Microscopic origin of the entropy of de Sitter spacetime

A bstract We construct an infinite family of semiclassical de Sitter (dS) microstates, realized as backreacted geometries of dS spacetime with a constant tension thin-shell brane located behind the dS event horizon. We show that wormhole contributions to the semiclassical dS Euclidean gravitational path integral lead to universal nonperturbative overlaps among the semiclassical dS microstates. By evaluating the nonperturbative overlaps, we count the dimension of the Hilbert space spanned by the semiclassical dS microstates and find that it precisely equals the exponential of the Gibbons-Hawking entropy of the dS spacetime. Our construction thus provides a gravitational state-counting derivation for the Gibbons-Hawking entropy of dS spacetime.