Panella, Emanuele;
(2025)
New insights on classical-quantum gravitational backreaction.
Doctoral thesis (Ph.D), UCL (University College London).
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Abstract
This thesis treats the backreaction of quantum degrees of freedom on classical systems, with a focus on gravitational physics. We consider both fundamental and effective classical subsystems. Assuming fundamental classicality of a subsystem leads to classical-quantum (CQ) dynam- ics, a framework that requires both decoherence and classical diffusion for consistency. We first apply the CQ formalism to study the evolution of two coupled oscillators – one classical, one quantum – with classical friction. Using path integrals, we show that the system relaxes to a unique non-equilibrium steady state, which becomes thermal in the high-diffusion limit. We derive the phase-space representation of hybrid dynamics and show that for harmonic potentials it maps exactly to a Fokker-Planck equation. We then examine the proposal that gravity could remain fundamentally classical. Consis- tency of the theory at all scales implies that the combined matter and gravitational evolution has be of CQ form. We first analyse a stochastic Klein-Gordon field (the classical sector of a CQ Yukawa model) as a toy model for linearised CQ gravity. We address the issue of unbounded diffusion and discuss the implication of the infinite energy production in the model. Next, we study a CQ model of cosmology. In a stochastic FLRW Universe, we show that diffusion during inflation, if strong enough, could mimic dark matter (CDM) effects. The second part discusses “braneworld holography” as a method to compute semiclassi- cal backreaction of conformal quantum fields on an effectively classical metric. We apply this framework and find an exact quantum Kerr–de Sitter solution to the (2+1)-dimensional semi- classical Einstein’s equations with higher-curvature corrections, and derive its thermodynamic properties. We compare the exact solution with the non-holographic, but limited, perturbative approach to the backreaction problem. We conclude with prospects for future work.
| Type: | Thesis (Doctoral) |
|---|---|
| Qualification: | Ph.D |
| Title: | New insights on classical-quantum gravitational backreaction |
| Open access status: | An open access version is available from UCL Discovery |
| Language: | English |
| Additional information: | Copyright © The Author 2025. Original content in this thesis is licensed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) Licence (https://creativecommons.org/licenses/by-nc/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request. |
| UCL classification: | UCL UCL > Provost and Vice Provost Offices > UCL BEAMS UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Physics and Astronomy |
| URI: | https://discovery.ucl.ac.uk/id/eprint/10212594 |
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