'Empty Black Holes, Firewalls, and the Origin of Bekenstein-Hawking Entropy'
http://arxiv.org/abs/1212.4176
"But why an incompressible fluid? The reason comes from an attempt to solve the (old) cosmological constant problem, which is arguably the most puzzling aspect of coupling gravity to relativistic quantum mechanics [13]. Given that the natural expectation value for the vacuum of the standard model of particle physics is ∼ 60 orders of magnitude heavier than the gravitational measurements of vacuum density, it is reasonable to entertain an alternative theory of gravity where the standard model vacuum decouples from gravity. Such a theory could be realized by coupling gravity to the traceless part of the quantum mechanical energy-momentum tensor. However, the consistency/covariance of gravitational field equations then requires introducing an auxiliary fluid, the so-called gravitational aether [14]. The simplest model for gravitational aether is an incompressible fluid (with vanishing energy density, but non-vanishing pressure), which is currently consistent with all cosmological, astrophysical, and precision tests of gravity [15, 16]:
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32πGN Gμν = Tμν − Tα gμν + Tμν ,
Tμν = p (uμ uν + gμν ), T μν;ν = 0,
where GN is Newtons constant, Tμν is the matter energy momentum tensor and Tμν is the incompressible gravitational aether fluid. In vacuum, the theory reduces to GR coupled to an incompressible fluid."
