Quantum field theories of the dark universe

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The successes of the Standard Models of particle physics and cosmology have been unprecedented. Together, they are able to explain our observations from the dynamics and interactions of subatomic particles on the smallest scales to the evolution of the Universe on its largest scales. However, important questions remain unanswered.Known particle physics describes only the visible 5% of the Universe. To explain how galaxies formed, and how the stars and gas that they are made of move, we need dark matter. To explain why distant galaxies are accelerating away from us, we need dark energy. The nature of this "dark universe" remains a mystery, and unravelling its secrets is one of the most important and challenging problems in fundamental physics.In addition, the Standard Models cannot explain how matter came to dominate over antimatter to leave behind the 5% of visible matter. Worse still, measurements of the masses of the heaviest known elementary particles - the Higgs boson and top quark - indicate that our universe may reside in an unstable state that could decay to a catastrophically different one due to the predictions of quantum theory.To resolve these problems and to explain the nature of the dark universe, we must modify the Standard Model of particle physics or Einstein's theory of gravity, or both.I aim to do precisely this by introducing new particles described by "scalar fields", which give rise to new forces of nature. Once coupled directly to gravity or, equivalently, to the Higgs boson, these "scalar-tensor theories" become sensitive to the density of their local environment. This allows them to evade tests of gravity in our Solar System but still produce new forces elsewhere in the universe, providing a rich array of behaviours and the potential to describe the dark universe. Moreover, new scalar fields can resolve the weaknesses in our model of particle physics, helping to stabilise the state of our universe or change the way that the hot plasma of the early universe evolved.I will develop new theoretical tools to confront these models with the full rigour of the mathematical framework that underpins fundamental physics known as quantum field theory. This will allow me to address key theoretical uncertainties that are preventing us from making accurate predictions for experiment and observation. My research will determine definitively whether models of the dark universe based on extra scalar particles can explain the observed content and evolution of the Universe, while standing up to experiment as consistent extensions of known particle physics.I will establish an internationally leading research group at the University of Nottingham, based within its Particle Cosmology Group, which is home to extensive and complementary expertise in areas of astrophysics and cosmology that will benefit this programme. Additional collaborators will include members of the University's Astronomy and Quantum Gravity Groups, and renowned researchers in theory and experiment from other leading research institutions in the UK and overseas, including the Institute for Particle Physics Phenomenology at Durham University and CERN. I will exploit existing and future data from particle-physics experiments, such as those at CERN's Large Hadron Collider; from ground-based and satellite observatories, such as the Dark Energy Survey and the LIGO gravitational-wave observatory; and from experiments using quantum measurement techniques to look for new forces of nature.My programme will pioneer a novel interdisciplinary approach to the dark universe, which simultaneously challenges theoretical models on their empirical consistency with data and their mathematical consistency within quantum theory. It will either rule these models out or provide a catalogue of viable ones, along with reliable predictions that will help to guide future experimental and observational efforts to uncover the mysteries of the dark universe.