Contrasting Quantum Cosmologies

Abstract

We compare the recent loop quantum cosmology approach of Bojowald and co-workers with earlier quantum cosmological schemes. Because the weak-energy condition can now be violated at short distances, and not necessarily with a high energy density, a number of possible instabilities are suggested: flat space unstable to expansion or baby universe production. Or else a Machian type principle is required to prevent such behaviour. Allowing a bounce to prevent an approaching singularity seems incompatible with other standard notions concerning the arrow of time and unitarity. Preventing rapid oscillations in the wavefunction appears in conflict with more general scalar-tensor gravity. Other approaches such as ``creation from nothing'' or from some quiescent state, static or time machine, are also assessed on grounds of naturalness and fine tuning.