Review article published as Chapter 8, pp. 283-352 in "Regular Black Holes: Towards a New Paradigm of Gravitational Collapse," C. Bambi editor (Springer), 69 pages, 8 figures, Typos fixed, References added and updated
Gravitational vacuum condensate stars, proposed as the endpoint of gravitational collapse consistent with quantum theory, are reviewed. Gravastars are cold, low entropy, maximally compact objects characterized by a surface boundary layer and physical surface tension instead of an event horizon. Within this thin boundary layer the effective vacuum energy changes rapidly, such that the interior of a non-rotating gravastar is a non-singular static patch of de Sitter space with eq. of state p=-rho. Remarkably, essentially this same result is obtained by extrapolating Schwarzschild's 1916 constant density interior solution to its compact limit, showing how the black hole singularity theorems and the Buchdahl compactness bound are evaded. The surface stress tensor on the horizon is determined by a modification of the Lanczos-Israel junction conditions for null hypersurfaces, which is applied to rotating gravastar solutions as well. The fundamental basis for the quantum phase transition at the horizon is the stress tensor of the conformal anomaly, depending upon a new light scalar field in the low energy effective action for gravity. This scalar conformalon field allows the effective value of the vacuum energy described as a condensate of an exact 4-form abelian gauge field to change at the horizon. The resulting effective theory thus replaces the fixed constant Lambda of classical general relativity, and its apparently unnaturally large sensitivity to UV physics, with a dynamical condensate whose ground state value in empty flat space is zero identically. This provides both a solution to the cosmological constant problem and an effective Lagrangian dynamical framework for the boundary layer and interior of gravitational condensate stars. The status of present observational constraints and prospects for detection of gravastars through their gravitational wave and echo signatures are discussed.
