Quantum vacua: momentum space topology of fermion zero modes
  Quantum vacua are characterized by the topological structure of their fermion zero modes. The vacua are distributed into universality classes protected by topology in momentum space. The vacua whose manifold of fermion zero modes has co-dimension 3 are of special interest because in the low-energy corner the fermionic excitations become the Weyl relativistic chiral fermions, while the dynamical bosonic collective modes of the fermionic vacuum interact with the chiral fermions as the effective gravity and gauge fields. The relativistic invariance, the chirality of fermions, the gauge and gravity fields, the relativistic spin, etc., are the emergent low-energy properties of the quantum vacuum with such fermion zero modes. The vacuum of the Standard Model and the vacuum of superfluid 3He-A belong to this universality class and thus they are described by similar effective theories. This allows us to use this quantum liquid for the theoretical and experimental simulations of many problems related to the quantum vacuum, such as the chiral anomaly and the cosmological constant problems. 
G.E. Volovik