Investigations in Gravitational Radiation

Einstein's General Relativity predicts that dynamical systems in strong gravity fields will emit vast amounts of energy in the form of gravitational waves (GW). Thse 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. Experiments aimed at detecting them have been in development for several decades, and are now reaching sensitivities where detection is expected within a few years.

The worldwide network of interferometric detectors includes the American advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO), the French-Italian-Dutch-Polish advanced Virgo (AdV) and the German-UK GEO600 that are being enhanced with a new detector (KAGRA) under construction in Japan. The former detectors have all reached sensitivities close to their design goals and have taken the most sensitive data to date. Cooperation amongst different projects has enabled continuous data acquisition, with sensitivity to a wide range of sources and phenomena, over most of the sky. Modelling GW sources has allowed deeper searches and data from LIGO, Virgo, and GEO have increased our understanding of astronomical phenomena. For example, we have built accurate models to describe the dynamics of spinning black hole binaries for improving efficiency of detection and accuracy of parameter estimation, initiated studies on distinguishing models of the formation and evolution of compact binaries and supernovae, ruled out merging neutron star binary as progenitor of the gamma ray burst (GRB) GRB070201, and shown that less than 1% of the Crab pulsar's radiated power is in GW.

We are now entering a new era as advanced detectors begin their first phase of operation and within a few years will, we expect, routinely observe GW. 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 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 initiated and led searches for astronomical sources, thanks to funding support received since first data taking runs began 12 years ago. Key ingredients of several searches (accurate waveforms models, geometric formulation of data analysis to optimise searches, algorithms to search for generic bursts, Bayesian search and inference techniques) were developed at Cardiff and Glasgow.

We propose a programme that leads to full exploitation of data from aLIGO and AdV, building on the analysis of data from the most recent LIGO/Virgo science runs and from GEO600 while the advanced detectors were under construction. In particular, we will refine waveform models and carry out deep and wide parameter space searches for coalescing binaries, GW emitted in coincidence with GRBs and supernovae, and continuous signals from rotating 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, both in the US and for a possible 3rd aLIGO detector in India, and in the development of enhancements and upgrades to the aLIGO detectors in the areas of mirror coatings for low thermal noise, silicon substrates, room temperature and cryogenic suspensions and improved interferometer topologies to combat quantum noise.