New Horizons in Gravity: Dark Energy and Condensate Stars

Black holes are an apparently unavoidable prediction of classical General Relativity, at least if matter obeys the strong energy condition ρ + 3p ≥ 0. However quantum vacuum fluctuations generally violate this condition, as does the eq. of state of cosmological dark energy ρ = −p > 0. When quantum effects are considered, black holes lead to a number of thermodynamic paradoxes associated with the Hawking temperature and assumption of black hole entropy, which are briefly reviewed. It is argued that the largest quantum effects arise from the conformal scalar degrees of freedom generated by the trace anomaly of the stress-energy tensor in curved space. At event horizons these can have macroscopically large backreaction effects on the geometry, potentially removing the classical event horizon of black hole and cosmological spacetimes, replacing them with a quantum phase boundary layer, where the effective value of the gravitational vacuum energy density can change. In the effective theory including the quantum effects of the anomaly, the cosmological term ? becomes a dynamical condensate, whose value depends upon boundary conditions at the horizon. By taking a positive value in the interior of a fully collapsed star, the effective cosmological term removes any singularity, replacing it with a smooth dark energy de Sitter interior. The resulting gravitational vacuum condensate star (or gravastar) configuration resolves all black hole paradoxes, and provides a testable alternative to black holes as the final quantum mechanical end state of complete gravitational collapse. The observed Λ_eff dark energy of our universe likewise may be a macroscopic finite size effect whose value depends not on Planck scale or other microphysics but on the cosmological Hubble horizon scale itself.