Cosmology is the branch of astronomy that deals with the origin, structure, evolution and space-time relationships of the universe. NASA defines cosmology as "The study of the structure and changes in the present universe". Another definition of cosmology is "the study of the universe, and humanity's place in it".
Though the word cosmology is recent (first used in 1730 in Christian Wolff's Cosmologia Generalis), the study of the universe has a long history.
Until recently, people thought that the universe was only the Milky Way galaxy. They thought this because they could only see the planets up to Saturn and stars. With the invention of the telescope, we could see more of the universe. Even in the 20th century, people thought that the Milky Way was the entire universe. With the Hubble Space Telescope, people could see things far away.
Modern cosmology is considered to have started in 1917 with the final paper of Albert Einstein's theory of general relativity. This paper was known as "Cosmological Considerations of the General Theory of Relativity" which made physicists start to change the assumption that the Universe never changed. When a scientific discipline begins to change an idea that is believed by many people, it is known as a paradigm shift. During this paradigm shift, the Great Debate took place. Many scientists debated if there were other galaxies. The debate ended when Edwin Hubble found Cepheid Variables in the Andromeda Galaxy in 1926.
The Big Bang model was then created by Belgian priest, Georges Lemaître in 1927. This was supported by Edwin Hubble's discovery of the redshift in 1929 and later by the discovery of cosmic microwave background radiation. This was found by Arzo Penzias and Robert Woodrow Wilson in 1964.
All of these discoveries have been further supported throughout the 20th century. Some more observations of the cosmic microwave background radiation were found by the COBE, WMAP, and Planck satellites. Some more observations of the redshift were found by the 2dfGRS and SDSS. These are known as redshift surveys. Survey is this context refers to an astronomical survey. An astronomical survey is a place in space. A redshift survey is a survey that looks for redshifts.
On 1 December 2014, at the Planck 2014 meeting in Ferrara, Italy, astronomers reported that the universe is 13.8 billion years old and is composed of 4.9% regular matter, 26.6% dark matter and 68.5% dark energy.
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