Microscopic origin of the entropy of de Sitter spacetime

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 outside the dS event horizon. We further show that wormhole contributions to the semiclassical Euclidean gravitational path integral lead to universal nonperturbative overlaps between these 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 dS spacetime. Our construction thus provides a state-counting derivation for Gibbons-Hawking entropy of dS spacetime.