Russo, Andrea;
(2025)
Quantum matter backreacting on classical spacetime.
Doctoral thesis (Ph.D), UCL (University College London).
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Abstract
This work explores the weak gravitational regime and the cosmological implications of a recently proposed unification framework known as classical-quantum (CQ) theory. CQ dynamics attempts to coherently unify quantum physics with classical general relativity by stochastically coupling the two theories. After an overview of the background material, which includes three different formulations of the CQ framework, this work explores the Newtonian limit of classical general relativity coupled with quantum matter. Due to the backreaction generated by the coupling, we find that the gravitational field diffuses around Poisson's equation while quantum matter decoheres into mass eigenstates. We study the bounds in the coefficients regulating diffusion and decoherence, and compare and contrast with previous works on the interaction between a classical Newtonian potential and quantum matter. Secondly, we focus on deriving the Newtonian limit of classical-quantum Nordström’s theory of gravitation to show that a self-consistent scalar theory of gravity leads to the same results obtained from its general relativistic counterpart. This model reinforces the previous results and highlights the consistency of the Newtonian limit with a diffeomorphism-invariant CQ theory of gravity. Lastly, we delve deeper into the gravitational sector of CQ theories of gravity to explore the consequences of stochasticity on the rotational curves of galaxies. We find evidence that the presence of stochastic noise could act to explain the phenomenology usually attributed to dark matter. A deviation from the expected general relativistic behaviour appears at low accelerations and connects it with cosmological parameters. A dark energy-like effect is expected even if the starting theory does not have a bare cosmological constant. We propose an explanation and compare our results with tabletop experiments to understand how large-scale diffusion and local noise might relate. We expect these results to be relevant to future theoretical and experimental tests of the quantum nature of gravity.
| Type: | Thesis (Doctoral) |
|---|---|
| Qualification: | Ph.D |
| Title: | Quantum matter backreacting on classical spacetime |
| 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/10207527 |
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