Cosmological constant, false vacua, and axions

It is suggested that the true ground state of the world has exactly vanishing vacuum energy and that the cosmological constant that seems to have been observed is due to our region of the universe being stuck in a false vacuum, whose energy is split from the true vacuum by nonrenormalizable operators that are suppressed by powers of the Planck scale. It is shown that conventional invisible axion models typically have the features needed to realize this possibility. In invisible axion models the same field and the same potential can explain both the cosmological constant (or dark energy) and the dark matter. It is also shown that the idea can be realized in nonaxion models, an example of which is given having $\Lambda {=M}_W^7{/M}_{\mathrm{Pl}}^3,$ which agrees well with the observed value.