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The question of the entropy of the universe is crucial and remains unanswered in cosmology. Assuming a flat universe, we derive 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, 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.