To answer the question: "what is cosmology?" we must define the word cosmology first. It comes from the Greek: 'cosmologia' (order, orderly arrangement, ornaments) + 'logos' (word, reason, plan). It is the study of the Universe in its totality. The study of the Universe has a long history, involving at least three main domains: religion, philosophy and science. This article concentrates only on scientific cosmology.
Also called physical cosmology, it is the branch of science that deals with the scientific study of the origins and evolution of the Universe and the nature of the Universe on its very largest scales.
In its earliest form physical cosmology was basically just celestial mechanics, the study of the 'heavens'. The Greek philosophers Aristarchus of Samos, Aristotle and Ptolemy proposed different theories for how the heavens work.
In particular, the 'earth-centric' Ptolemaic system with it's perfect circles and small 'epicycles' was the accepted theory to explain the motion of the heavenly bodies. In the picture above, the Earth is at the centre with the Sun, Moon and planets all revolving around Earth.
It was the best theory until Copernicus, Kepler and Galileo proposed a 'sun-centric' system in the 16th century. Although Greek, Indian and Muslim savants formulated the sun-centric theory centuries before Copernicus, his reiteration that the Sun, rather than the Earth, is at the center of the solar system is considered among the most important landmarks in the history of modern astronomy.
With Isaac Newton's 1687 publication of 'Principia Mathematica', the problem of the motion of the heavenly bodies was solved. Newton provided a physical mechanism for Kepler's laws of planetary motion. His law of universal gravitation resolved the anomalies caused by gravitational interaction between the planets in the previous systems.
Despite the words 'universal gravitation', Newton did not consider the implications of his theory on the Universe at large, although it implied that gravity works the same everywhere. The universal application of gravity was left to Albert Einstein, who formulated it more than two centuries later.
Heber D. Curtis, on the other hand, suggested that the observed spiral nebulae were star systems in their own right, or 'island universes'. In 1923-24 Edwin Hubble detected novae in the Andromeda galaxy and then showed its distance to be way beyond the Milky Way's boundaries. This settled the debate and it was accepted that the Milky Way was not all the universe there is. However, the cosmos was still thought to be static and unchanging.
Subsequent modeling of the universe explored the possibility that the cosmological constant introduced by Einstein may result in an expanding universe, depending on its value. In 1929, Edwin Hubble's discovered the red shift of the light of distant galaxies, indicating that they move away from the Milky Way. Hence, the universe must be expanding.
Scientific cosmology really began in 1917, when Albert Einstein's published the final modification to his theory of gravity in the paper 'Cosmological Considerations of the General Theory of Relativity'. This paper prompted early cosmologists such as Willem de Sitter, Karl Schwarzschild and Arthur Eddington to explore the astronomical consequences of the theory of relativity.
Mount Wilson astronomer Harlow Shapley championed the model of a cosmos made up of the Milky Way star system only – a static unchanging universe. Einstein also believed this and 'fudged' his equations of general relativity to represent this static state. He introduced a 'cosmological constant' that prevented the universe from contracting or expanding, which was what his original equations told him.
Einstein promptly repudiated his earlier 'fudge factor' and declared that the introduction of the cosmological constant was the "biggest blunder of his scientific career". His original 1917 equations support an expanding universe without the cosmological constant anyway.
In 1931 Georges Lemaitre, a Belgian priest, postulated that an expanding universe meant a creation event at some time in the past. He postulated that the Universe was created from a small 'cosmic egg' and set to expand until today. This theory was later labeled the 'big bang'.
There were many rival theories to the big bang. The 1964 discovery of the cosmic microwave background radiation by Arno Penzias and Robert Woodrow Wilson was a first step in ruling out many alternative physical cosmologies.
The Golden Age of Cosmology
Recent observations made by the COBE and WMAP satellites observing and accurately measuring this background radiation have effectively, transformed cosmology from a highly speculative science into a predictive science. This has led many to refer to modern times as the "golden age of cosmology".
Present observations match predictions made by a theory called Cosmic Inflation, first proposed by American physicist and cosmologist Alan Guth in 1981. It is a modification of the standard big bang theory that explains how the universe obtained the expansion and solves some problem areas of the big bang satisfactorily.
Since there are still problems with scientifically explaining how the universe came forth 'out of nothing', modern research are mainly concentrated on 'quantum cosmology'. This is a branch of 'quantum gravity', which strives to reconcile general relativity with quantum physics. No satisfactory reconciliation has been possible yet.