Temperature of universe
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Understanding the Temperature of the Universe
Cosmic Microwave Background Radiation Temperature
The temperature of the universe, as measured by the Cosmic Background Explorer satellite, is approximately 2.726 Kelvin. This precise measurement has led to significant insights into the fundamental characteristics of the universe, such as the early radiation-dominated epoch, the synthesis of light elements, and the matter-antimatter asymmetry . The current temperature of the cosmic microwave background (CMB) radiation is a remnant from the early universe, providing a snapshot of the universe's state shortly after the Big Bang.
Evolution of the Universe's Temperature
In the early stages of the universe, there were no stars or galaxies, only a hot plasma of free electrons and nuclei. The temperature of this early universe was governed by the Stefan-Boltzmann law and general relativistic cosmological theory. Over time, as the universe expanded, the energy from this hot plasma transitioned into matter and other forms of energy, resulting in the current background radiation temperature of 2.73K .
Matter Temperature During Recombination
During the cosmological recombination period, the temperature of atomic matter in the universe was closely tied to the CMB radiation until decoupling occurred at a redshift of approximately 100. After decoupling, the matter cooled more rapidly than the radiation, and without astrophysical feedback processes, the temperature of the intergalactic medium would have dropped to about 20 millikelvin today .
Reheat Temperature and Its Implications
The thermal history of the universe before nucleosynthesis is not well understood, but the maximum temperature during the radiation-dominated era, known as the reheat temperature, has significant cosmological implications. This temperature could have been as low as 0.7 MeV. A low reheat temperature affects the production and abundance of weakly interacting massive particles (WIMPs) and alters the constraints on various dark matter candidates, including supersymmetric dark matter, axions, and massive neutrinos .
Constraints on Reheating Temperature
Recent analyses of cosmic microwave background data from the WMAP five-year survey have provided new constraints on the reheating temperature of the universe. The data suggest an upper limit of the reheat temperature at approximately 2 MeV, which can be improved to 3.2 MeV when including external priors from other surveys. Future combined analyses from CMB and weak lensing surveys are expected to further refine these constraints .
Lower Bound on Reheating Temperature
Studies have also established a lower limit on the reheating temperature, suggesting it must be greater than 4 MeV. However, if the decaying heavy particles that reheat the universe have a direct decay mode to neutrinos, the reheat temperature could be as low as 1 MeV. These findings also impose stringent bounds on models with large extra dimensions, providing the strongest available constraints on such models .
Conclusion
The temperature of the universe, currently measured at around 2.726 Kelvin, offers profound insights into its early conditions and subsequent evolution. Understanding the reheat temperature and its constraints is crucial for developing accurate models of the universe's thermal history and the behavior of dark matter and other fundamental particles. As observational techniques and data analysis methods improve, our comprehension of these cosmic temperatures and their implications will continue to deepen.
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