Conventional equations of state suggest that in complete gravitational collapse a singular state of matter with infinite density could be reached finally, to what is popularly called a "black hole,” the characteristic feature of which is its apparent horizon, where light rays are first trapped. The loss of information to the outside world this implies gives rise to serious difficulties with well-established principles of quantum mechanics and statistical physics.
The formation of a gravitational vacuum condensate star with a p=−ρ interior solves these problems and remarkably, actually follows from Schwarzschild's second paper over a century ago. The advent of Gravitational Wave (GW) Astronomy will soon allow for observational tests of this hypothesis, in the discrete surface modes of oscillation and GW “echoes,” which should be detectable by advanced LIGO and related instruments. The imaging of Sgr A* by the Event Horizon Telescope will furnish additional tests of the gravastar hypothesis. The same Standard Model effects responsible for the formation of a gravastar surface lead also to the prediction of addition Scalar GWs from Neutron Star mergers.
Thus the new era of GW and multi-messenger astronomy holds the promise of a breakthrough in understanding of the physics of compact objects, and how quantum effects may be compatible with Einstein's General Relativity.