Energy in Radiation in the Early UniverseElectromagnetic radiation and the flux of neutrinos were the dominant form of energy in the early universe, becoming more dominant as one models earlier times in the big bang. It is reminiscent of the Biblical phrase "Let there be light". But of course this is not visible light but far beyond gamma rays since the average photon energy increases as one models earlier times and photon energies become high enough to accomplish pair production of all known particles. For example at a temperature of 10^{11} K, modeled at a time of about 20 milliseconds into Weinberg's model of the big bang, the thermal energy kT = 8.6 MeV. At this temperature, the peak energy given by the Wien displacement law is 42.8 MeV. The radiation energy was sufficient not only for electronpositron pair production, but also to maintain essentially equal populations of protons and neutrons. So in that era, radiation was truly dominant. The radiation energy density in astronomy texts is usually put in the form For consistency with other particle quantities that are expressed in terms of the number of degrees of freedom, this is sometimes written in the form To assess the role of radiation in the expansion of the early universe, one must take a step beyond the simple Newtonian expansion model and include the radiation pressure. When the radiation pressure is included, the effective density represented by the radiation as a function of the scale factor R is The dependence on the fourth power of R distinguishes the radiation energy density from the mass density, which depends upon the third power of R,

Index Reference Carroll & Ostlie Ch 29  

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Neutrinos in the Early UniverseNeutrinos joined Electromagnetic radiation as the dominant form of energy in the early universe, becoming more dominant as one models earlier times in the big bang. To assess the energy density in neutrinos, it would be helpful to place it in a form similar to the energy density in radiation, Applying all the statistical factors gives a neutrino energy density expression Another difference for the neutrinos is the fact that their effective temperature at the present time is different from the 2.725K of the cosmic microwave background. Analysis of the neutrino temperature leads to the prediction that the neutrino temperature is

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Relativistic Particle Density Ω_{Rel}Neutrinos joined Electromagnetic radiation as the dominant form of energy in the early universe, becoming more dominant as one models earlier times in the big bang. In characterizing the effective density of the present universe, the mass of the neutrinos is generally neglected, and they are combined with photons as the effective density of the relativistic particles. The combination of the radiation energy density

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