Physical Interpretation of Gibbons–Hawking Entropy

Ascertain which side of the cosmological horizon is accounted for by the Gibbons–Hawking entropy in de Sitter spacetime and clarify its thermodynamic interpretation in observer-dependent settings.

Background

While black hole entropy naturally counts exterior ignorance of the interior, the cosmological horizon is observer-dependent, complicating the interpretation of Gibbons–Hawking entropy. The thesis notes that outside static patch holography it is unclear whether the entropy refers to the interior or exterior side of the horizon.

References

In the case of the cosmological horizon, it is not clear which side of the horizon is described by the Gibbons-Hawking entropy.

— Information-theoretic constraints in quantum gravity and cosmology  (2510.15787 - Franken, 17 Oct 2025) in Introduction, Section: Holography for cosmology

The microscopic interpretation of the de Sitter entropy remains unclear. In particular it is not known what degrees of freedom are being counted or whether it has a completely different interpretation. Understanding the origin of the de Sitter entropy therefore remains a central open problem. This motivates the question: What are the macroscopic constraints on a microscopic realization of the de Sitter entropy?

— Notes on de Sitter space  (2609.19454 - Mühlmann, 16 Sep 2026) in Section 6, subsection 'The cosmological horizon'