Quantum effects are expected to modify the cosmological dynamics of the early universe while maintaining some (potentially discrete) notion of space-time structure. In one approach, loop quantum cosmology, current models are shown here to either be incompatible with a consistent space-time structure, or to have physical singularities. The latter happens in spite of a nonzero scale factor in the isotropic background dynamics. A new effective Friedmann equation shows that a bounce is obtained at sub-Planckian densities, preceded by a physical singularity at infinite scale factor that resembles a time-reversed big rip, or a big fix. The entire phase is accompanied by rapid changes of the Hubble radius. In addition, a new version of perturbative inhomogeneity in loop quantum cosmology is introduced that maintains a consistent space-time structure and has a nonsingular background dynamics.

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• Canonical quantum gravity
• Loop quantum gravity
• Quantum cosmology