A Cooling Universe

In order to understand the types of reactions taking place in the early universe, scientists need to have some estimates of the amount of energy present. Another way to express that concept is to say that scientists must know the approximate temperature of the universe, since temperature is a measure of the average kinetic energy present. Discussions of the evolution of the young universe are, therefore, often phrased in terms of the temperatures present at various stages of cosmic evolution. 

Scientists now believe that the temperature of the universe a few microseconds after the big bang was probably about 1014 K. The symbol K stands for degrees kelvin, a measure of absolute temperature. At very high temperatures, the kelvin, centigrade, and Fahrenheit temperatures are roughly the same. By comparison, the temperature at the interior of our Sun is about 107 K. 

The original fireball created by the big bang expanded and cooled very rapidly. Within a few microseconds, the temperature had dropped to less than 1013 K, an environment in which heavy quarks 

Antiparticles are formed by reactions similar to those hy which protons, muons, and electrons are formed. The only difference is that such reactions involve the use of antineutrinos rather than neutrinos. For example, an antiproton is formed in the following reaction  

The bar over the neutrino symbol in this equation represents an antineutrino in its reaction with a gamma ray to produce an antiproton. Similarly, an antielectron is formed in the reaction between a gamma ray and an electron antineutrino  

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