  We discuss phenomenology of quantum vacuum. Phenomenology of macroscopic
systems has three sources: thermodynamics, topology and symmetry.
Thermodynamics of the self-sustained vacuum allows us to treat the problems
related to the vacuum energy: the cosmological constant problems. The natural
value of the energy density of the equilibrium the self-sustained vacuum is
zero. Cosmology is discussed as the process of relaxation of vacuum towards the
equilibrium state. The present value of the cosmological constant is very small
compared to the Planck scale, because the present Universe is very old and thus
is close to equilibrium. Momentum space topology determines the universality
classes of fermionic vacua. The Standard Model vacuum both in its massless and
massive states is topological medium. The vacuum in its massless state shares
the properties of superfluid 3He-A, which is topological superfluid. It belongs
to the Fermi-point universality class, which has topologically protected
fermionic quasiparticles. At low energy they behave as relativistic massless
Weyl fermions. Gauge fields and gravity emerge together with Weyl fermions at
low energy. This allows us to treat the hierarchy problem in Standard Model:
the masses of elementary particles are very small compared to the Planck scale
because the natural value of the quark and lepton masses is zero. The small
nonzero masses appear in the infrared region, where the quantum vacuum acquires
the properties of another topological superfluid, 3He-B, and 3+1 topological
insulators. The other topological media in dimensions 2+1 and 3+1 are also
discussed. In most cases, topology is supported by discrete symmetry of the
underlying microscopic system, which indicates the important role of discrete
symmetry in Standard Model.
