Exploring the gravitational-wave universe

Gravitational waves (GWs) have provided us with a new means to study the universe. Since 2015 over 90 confirmed GW detections of merging black holes and neutron stars have transformed our understanding of astrophysics, uncovering a vast new population of massive black holes that has challenged our understanding of the evolution of massive stars; demonstrated that neutron-star mergers are the source of short gamma-ray bursts; tested general relativity to unprecedented accuracy; starte to uncover the black-hole mass distribution and provided new independent measurements of the Hubble constant. This is only the beginning: over the course of this grant period planned improvements in detector sensitivity will lead to daily observations, taking us into a new era towards precision GW astronomy. The Cardiff Gravity Exploration Institute (GEI) has been a pioneer in GW research for 50 years. We have developed the methods now routinely used to detect signals, the theoretical signal models and analysis codes necessary to measure the properties of the signals’ sources, and the most advanced machine-learning methods to discover entirely new sources. With these data we constrain state-of-the-art astrophysical models. In 2018 we started an experimentation group, whose members have been key contributors to instrument innovation for several decades and now in Cardiff contribute directly to improving Advanced LIGO detector sensitivity. In the eight themes (including Operations) in this grant we will develop improved gravitational-wave (GW) detector technology, GW signal models, detection and source measurement tools, and astrophysical modelling, to exploit the data from the fourth and fifth observing runs (O4 and O5) of the Advanced LIGO, Virgo and KAGRA detectors. The grant period will include the data release from O4, and the beginning of O5. During this time we expect the observation of several hundred compact-binary mergers. The research themes in this project help make possible this number of observations, and will ensure that we extract the maximum possible scientific information from them. We will further use that information to understand the formation and evolution of binary systems in the universe. These research projects address several STFC Science Challenges in Frontier Physics: A5 (how do stars and galaxies evolve?), A6 (how do nuclear reactions power astrophysical processes and create the chemical elements?), A7 (what is the true nature of gravity?), A8 (what can gravitational waves and high-energy particles from space tell us about the universe?) and C3 (what is the nature of space-time?) We will develop a new interferometer modelling approach that will make possible high-power laser operation in O5, and a novel method for detector calibration. We will construct theoretical signal models that meet the far higher accuracy requirements that are essential for O5 observations, and methods to fully characterise any systematic uncertainties that remain. In source measurement we will develop tools to rapidly characterise exceptional sources that include precession or eccentricity, improved localisation for EM follow-up, and a new framework for parameter estimation that accounts for real detector noise. Our machine-learning search pipeline will allow low-latency detections in <15s for multi-messenger astronomy. Finally, we will develop tools to characterise black-hole populations and to identify multiple sub-populations.