  We consider the Hawking-Ellis (Segre-Plebanski) classification of
stress-energy tensors, both in the test-field limit, and in the presence of
back-reaction governed by the usual Einstein equations. For test fields it is
not too difficult to get a type~IV stress-energy via quantum vacuum
polarization effects. (For example, consider the Unruh quantum vacuum state for
a massless scalar field in the Schwarzschild background.) However, in the
presence of back-reaction driven by the ordinary Einstein equations the
situation is often much more constrained. For instance: (1) in any static
spacetime the stress-energy is always type I in the domain of outer
communication, and on any horizon that might be present; (2) in any stationary
axisymmetric spacetime the stress-energy is always type I on any horizon that
might be present; (3) on any Killing horizon that is extendable to a
bifurcation 2-surface the stress-energy is always type I; (4) in any stationary
axisymmetric spacetime the stress-energy is always type I on the axis of
symmetry; (5) some of the homogeneous Bianchi cosmologies are guaranteed to be
Hawking-Ellis type I (for example, all the Bianchi type I cosmologies, all the
FLRW cosmologies, and all the "single mode" Bianchi cosmologies). That is, in
very many physically interesting situations once one includes back-reaction the
more unusual stress-energy types are automatically excluded.
