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Early Universe Temperature: Big Bang and Nucleosynthesis
The temperature of the universe immediately after the Big Bang was extremely high. During the first fractions of a second, the universe was in a hot, dense state, with temperatures well above 10^10 Kelvin (K) . As the universe expanded, it cooled rapidly. By the time of Big Bang nucleosynthesis, which occurred from about 0.01 seconds to a few minutes after the Big Bang, the temperature had dropped from around 10 MeV (about 10^11 K) to below 0.1 MeV (about 10^9 K) . This cooling allowed protons and neutrons to combine and form light elements such as helium, deuterium, and lithium .
Cosmic Microwave Background (CMB) Temperature Evolution
The residual heat from the Big Bang is observed today as the cosmic microwave background (CMB), which has a temperature of about 2.7 K . The temperature of the CMB increases with redshift, meaning it was much hotter in the past. For example, at a redshift of z = 0.89, the CMB temperature was measured to be about 5.08 K, and at z = 2.34, it was between 6.0 and 14 K, matching predictions from the standard Big Bang model 26. These measurements confirm that the universe has cooled steadily as it expanded.
Reheating and Maximum Temperatures
The "reheating" phase after cosmic inflation set the initial temperature for the hot Big Bang. Theoretical models suggest that the reheating temperature could range from the MeV scale up to as high as 10^16 GeV, depending on the specifics of inflation and particle production 35910. Some models predict a maximum temperature in the TeV regime, while others allow for much higher temperatures, especially if inflation started at grand unified theory (GUT) scales . The exact value is still uncertain, but it is clear that the early universe reached extremely high temperatures before cooling down to allow for nucleosynthesis and, eventually, the formation of atoms and galaxies 35910.
Alternative Theories and Debates
While the mainstream view holds that the Big Bang began with an extremely high temperature, some alternative theories propose different initial conditions. For example, one hypothesis suggests the universe started at absolute zero and slowly warmed up, but this view is not widely supported by observational evidence .
Conclusion
In summary, the temperature of the universe at the moment of the Big Bang was extremely high, likely exceeding 10^10 K, and cooled rapidly as the universe expanded. This cooling is directly observed in the cosmic microwave background, which provides a snapshot of the universe at a much later, cooler stage. Theoretical models and observations together support the picture of a hot, dense early universe that has been cooling ever since 12356789+1 MORE.
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