Entropies of the various components of the universe

Abstract

In this work, we study the entropies of photons, dust (baryonic matter), dark matter, and dark energy in the context of cosmology. When these components expand freely with the universe, we calculate the entropy and specific entropy of each component from the perspective of statistics. Under specific assumptions and conditions, the entropies of these components can satisfy the second law of thermodynamics independently. Our calculations show that the specific entropy of matter cannot be a constant during the expansion of the universe except for photons. When these components interact with the space-time background, there could exist the phenomenon of particle production (annihilation). We study the influence of the interaction on the entropies of these components, and obtain the conditions guaranteeing that the entropy of each component satisfies the second law of thermodynamics.