The weak-gravity bound and the need for spin in asymptotically safe\n matter-gravity models

Abstract

We discover a weak-gravity bound in scalar-gravity systems in the\nasymptotic-safety paradigm. The weak-gravity bound arises in these systems\nunder the approximations we make, when gravitational fluctuations exceed a\ncritical strength. Beyond this critical strength, gravitational fluctuations\ncan generate complex fixed-point values in higher-order scalar interactions.\nAsymptotic safety can thus only be realized at sufficiently weak gravitational\ninteractions. We find that within truncations of the matter-gravity dynamics,\nthe fixed point lies beyond the critical strength, unless spinning matter,\ni.e., fermions and vectors, is also included in the model.\n