A String Theoretic Derivation of Gibbons-Hawking Entropy

We describe an attempt of string theoretic derivation of the Gibbons-Hawking entropy. Despite not admitting a de Sitter vacuum, the string theory, by the power of open-close correspondence, captures the Gibbons-Hawking entropy as the entropy of Chan-Paton species on a de Sitter-like state obtained via $D$-branes. Moreover, this derivation sheds a new light at the origin of the area-form, since the equality takes place for a critical 't Hooft coupling for which the species entropy of open strings saturates the area-law unitarity bound.