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Abstract

At the Planck scale, matter, space, and time fluctuate collectively. This thesis explores the phenomenology of a suggested joint theory of quantum gravity and matter. The discovery of the Higgs boson has completed the Standard Model of particle physics, realizing a delicate balance of the measured masses and couplings for which the Higgs potential provides a strong hint for Planckian quantum scale symmetry. The latter could also tame gravitational and Abelian interactions and render both General Relativity and the Standard Model asymptotically safe. A pivotal weak-gravity mechanism could facilitate a gravitationally induced UV-completion of the Standard Model. Within this scenario, the asymptotic-safety paradigm potentially enhances the predictive power of the Standard Model. It could uniquely fix the Abelian gauge and various Yukawa couplings from first principles. We uncover mechanisms which could link the mass difference of top and bottom quark to their charge ratio, could dynamically favor small Dirac neutrino masses, and might allows for phenomenologically appealing transitions between different fixed points of the CKM-mixing matrix. In the absence of intermediate scales, those Planckian predictions are connected to the electroweak scale by Renormalization Group flows. This could permit testing quantum gravity at accessible energy scales. Thereupon, we generalize the paradigm of quantum scale symmetry and the associated enhanced predictivity to grand unification where it potentially restores the predictivity of the complicated chain of spontaneous symmetry breaking. Asymptotically safe quantum fluctuations could also resolve the singularity at the center of black holes. We obtain the shadow boundary for nonspinning and spinning regular black holes. In comparing to the shadow image obtained by the Event Horizon Telescope, we find that horizonless objects can not yet be excluded.