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Hot Big Bang theory helped establish the universe.
Einstein had developed his general theory of relativity from the assumption that it is impossible to tell acceleration from a gravitational field. This theory became a more thorough, but more complex, replacement for Newton's Theory of Gravity. Like Newton's theory it was expected that everything would attract everything else and the universe would collapse.
Einstein had resisted the idea of a changing universe and had introduced his 'cosmological constant' into the equations of General relativity to avoid the universe collapsing in on itself. Many other scientists did not follow Einstein's lead with the cosmological constant, but were happy to accept general relativity. Willem de Sitter was the first to show that the universe must expand. His prediction was improved by Alexander Alexandrovich Friedmann in 1922, and further refined by Arthur Stanley Eddington in 1930. Georges Lemaître, in 1927, thought about the consequences in a slightly different way.
If the universe was alread expanding gravity could slow the expansion, but not necessarily stop it and cause collapse. He realised that an expanding universe would have been smaller yesterday than today and so on, all the way back to a 'day that would not have had a yesterday'. He argued that that instant would have been the moment of creation, and as he was also an abbot of the Roman Catholic church argued that God had created 'a primeaval atom' which had grown to become the Universe. Lemaître's main interest in the primæval atom was as a source of all other atoms, which he imagined taking place by a process of continual fission. Initially Einstein thought that Lemaître did not understand the physics properly and dismissed the idea, but later after Hubble's discovery of the expansion, Lemaître gave a lecture at which both Hubble and Einstein were present in which Einstein proclaimed, " [what he had just heard] was the most beautiful and satisfying interpretation I have ever listened to" and admitted that the cosmological constant hadbeen an error.
Not everyone accepted the idea of a moment of creation. Fred Hoyle was unhappy about accepting a God given creation, and as an atheist attempted to develop a theory without such an intervention. This was to become the steady state theory . His principal challenge to Lemaître's theory was that there was no 'fossil' record. In an attack on the theory he had dismissively referred to "this hot Big Bang" and the name stuck. In the same year as steady state was published, 1948, George Gamow and his student Ralph Alpher, proposed that if the universe was created in a gigantic explosion that the various elements observed today would be produced within the first few minutes after the big bang, when the extremely high temperature and density of the universe would fuse subatomic particles into the chemical elements.
This theory provided a basis for understanding the earliest stages of the universe and its subsequent evolution. The extremely high density would cause the universe to expand rapidly. As t expanded, the Hydrogen and helium would cool and condense into stars and galaxies. This could explain the expansion of the universe and Hubble's law. As the universe expanded, it would continue to cool, until now it should be a temperature of about 3 K ( -270º C). Much of Gamow's early work was forgotten perhaps because of Gamow's sense of humour. Just for a joke he added the name of his friend Hans Bethe to the paper, so that it read Alpher, Bethe, Gamow; the paper was referred to as the Alpha Beta Gamma paper.
Gamow's idea was re-examined by Bob Dicke and Jim Peebles at Princeton University. They agreed with Gamow's work but instead of considering cooling they considered that the original radiation would have been red-shifted to the microwave region of the spectrum, entirely consistent with the cooling hypothesis. They started to look for this radiation, at about 7 cm, with a simple receiver.
Unbeknown to them this radiation had already been detected by Arno Penzias and Robert Wilson in 1965. With further work to confirm that the radiation was consistent with that from a black body, most scientists now accept this observation as one of the most important confirmations of the big bang theory, (and provides Hoyle with his fossil record.)
The other important property of the theory is its correct prediction of the hydrogen - helium ratio of the Universe. As the universe cooled the energy involved in reactions dropped. After about 2 to 3 minutes the temperature would have dropped to about 109 K. At this temperature nucleons could strike each other and fuse to form nuclei, without being immediately broken apart again by other collisions. The nucleons would have had a ratio of protons to neutrons of about 16 to 1 because Neutrons are slightly heavier than protons and thus unstable, having a half life of about 4 minutes. These protons and neutrons could join together to make light nuclides such as4He,3He,2H and7Li in just the abundances we see today,of 75 - 80 % hydrogen, 25 - 20 % helium and traces of lithium. After only a few minutes, possibly less than half an hour, the temperature would have dropped even further and this nucleosynthesis would stop.
The expansion would now continue and cool until stars and galaxies were formed, allowing nuclear reactions to start up again to re-illuminate the Universe, and provide energy for life on Earth, and ultimately the observers to discover it all.
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