4d Simplicial Quantum Gravity: Matter Fields and the Corresponding Effective Action

Four-dimensional simplicial quantum gravity is modified either by coupling it to U(1) gauge fields or by introducing a measure weighted by the orders of the triangles. Strong coupling expansion and Monte Carlo simulations are used. Although the two modifications of the standard pure-gravity model are apparently very distinct, they produce strikingly similar results, as far as the geometry of random manifolds is concerned. In particular, for an appropriate choice of couplings, the branched polymer phase is replaced by a crinkled phase, characterized by the susceptibility exponent $\gamma < 0$ and the fractal dimension $d_H > 2$. The quasi-equivalence between the two models is exploited to get further insight into the extended phase diagram of the theory.