Criticality and Scaling in 4 D Quantum Gravity ⋆

We present a simple argument which determines the critical value of the anomaly coefficient in four dimensional conformal factor quantum gravity, at which a phase transition between a smooth and elongated phase should occur. The argument is based on the contribution of singular configurations (“spikes”) which dominate the partition function in the infrared. The critical value is the analog of c = 1 in the theory of random surfaces, and the phase transition is similar to the BerezenskiiKosterlitz-Thouless transition. The critical value we obtain is in agreement with the previous canonical analysis of physical states of the conformal factor and may explain why a smooth phase of quantum gravity has not yet been observed in simplicial simulations. We also rederive the scaling relations in the smooth phase in light of this determination of the critical coupling. Research supported in part by the EEC contract CHRX-CT93-0340. Laboratoire Propre du CNRS UPR A.0014. November 1996