Quantum Universe

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The universe arrayed around us provides a marvellous natural physics laboratory. As we look outwards, we see amazing objects like neutron stars and black holes, undergoing extreme physical processes in which the fundamental laws of physics play out in front of our eyes. We see galaxies and stars in the process of forming, developing and aging. We detect the light, radio waves, X--rays, neutrinos and even gravitational waves emitted from each of these, and many other dramatic sources. Seen from our viewpoint outside them, black holes are extremely simple yet paradoxical objects. Reconciling them with quantum physics could lead to revolutionary advances in our understanding of space and time. Observed from our vantage point within it, the universe is likewise remarkably minimal in form, yet equally puzzling. Physics' greatest challenge is to understand this simplicity. To do so we must combine the laws of quantum physics, which are extremely well verified in laboratory experiments, with Einstein's theory of gravity, which accurately describes the structure and evolution of the cosmos. This proposal will support an exceptional cohort of students working to develop Feynman's famous path integral formulation of quantum mechanics with the inclusion of gravity. The proposer pioneered the use of  Picard--Lefschetz theory, an advanced mathematical technique, in combination with ideas such as weak measurement, coming from quantum foundations, and geometrodynamics - the description of quantum spacetime.  These approaches will be combined in a fully quantum description, to understand how classical spacetime emerges from constructive interference and how quantum fluctuations perturb its fabric. One PhD student will work on each of the following objectives. O1: proving the existence of Feynman's path integral for real-time quantum systems, both with and without gravity, and applying this to various systems, O2: modelling the interference patterns produced by coherent sources such as pulsars and fast radio bursts and attempting to reconstruct the source and the lens, O3: studying the quantum behaviour of spacetime including the big bang singularity, the emergence of time and the quantum evaporation of black holes, and O4: developing our recent explanation - the simplest yet proposed - for the cosmological dark matter. The latter rests on imposing a profound symmetry (CPT symmetry) on the vacuum state of the neutrino sector of the standard model of particle physics. Comparison with leading edge observations and predictions for new experiments will be a hallmark of our work.  Our overarching goal is to obtain a consistent quantum understanding of the universe, across all scales, consistent with the known laws of physics as established through experiment and observation. The powerful mathematical tools and techniques we develop are likely to have wide application, for example assisting with the design and data analysis tasks of revolutionary new radio telescopes.