Spectral functions in quantum gravity and gauge theories
Manuel Reichert (University of Sussex)

Over the past decades, the asymptotic safety scenario has matured into a viable contender for a consistent theory of quantum gravity. A pressing open question concerns the unitarity of the theory. I will present important steps towards tackling this issue and show the first direct computation of the graviton spectral function in asymptotically safe quantum gravity with a novel Lorentzian renormalisation group approach. The resulting graviton spectral function displays a massless one-graviton peak and a positive multi-graviton continuum with an asymptotically safe scaling for large spectral values, though being gauge-dependent. To understand the intricacies, I will contrast this to a perturbatively well-controlled computation of spectral functions in gauge theories with an infrared Banks-Zaks fixed point. I will indicate consequences for scattering amplitudes and unitarity.