Nonzero cosmological constant and the many vacua world

The idea of the quantum state of the Universe described by some density matrix, i.e mixture of at least two vacua, the trivial symmetric and the nontrivial one with spontaneously broken symmetry is discussed. Nonzero cosmological constant necessarily arises for such a state and has the observable value if one takes the axion mass for the vacuum expectation value. The Higgs model, Nambu's model and discrete symmetry breaking are considered. Human observers can observe only the world on the nonsymmetric vacuum, the world on the other vacuum is some dark matter. Gravity is due to action of two worlds. Tachyons nonobservable for visible matter can be present in the dark matter, leading to some effects of nonlocality in the space of the Universe