Investigations in Gravitational Radiation

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Einstein's General Relativity predicts that dynamical systems in strong gravitational fields will emit vast amounts of energy in the form of gravitational waves (GW). These are ripples in the very fabric of spacetime that travel from their sources at the speed of light, carrying information about physical processes responsible for their emission. They are among the most elusive signals from the deepest reaches in the Universe.In September 2015, during the 1st Advanced LIGO observing run, gravitational waves from the collision of two black holes were discovered using the LIGO observatories. The detection of GW150914 resulted in the award of the 2017 Nobel Prize in Physics with explicit recognition of the role of the UK as a critical part of the global team.In August 2017, during the 2nd observing run, LIGO and Virgo detected the first gravitational wave signal from the collision of two neutron stars. GW170817 was observed in coincidence with a gamma-ray burst (GRB) as well as signals across the electromagnetic spectrum, including the optical and infra-red signature of a kilonova. These discoveries have established a new paradigm of multi-messenger astrophysicsThe 3rd observing run of Advanced LIGO and Advanced Virgo (AdV), O3, started on 1st April 2019 and ended in March 2020 during the end of which time the Japanese KAGRA instrument joined the observing network. Modelling GW sources has allowed deeper searches and data from LIGO, Virgo, and GEO have increased our understanding of astronomical phenomena. We are now able to make regular observations of GWs. To date close to 60 observations of coalescing objects, with an unexpectedly wide range of masses, have been made, with event rates being approximately 1 per week. We now have evidence for the existence of black hole/neutron star binaries, the existence of objects in the mass gap between accepted neutron star masses and black hole masses and the first real experimental evidence for the existence of intermediate mass black holes.The aLIGO detectors are based on the quasi-monolithic silica suspension concept developed in the UK for GEO600 and on the high-power lasers developed by our German colleagues in GEO600. The AdV detector also uses a variant of the silica suspension technology. Further, KAGRA is being built with input on cryogenic bonding technology from the UK groups.The consortium groups have led searches for astronomical sources, thanks to funding support received, since first data taking runs began 18 years ago. Key ingredients of several searches were developed at Glasgow.We propose a programme that exploits data from aLIGO, AdV, and KAGRA building on our analysis of data from the most recent LIGO/Virgo science runs. In particular we will observe and analyse signals from the LIGO / Virgo/ KAGRA detector network with particular emphasis on compact binary inference, population and cosmological measurements - measurement of the Hubble Constant and tests of General Relativity, application of machine learning techniques for increased efficiency in modelling signals. performing searches etc and the search for gravitational wave emission from neutron stars.In parallel, we propose essential detector R&D. Detector sensitivity is mainly limited by thermal noise associated with the substrates of the mirrors, their reflective coatings, and their suspension elements, as well as by noise resulting from the quantum nature of the light used in sensing. Our research is targeted towards making innovative improvements in these areas, essential to maximize the astrophysical potential of GW observatories. We have major responsibilities for the silica suspensions in aLIGO, and in the development of enhancements and upgrades to the aLIGO detectors (to form aLIGO+), along with R&D in the areas of mirror coatings for low thermal noise, silicon substrates, cryogenic suspensions and improved interferometer topologies to combat quantum noise.