Exploring the Gravitational-wave Universe

The era of gravitational-wave (GW) astronomy began in 2015 with LIGO's detection of the binary black-hole merger event GW150914, at the time the most energetic event ever observed by humanity. The announcement of this spectacular event generated headlines worldwide and led to the award of the 2017 Nobel Prize in Physics. More detections followed, including the now-famous binary neutron star merger event GW170817 associated with GRB170817A - the first multi-messenger gravitational-wave event. To date LIGO and VIRGO have published the details of 15 high-confidence detections and released public alerts for more than 50 additional candidate signals. Cardiff University researchers in the Gravity Exploration Institute (GEI) have made critical contributions to these detections and to the opening of the gravitational-wave sky. Together, the research of the GEI spans the entire breadth of gravitational-wave astronomy, from fundamental research into instruments, the modelling, detection and interpretation of events, and implications for fundamental physics, astrophysics, and cosmology. The GEI has grown to become the fourth largest group in the LIGO Scientific Collaboration (LSC), and our members provide leadership through a number of key strategic roles. We propose an ambitious programme that will have impacts across the breadth of gravitational-wave astronomy. This includes the development of advanced technology for future detectors, the creation of the most accurate state-of-the-art models for binary black hole signals, cutting edge techniques for real-time analysis of the LIGO-Virgo-KAGRA data, fast and accurate characterisation of sources, and mapping gravitational-wave observations into new insights into astrophysics, cosmology, and fundamental physics. Our experimental program focuses on technologies critical for upgrades to the current LIGO detectors and for next-generation observatories. We will increase the detector up-time and data quality by improving the seismic feedback control systems and implementing real time adaptive controls. We will develop lower-loss methods to reduce the noise in LIGO below the standard quantum limit and explore how co-located interferometers may further improve sensitivity. And we will pave the way for next-generation cryogenic detectors by building an input-output optics prototype operating at longer wavelengths, requiring the exploration of new lasers and optics. Our modelling, analysis, and astrophysics programme will benefit from synergy between the projects, yielding more precise source characterisation and astrophysical interpretations. We will perform the deepest analysis of the LIGO-Virgo data for gravitational-wave counterparts to gamma-ray bursts like GRB170817A. We will push the limits of multi-messenger astronomy by using machine learning techniques to detect generic transient signals such as supernovae, binary mergers, and accretion disk instabilities with sub-second latencies, enabling follow-up observations to catch the earliest electromagnetic emissions from these events. The ever increasing detection rates will allow us to explore further across the parameter space of compact binaries populations, including more rare events, but also require ever faster means of characterising detected signals. We will develop semi-analytical tools to provide an intuitive understanding of parameter estimation, allowing the prompt identification of exceptional events. We will implement complete, accurate, and fast inference techniques pioneered at Cardiff and apply then to infer neutron star equation of state. We will further develop tools to infer astrophysical populations from our observations and provide real-time updates to population models. Finally, we will compare the observed populations with astrophysical formation scenarios, exploiting the unprecedented data set and physically motivated models of binary populations, to infer the origins of gravitational waves.