  We discuss phenomenology of quantum vacuum. Phenomenology of macroscopic
systems has three sources: thermodynamics, topology and symmetry. Momentum
space topology determines the universality classes of fermionic vacua. The
vacuum in its massless state 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. 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 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.
