Elementary Particle PopulationsWhen the temperature is well above a threshold temperature defined by then particles may be treated statistically. Here k is the Boltzmann constant, and this definition of threshold temperature involves setting the average thermal energy equal to the rest mass energy. When the temperature is much higher than the threshold temperature, then one can presume that particleantiparticle pairs can be freely created from the thermal energy, and will therefore exist in thermal equilibrium with their surroundings. The contribution of a given type of particle will then be proportional to the "effective number of species" of that particle.
The effective number of species is the product of three factors:
The effective number of species is important for the calculation of the energy when a collection of particle is at a high enough temperature so that it can be considered to act like radiation. Under those condition, such as in modeling the early processes in the big bang, the Stefan Boltzmann law can be used to calculate the energy associated with the particles.

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