An Exact Formula for Cosmic Entropy in Rh=Ct Cosmological Model

The question of the entropy of the universe is crucial and remains unanswered in cosmology. Assuming a flat universe, we derive, which we then demonstrate, an exact heuristic formula for the entropy of the apparent universe, i.e. at the Hubble radius. This approach forms part of a quantum thermodynamic cosmology framework of the Rh = ct type, which is in the field of classical mechanics, and could help to quantify the Planck era of Big Bang theory. It assumes that the universe would exist before Planck time at Planck temperature. Furthermore, it could shed new light on the standard cosmological model with regard to entropy.