MG11 
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Dark Energy and Condensate Stars: A Quantum Alternative to Classical Black Holes

It is suggested that the dark energy apparently pervading our universe is vacuum energy due to a causal boundary effect at the cosmological horizon. A simple model with a purely vacuum energy de Sitter interior and Schwarzschild exterior, separated by a thin boundary layer is outlined. The boundary layer is a quantum transition region which replaces the event horizons of the classical solutions, through which the vacuum energy changes. This suggests that a quantum BEC phase transition in gravity in which the local vacuum energy changes could lead to a new kind of non-singular endpoint of collapse, a GBEC star. Like a black hole, a collapsed object of this kind would be cold and dark, but unlike a classical black hole, a GBEC star has no singularities, no event horizons, and a modest entropy.

Macroscopic Effects of the Quantum Trace Anomaly

The low energy effective action of gravity in any even dimension generally acquires non-local terms associated with the trace anomaly, generated by the quantum fluctuations of massless fields. The local auxiliary field description of this effective action in four dimensions requires two additional scalar fields, not contained in classical general relativity, which remain relevant at macroscopic distance scales. The auxiliary scalar fields depend upon boundary conditions for their complete specification, and therefore carry global information about the geometry and macroscopic quantum state of the gravitational field. The scalar potentials also provide coordinate invariant order parameters describing the conformal behavior and divergences of the stress tensor on event horizons. We compute the stress tensor due to the anomaly in terms of its auxiliary scalar potentials in a number of examples, with qualitatively correct global approximations to the renormalized expectation value of the quantum stress tensor.