What is the entropy of the universe?

TL;DR

This paper questions the conventional understanding of the universe's entropy dominated by black holes, highlighting potential discrepancies between area entropy and precursor matter entropy under unitarity assumptions.

Contribution

It challenges the standard black hole entropy calculations by examining the implications of unitarity on the entropy of Hawking radiation and black hole precursors.

Findings

Black hole entropy is dominated by area in Planck units.

Precursor matter entropy is much less than area entropy.

Unitarity may imply different entropy estimates for Hawking radiation.

Abstract

Standard calculations suggest that the entropy of our universe is dominated by black holes, whose entropy is of order their area in Planck units, although they comprise only a tiny fraction of its total energy. Statistical entropy is the logarithm of the number of microstates consistent with the observed macroscopic properties of a system, hence a measure of uncertainty about its precise state. Therefore, assuming unitarity in black hole evaporation, the standard results suggest that the largest uncertainty in the future quantum state of the universe is due to the Hawking radiation from evaporating black holes. However, the entropy of the matter precursors to astrophysical black holes is enormously less than that given by area entropy. If unitarity relates the future radiation states to the black hole precursor states, then the standard results are highly misleading, at least for an observer that can differentiate the individual states of the Hawking radiation.