Scale symmetry - a link from quantum gravity to cosmology
Prof. Christof Wetterich (Universität Heidelberg)

We explore dilaton quantum gravity as a quantum field theory for the metric and a scalar field, with ultraviolet and infrared fixed points. The ultraviolet fixed point makes gravity asymptotically safe and non-perturbatively renormalizable. The approximate scale invariance close to the fixed point is reflected in cosmology in the almost scale invariant spectrum of primordial fluctuations. The approach to the infrared fixed point in the asymptotic future is characterized by the dynamics of an almost massless cosmon - the pseudo Goldstone boson of spontaneously broken scale symmetry. It is responsible for dynamical dark energy.

Both inflation and the present dynamical dark energy arise from the same scalar field. The history of the Universe describes a crossover from a „past fixed point“ where all particles are massless, to a „future fixed point“ where exact scale invariance is spontaneously broken, generating the particle masses. The cosmological solution can be extrapolated to the infinite past in physical time - the Universe has no beginning and no physical singularity. A simple model is compatible with all present cosmological observations. It could be tested by the observation of huge lumps in the cosmic neutrino background , the detection of early dark energy , or rather large primordial graviton fluctuations generated during inflation.