Temperature of the universe
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Cosmic Microwave Background Temperature: Current Value and Measurement
The temperature of the universe today is most precisely measured through the cosmic microwave background (CMB) radiation. The most accurate measurement to date is 2.726 ± 0.01 Kelvin, as determined by the Cosmic Background Explorer satellite, and this value is widely accepted in the scientific community . This measurement is consistent with the earlier Nobel Prize-winning discovery by Penzias and Wilson, who found the temperature of outer space to be about 3 Kelvin, providing strong support for the Big Bang model . Other analyses and models, including alternative cosmologies, also predict a CMB temperature very close to this value, such as 2.736 K .
Evolution of the Universe’s Temperature Over Time
Early Universe: High Temperatures
At the very beginning, the universe was extremely hot. The maximum temperature during the radiation-dominated era, often called the "reheat temperature," could have been as high as 10^16 GeV (or even up to the Planck temperature, about 10^19 GeV) depending on the cosmological model Hu2020Muller2012. Some models suggest that the temperature at the formation of protons was around 4.3 × 10^13 K . The exact maximum temperature is still a subject of research and depends on the details of inflation and reheating scenarios Hu2020Muller2012Komatsu2015.
Cooling with Expansion
As the universe expanded, its temperature dropped. During the epoch of cosmological recombination (when atoms first formed), the temperature of atomic matter was tightly coupled to the CMB until a redshift of about z ≈ 100. After this point, matter cooled faster than radiation, and without astrophysical heating, the matter temperature today would be much lower than the CMB, around 20 millikelvin .
Temperature-Redshift Relation
The temperature of the CMB increases linearly with redshift, as predicted by the standard cosmological model. For example, at a redshift of z = 0.89, the CMB temperature was measured to be 5.08 ± 0.10 K, matching the predicted value of 5.14 K . This linear relationship is a key test of the Big Bang theory and is supported by observations Crawford2022Hannestad2004. Some models explore deviations from this law, but current data favor a low-dissipation, standard cosmological scenario .
Implications and Open Questions
The current temperature of the universe and its evolution provide insights into fundamental processes such as the early radiation-dominated epoch, the synthesis of light elements, and the development of cosmic structure . The maximum temperature reached in the early universe has important implications for particle physics, dark matter production, and baryogenesis Hu2020Muller2012Komatsu2015. While alternative cosmological models exist, the standard model with a CMB temperature of about 2.7 K remains consistent with a wide range of observations Turner1993Giudice2000Crawford2022+2 MORE.
Conclusion
The temperature of the universe today is about 2.7 Kelvin, as measured by the CMB. This value and its evolution with redshift are key pillars of modern cosmology, supporting the Big Bang model and informing our understanding of the universe’s history from its hottest moments to the present cold, expanding cosmos Turner1993Giudice2000Crawford2022+2 MORE.
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