Why there is something rather than nothing: cosmological constant from summing over everything in lorentzian quantum gravity.
The density matrix of the Universe for the microcanonical ensemble in quantum cosmology describes an equipartition in the physical phase space of the theory (sum over everything), but in terms of the observable spacetime geometry this ensemble is peaked about the set of recently obtained cosmological instantons limited to a bounded range of the cosmological constant. This suggests the mechanism of constraining the landscape of string vacua and a possible solution to the dark energy problem in the form of the quasiequilibrium decay of the microcanonical state of the Universe.
Barvinsky
A O
AO
Theory Department, Lebedev Physics Institute, Leninsky Prospect 53, 119991 Moscow, Russia.