| Abstract |
Cosmologists have made remarkable progress in understanding billions of years of the Universe's evolution, but the first fraction of a second after the Big Bang is still mysterious. The details of what happened in this fraction of a second hold the key to some of the most fundamental questions in physics. Theorists have long thought that the Universe may have undergone one or more dramatic, discontinuous changes in this time, called first-order phase transitions, in which a field reaches a lower-energy state by nucleating 'bubbles' of a new phase. These transitions could solve a variety of cosmic puzzles, such as why there is more matter than antimatter, and whether our Universe is part of a larger 'multiverse'. For decades there was no way to empirically test these ideas, as the earliest light we can ever hope to observe with our telescopes was emitted more than 300,000 years after the Big Bang. That situation has changed with the recent advent of gravitational-wave astronomy, which allows us to observe signals from the very first moments of the Universe's history. First-order phase transitions are a powerful source of gravitational waves, and are thus a prime observational target for current and future gravitational-wave missions. However, there are serious theoretical challenges in modelling these transitions, as they occur in the strongly non-perturbative, out-of-equilibrium regime where our physical understanding is limited. These challenges mean that various existing approaches all fail to capture some of the crucial aspects of the problem, and as a result there are significant theoretical uncertainties in the resulting observational imprints, including gravitational waves. I will use two innovative new approaches to tackle this problem. The first is cold-atom analogue experiments, which can be used to create quantum fields in the lab that emulate the behaviour of fundamental fields in the early Universe, allowing us to study cosmic bubble nucleation empirically, in real time, and in a controlled and reproducible manner. The second is semiclassical lattice simulations, which (unlike existing simulations) capture the dynamics of bubble nucleation, and reveal new observable phenomena such as correlations between bubbles. Together, these new approaches promise to unlock valuable new insights into cosmic bubble nucleation. This will be an ambitious research program in which I will combine (i) theoretical modelling and interpretation for cold-atom analogue experiments, with the goal of extracting new insights on bubble nucleation from experimental data; (ii) establishing semiclassical simulations as a new tool for studying gravitational waves from phase transitions, by developing the necessary numerical frameworks and extending the simulation method to new physical regimes; (iii) translating these results into improved analyses of gravitational-wave data, and forecasts for future missions. I will complement this research with a program of public engagement and outreach, built around an interactive bubble-nucleation app that will provide an intuitive and engaging visual tool for communicating ideas about the early Universe. My public engagement work will target secondary-school children, particularly from groups that are under-represented in STEM, with the goals of inspiring interest in science and tackling educational inequality. As well as being crucial and timely for gravitational-wave cosmology, my work will provide insights into a broad range of questions about early Universe physics. I will also strengthen emerging links between the cosmology and cold-atom communities, helping to drive further progress in this fruitful new area of interdisciplinary collaboration. |
