Temperature of the universe
Pro Research Analysisby 
Searched over 200M research papers
Temperature of the Universe: An Overview
Cosmic Microwave Background Radiation (CMB) Temperature
The temperature of the universe is primarily measured through the Cosmic Microwave Background (CMB) radiation. The most accurate measurement of the CMB temperature has been determined to be 2.726 ± 0.01 Kelvin by the Cosmic Background Explorer satellite . This temperature is a remnant from the early universe, specifically from the time of recombination when electrons and protons first combined to form neutral hydrogen atoms, allowing photons to travel freely.
Early Universe and Temperature Decline
In the early stages of the universe, it was a hot, dense plasma composed of free electrons and nuclei. The temperature of this early universe was governed by the Stefan-Boltzmann law and general relativistic cosmological theory . As the universe expanded, it cooled down, leading to the current observed temperature of the CMB at 2.73K .
Matter Temperature During Recombination
During the epoch of recombination, the temperature of atomic matter in the universe was closely tied to the CMB temperature until decoupling occurred at a redshift of approximately z ≈ 100. After decoupling, the matter temperature began to cool faster than the radiation, following a different cooling rate . Without astrophysical feedback processes, the matter temperature would have fallen to about 20 millikelvin today .
Reheating Temperature and Cosmological Implications
The thermal history of the universe before nucleosynthesis is not well known, but the maximum temperature during the radiation-dominated era, known as the reheat temperature, could have been as low as 0.7 MeV . This low reheat temperature has significant implications for cosmology, including the production of weakly interacting massive particles (WIMPs) and the constraints on dark matter models .
Constraints from WMAP-5 Data
Recent data from the WMAP five-year survey has provided new constraints on the reheating temperature of the universe. The analysis suggests an upper limit of the reheat temperature to be greater than 2 MeV, which can be improved to 3.2 MeV when including external priors from other surveys . These constraints are crucial for understanding the thermal history and the formation of the universe.
Supersymmetric Dark Matter and Reheating Temperature
The reheating temperature also plays a critical role in the context of supersymmetric dark matter. Studies indicate that for reheating temperatures below 30 GeV, the parameter space compatible with neutralino dark matter is significantly enhanced . However, if the reheating temperature is below 1 GeV, neutralinos cannot be efficiently produced, making it difficult to account for the present amount of dark matter .
Conclusion
The temperature of the universe, as measured by the CMB, provides critical insights into the early stages of the universe and its subsequent evolution. The decline in temperature from a hot plasma state to the current 2.73K CMB temperature is a testament to the universe's expansion and cooling. Understanding the reheating temperature and its implications helps refine our models of dark matter and the overall thermal history of the cosmos.
Sources and full results
Most relevant research papers on this topic