his [[law of universal gravitation]] allowed the [[anomalies]] in previous systems, caused by gravitational interaction between the planets, to be resolved. A fundamental difference between Newton's cosmology and those preceding it was the [[Copernican principle]] that the bodies on earth obey the same [[physical law]]s as all the celestial bodies. This was a crucial philosophical advance in physical cosmology.
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Modern scientific cosmology is usually considered to have begun in 1917 with [[Albert Einstein]]'s publication of his final modification of [[general relativity]] in the paper "Cosmological Considerations of the General Theory of Relativity," (although this paper was not widely available outside of Germany until the end of [[World War I]]). General relativity prompted [[cosmogony|cosmogonists]] such as [[Willem de Sitter]], [[Karl Schwarzschild]] and [[Arthur Eddington]] to explore the astronomical consequences of the theory, which enhanced the growing ability of [[astronomy|astronomers]] to study very distant objects. Prior to this (and for some time afterwards), physicists assumed that the Universe was static and unchanging. In parallel to this dynamic approach to cosmology, a debate was unfolding regarding the nature of the cosmos itself. On the one hand, Mount Wilson astronomer Harlow Shapley championed the model of a cosmos made up of the [[Milky Way]] star system only. [[Heber D. Curtis]], on the other hand, suggested spiral nebulae were star systems in their own right, island universes. This difference of ideas came to a climax with the organization of the [[Great Debate]] at the meeting of the (US) National Academy of Sciences in Washington on 26 April 1920. The resolution of the debate on the structure of the cosmos came with the detection of novae in the [[Andromeda galaxy]] by [[Edwin Hubble]] in 1923 and 1924. Their distance established spiral nebulae well beyond the edge of the Milky Way and as galaxies of their own. Subsequent modeling of the universe explored the possibility that the [[cosmological constant]] introduced by Einstein in his 1917 paper may result in an [[expanding universe]], depending on its value. Thus the [[big bang]] theory was proposed by the [[Belgium|Belgian]] priest [[Georges Lemaître]] in 1927 which was subsequently corroborated by [[Edwin Hubble]]'s discovery of the [[Redshift|red shift]] in 1929 and later by the discovery of the [[cosmic microwave background radiation]] by [[Arno Penzias]] and [[Robert Woodrow Wilson]] in 1964. These findings were a first step to rule out some of many [[Non-standard cosmology|alternative physical cosmologies]].
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Recent observations made by the [[COBE]] and [[WMAP]] satellites observing this background radiation have effectively, in many scientists' eyes, transformed cosmology from a highly speculative science into a predictive science, as these observations matched predictions made by a theory called [[Cosmic inflation]], which is a modification of the standard [[big bang]] theory. This has led many to refer to modern times as the "Golden age of cosmology". <ref> [[Alan Guth]] is reported to have made this very claim in an [[Edge Foundation]] interview [http://www.edge.org/documents/day/day_guth.html]. </ref>
