  The condensed matter examples, in which the effective gravity appears in the
low-energy corner as one of the collective modes of quantum vacuum, provide a
possible answer to the question, why the vacuum energy is so small. This answer
comes from the fundamental ``trans-Planckian'' physics of quantum liquids. In
the effective theory of the low energy degrees of freedom the vacuum energy
density is proportional to the fourth power of the corresponding ``Planck''
energy appropriate for this effective theory. However, from the exact ``Theory
of Everything'' of the quantum liquid it follows that its vacuum energy density
is exactly zero without fine tuning, if: there are no external forces acting on
the liquid; there are no quasiparticles which serve as matter; no space-time
curvature; and no boundaries which give rise to the Casimir effect. Each of
these four factors perturbs the vacuum state and induces the nonzero value of
the vacuum energy density of order of energy density of the perturbation. This
is the reason, why one must expect that in each epoch the vacuum energy density
is of order of matter density of the Universe, or/and of its curvature, or/and
of the energy density of smooth component -- the quintessence.
