An approach to the cosmological constant problem(s)

We argue that in the context of string theory a large number N of connected degenerate supersymmetric vacua will lead to a ground state for the universe with a small, non-zero cosmological constant. For concreteness, we imagine a history where quantum fluctuations in any one vacuum give an energy density ∼H2mpl2 but the universe quickly cascades to a state of energy density ∼H2mpl2/N at the beginning of inflation. A similar process can occur at the electroweak and other phase transitions. The wavefunction of the universe becomes a superposition of many string vacua.