  A spacetime-independent variable is introduced which characterizes a
Lorentz-invariant self-sustained quantum vacuum. For a perfect
(Lorentz-invariant) quantum vacuum, the self-tuning of this variable nullifies
the effective energy density which enters the low-energy gravitational field
equations. The observed small but nonzero value of the cosmological constant
may then be explained as corresponding to the effective energy density of an
imperfect quantum vacuum (perturbed by, e.g., the presence of thermal matter).
