Understanding Quantum Gravity: from Scattering to Cosmology
Lead Research Organisation:
University of Sussex
Department Name: Sch of Mathematical & Physical Sciences
Abstract
Albert Einstein developed the theory of General Relativity more than 100 years ago. The predictions of the theory are still being verified today. Recently, the Event Horizon Telescope obtained the first picture of a black hole and the Laser Interferometer Gravitational-Wave Observatory detected gravitational waves from black-hole and neutron-star mergers. These experiments confirmed once more the tremendous success of General Relativity. Nonetheless, General Relativity cannot describe physics at tiny length scales, the quantum regime. Examples are the centre of a black hole or the beginning of the Universe. The central question of this research proposal is how to unify General Relativity with quantum physics.
To this day, even the question of whether the metric field can be used as a fundamental force carrier in quantum field theory remains unanswered. Using only the basic assumptions of diffeomorphism invariance and the metric field, the asymptotic safety community has put forward a tremendous amount of evidence that the theory is renormalisable with non-perturbative methods. The key result is the existence of an interacting ultraviolet fixed point of the renormalisation group flow. This ultraviolet fixed point provides a candidate theory of quantum gravity that needs to be confronted with self-consistency and observable consequences. I will spearhead this testing of the theory in three major physics realms: graviton-mediated scattering, the relation to the Standard Model of Particle Physics, and cosmological inflation.
This work uses modern renormalisation group methods, which allow to capture physics on all length scales, from General Relativity over effective field theory to deep into the quantum gravity regime. Using these methods on gravitational correlation functions, for which I am a world-leading expert, I will integrate out quantum fluctuations and gain access to the quantum effective action. Building upon my recent breakthrough in Lorentzian renormalisation group computations, I will be able to compute non-perturbative graviton-mediated scattering. Scattering amplitudes are key observables and give insights into the unitarity of the time evolution testing the self-consistency of the theory.
Using the same methods, I will constrain the matter content and interactions that are compatible with quantum gravity. Thereby, I will explore if the Standard Model of Particle Physics with gravity can be a fundamental theory or if extensions are necessary. This will also allow me to constrain physics beyond the Standard Model from the quantum gravity perspective. Lastly, I will extend the correlation functions to cosmological backgrounds and make contact with the Standard Model of Cosmology. I will test under which circumstances quantum gravity gives rise to inflation and I will compute the effects on the primordial tensor power spectrum. This work will be a milestone in the question of whether the Universe can be described fully in terms of a quantum field theory with the metric as a fundamental degree of freedom.
To this day, even the question of whether the metric field can be used as a fundamental force carrier in quantum field theory remains unanswered. Using only the basic assumptions of diffeomorphism invariance and the metric field, the asymptotic safety community has put forward a tremendous amount of evidence that the theory is renormalisable with non-perturbative methods. The key result is the existence of an interacting ultraviolet fixed point of the renormalisation group flow. This ultraviolet fixed point provides a candidate theory of quantum gravity that needs to be confronted with self-consistency and observable consequences. I will spearhead this testing of the theory in three major physics realms: graviton-mediated scattering, the relation to the Standard Model of Particle Physics, and cosmological inflation.
This work uses modern renormalisation group methods, which allow to capture physics on all length scales, from General Relativity over effective field theory to deep into the quantum gravity regime. Using these methods on gravitational correlation functions, for which I am a world-leading expert, I will integrate out quantum fluctuations and gain access to the quantum effective action. Building upon my recent breakthrough in Lorentzian renormalisation group computations, I will be able to compute non-perturbative graviton-mediated scattering. Scattering amplitudes are key observables and give insights into the unitarity of the time evolution testing the self-consistency of the theory.
Using the same methods, I will constrain the matter content and interactions that are compatible with quantum gravity. Thereby, I will explore if the Standard Model of Particle Physics with gravity can be a fundamental theory or if extensions are necessary. This will also allow me to constrain physics beyond the Standard Model from the quantum gravity perspective. Lastly, I will extend the correlation functions to cosmological backgrounds and make contact with the Standard Model of Cosmology. I will test under which circumstances quantum gravity gives rise to inflation and I will compute the effects on the primordial tensor power spectrum. This work will be a milestone in the question of whether the Universe can be described fully in terms of a quantum field theory with the metric as a fundamental degree of freedom.
