| Our quantum vacuum most probably belongs to strongly correlated and strongly interacting systems, similar to that which we have in complicated condensed matter systems, such as unconventional superconductors. As a rule these systems cannot be treated perturbatively. However, there are universal robust features which do not depend on details of the system, but are determined by universality classes of fermionic quantum vacua in terms of the topology in momentum space. Here we illustrate this approach on some examples of quantum phase transitions, in which the momentum space topology of the system changes. Such topological quantum phase transitions may occur in the quantum vacuum of our Universe or in the ground state of the fermionic condensed matter systems. Systems of different dimensionalities are considered. |
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