Physical cosmology is the branch of physics and astrophysics that
deals with the study of the physical origins and evolution of the
Universe. It also includes the study of 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 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.
With Isaac Newton and the 1687 publication of 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 laws as all the celestial bodies. This was a crucial philosophical advance in physical cosmology.
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 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 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, one long-standing
debate about the structure of the cosmos was coming to a climax. Mount
Wilson astronomer Harlow Shapley championed the model of a cosmos made up of the Milky Way star system only; while Heber D. Curtis
argued for the idea that 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 this debate 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.
Subsequent modelling 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 model was proposed by the Belgian priest Georges Lemaître in 1927 which was subsequently corroborated by Edwin Hubble's discovery of the 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 alternative physical cosmologies.
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 model.
This has led many to refer to modern times as the "Golden age of cosmology".
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