Quantum-chromodynamic vacuum as a glueball condensate

It is argued semiquantitatively that the magnetic instability of the perturbative QCD vacuum leads to the formation of a Bose-Einstein condensate that locally can be described as a $J^{\mathrm{PC}}=0^{++}$ gluon color-singlet bound state of "glueball." We also indicate how this result might be obtained by a more rigorous field-theoretical treatment. A general discussion of the global properties of the "glueball vacuum" is given, and as an illustration of our ideas we construct a phenomenological model in which the bag constant is related to the QCD scale $\Lambda$.