The thermodynamics of the universe is dictated by which form of energy dominates it - relativistic particles which are referred to as radiation, or non-relativistic particles which are referred to as matter. The former are particles whose rest mass is zero or negligible compared to their energy, and therefore move at the speed of light or very close to it; the latter are particles whose kinetic energy is much lower than their rest mass and therefore move much slower than the speed of light. The intermediate case is not treated well analytically.
Read more about Thermodynamics Of The Universe: Energy Density in The Expanding Universe, Rate of Expansion of The Universe, Dark Energy and Cosmic Inflation, See Also
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“Nothing in the universe is contingent, but all things are conditioned to exist and operate in a particular manner by the necessity of the divine nature.”
—Baruch (Benedict)