Scientific discussion meeting organised by Dr Iain Martin, Professor Nils Andersson, Professor Carole Mundell and Professor James Hough OBE FRS.
Scientific discussion meeting organised by Dr Iain Martin, Professor Nils Andersson, Professor Carole Mundell and Professor James Hough OBE FRS.
The recent detection of gravitational waves from astrophysical sources opened a new window to the universe. This breakthrough has generated enormous excitement, across the scientific community and among the general public. The aim of this meeting, which brings together gravitational-wave experts and leading astrophysicists, is to discuss the current status and future prospects of gravitational-wave astronomy.
Recorded audio of the presentations will be available on this page soon.
The black-hole coalescences detected so far by LIGO have begun to deliver the promises of the long development of such detectors, going back to the 1970s. As with many fields of astronomy, gravitational waves have already revealed systems we did not know about: binaries of black holes with total masses up to 70 solar. Virgo has now joined LIGO in observing, and as the two projects improve their sensitivity, and as new detectors in Japan and India join, we can expect many more surprises, and the start of multimessenger GW astronomy. But the range of these detectors is still limited to our cosmological near neighbourhood, so the field is planning ambitious new instruments, including third-generation (3G) detectors that will require worldwide collaborations just to build. And ESA, impressed by the success of LISA Pathfinder, is now fast-tracking LISA, with NASA as an expected junior partner. Complemented at low frequencies by pulsar timing observations by SKA, LISA and 3G detectors will reach to very high redshifts, and will really deliver on the promises.
The field of gravitational-wave astronomy has been launched by LIGO’s recent detections of multiple binary black hole coalescences. The instruments capable of detecting the minute strains due to these distant cataclysmic events must operate at the limits of fundamental measurement science, exploiting Einstein’s contributions to physics in several domains to enable the verification and exploitation of his theory of general relativity. The future holds great promise. The Advanced LIGO instruments are still a rough factor of two less sensitive than their design. Because the amplitude of the strain is sensed, at design sensitivity the volume of space within reach – and the rate of detections – will be roughly 8 times greater. Advanced Virgo in Italy has just started observing, and the data from the Virgo and LIGO instruments are analyzed together, enabling better localization of sources and offering the potential to measure polarization. KAGRA in Japan is expected to join the network in a few years, and LIGO India is planned to come on line in 2024. Incremental improvements in all these detectors is expected, offering another factor of 2 to 3 in sensitivity on this time scale. The next step forward, to reach a factor of 10 improvement (so 1000-fold in rate) over the current instruments, will require new observatories. As the signal scales with length, but most noise sources remain unchanged, moving beyond the current 3-4 km arm length is one necessary element in the new observatories, along with supporting the capability of cryogenics (to reduce thermal noise) and multiple parallel instruments to focus on different signal frequencies. This will require a major investment by participating countries, of the scale of Large telescopes, and requires a broad gravitational-wave science contribution to, and appreciation by, the scientific community.
Suspension systems for the test masses are critical systems in ground based gravitational wave as they isolate them from ground vibrations and other external motion as well as minimising thermally induced motion, allowing the test mass motion to be sensitive to gravitational wave signals. Glasgow University has been at the forefront of developing quasi-monolithic fused silica suspensions for more than 20 years. These mirror suspensions are fused silica test masses suspended on silica fibres made with dedicated in-house tooling to the correct shape and dimensions, which are welded to interface pieces called ‘ears’. These interface pieces are jointed onto the test masses using hydroxide catalysis bonding. Such mirror suspensions are developed for a range of fringe experiment, such that mirrors ranging from 1 g to 160 kg can be suspended in this way. Now Glasgow researchers are also using their expertise to develop similar quasi-monolithic mirror suspensions in silicon and sapphire for future generation detectors that will operate cryogenically. In this presentation an overview of the development and status of silica, sapphire and silicon quasi-monolithic suspensions will be given with some detail on the technology.
Gravitational waves are detected by measuring length changes between the test-mass mirrors in the detector arms. Brownian thermal noise is a motion of the atoms of a material due to it’s intrinsic temperature. In future gravitational wave detectors, Brownian thermal noise of the mirrors and in particular of the highly-reflective coatings, will limit the sensitivity to distance changes and therefore the ability to measure more gravitational wave signals from more distant sources. Therefore, the development of coatings with low thermal noise which at the same time meet the requirements on the optical properties is of great importance. This talk will give an overview about the current status of coatings and of the different approaches for coating improvement.
How did LIGO’s black holes form? What made it possible for them to be so heavy and so close, such that angular momentum loss through gravitational waves made them spiral in and merge? What are the fundamental lessons that they can teach us about the lives of the massive stars were their progenitors? The talk will briefly review the main formation scenarios that have been proposed to date and discuss the generic challenges that all of them face, followed by an expansion on the more recently proposed chemically homogeneous formation scenario and briefly mention some of the ideas for EM counterparts for binary black hole mergers.
For decades, the numerical relativity community worked to prepare for gravitational wave observations. Gravitational wave astronomy has arrived. I will review the role that numerical relativity played in the first detections and characterizations of gravitational waves. I will also provide my views on the future of numerical relativity and its potential for producing unique tools for astronomical discoveries.
Neutron stars are born in the supernova explosion of massive stars. They are more massive than the Sun but only the size of a city. Neutron stars possess densities exceeding that of atomic nuclei and magnetic fields millions to billions of times stronger than those created in laboratories on Earth, and they rotate as stably as atomic clocks. The compactness and gravity of neutron stars make these stars close relatives of black holes and thus potentially strong sources of gravitational waves. But the fact that neutron stars have a hard surface and are composed of normal nuclear particles makes them unique tools for advancing our knowledge of many areas of physics, including nuclear, particle, and plasma physics and condensed matter and low temperature physics. The physical properties of neutron stars feed into predictions of the gravitational waves emitted from these stars, and thus detection of gravitational waves will reveal invaluable insights into fundamental physics. In this talk, Wynn Ho describes some of the physics and astrophysics of neutron stars and how traditional electromagnetic wave observations already provide clues to the sorts of gravitational waves we expect to be detected from neutron stars.
Interferometric gravitational-wave detectors are some of the most complex instruments ever built; understanding their output is a huge challenge. Detector performance will change on a daily basis due to environmental, hardware and software issues. As such, data from the Laser Interferometer Gravitational-wave Observatory (LIGO) are typically non-stationary, containing many long and short duration artefacts. Long duration searches, such as those for the gravitational-wave background or continuous waves, are most affected by elevated noise at a given frequency, such as variations in spectral line amplitudes. However modelled and un-modelled transient searches are most sensitive to short duration noise events or ‘glitches’ which can mask or mimic a true gravitational-wave signal. These glitches can not only limit the detection of gravitational waves from the merger of black holes and neutron stars, but can also affect the ability to estimate the source parameters accurately. It is therefore important to mitigate these sources of noise at the gravitational-wave interferometers or from already recorded data. This talk will discuss the recent challenges of characterising the LIGO detectors in both the first and second observing runs (September 2015 - January 2016 and November 2016 - August 2017 respectively) and how glitches have affected the search for gravitational-waves from compact binary coalescences.
Gravitational waves from merging black hole binaries are the most powerful events ever witnessed. LIGO's observations of binary black holes have already begun to impact astrophysical models of the formation and evolution of compact binaries and have facilitated tests of general relativity in a regime where the theory had not been tested before. In addition to binary black holes we also expect to observe binaries in which one or both the companions is a neutron star and the other is possibly a black hole. Observing the full spectrum of binaries will help us understand the origin of short gamma-ray bursts, measure the equation-of-state of dense nuclear matter, test the no-hair theorem for black holes and map the cosmic history of the formation and growth of light black hole seeds. Gravitational wave observations could also reveal processes that operate in core collapse supernovae and the mechanism of core bounce and formation of neutron stars and black holes.
ESA has selected The Gravitational Universe as the Science Theme for the L3 large mission flight opportunity with a foreseen launch in 2034. The LISA Consortium has proposed the LISA mission concept for this Science Theme and recently the Science Programme Committee (SPC) of ESA has selected LISA as the L3 mission. LISA will comprise 3 spacecraft at the corners of an equilateral triangle with 2.5 million km arms in a heliocentric orbit trailing the earth. It will form a laser interferometer with 3 arms and 6 laser links, observing low-frequency gravitational waves with frequencies from less than 0.1 mHz up to more than 0.1 Hz. The LISA Pathfinder mission has concluded science operations on July 18th, 2017. It has demonstrated an acceleration noise performance more than a factor of three better than required for the full LISA mission, paving the way for a timely start of LISA.
Interferometric gravitational wave detectors (such as Advanced LIGO) employ high power solid-state lasers to maximise their detection sensitivity and hence their “reach” into the universe. These sophisticated light sources are ultra-stabilised with regard to output power, emission frequency, and beam geometry; this is crucial to obtain low detector noise. However, even when all laser noise is reduced as far as technically possible, unavoidable quantum noise of the laser still remains. This is a consequence of the Heisenberg Uncertainty Principle, the basis of quantum mechanics: In this case, it is fundamentally impossible to simultaneously reduce both the phase noise and the amplitude noise of a laser to arbitrarily low levels. This fact manifests in the detector noise budget as two distinct noise sources – photon shot noise and quantum radiation pressure noise – which together form a lower boundary for current-day gravitational wave detector sensitivities, the standard quantum limit of interferometry. In order to overcome this limit, various techniques are being proposed, among them different uses of non-classical light, and alternative interferometer topologies. MH will explain how quantum noise enters and manifests in an interferometric gravitational wave detector, and will give an overview of proposed schemes to overcome this seemingly fundamental limitation, all aimed at the goal of higher gravitational wave event detection rates.
The ability to control cell behaviour and cell fate remains a significant challenge within various fields of medical research, such as for manufacturing engineered tissues and for drug discovery. The use of mechanotransduction (how cells convert mechanical forces to biochemical cues) could provide an alternative route for providing high quality controlled/specialised cells without the requirement for chemical induction factors or complex cell scaffolds. Measurement and analytical techniques developed within the field of gravitational wave astronomy have recently been used to develop a novel technique for cell stimulation based on nanoscale vibration, referred to as "nanokicking". The technique has been demonstrated to differentiate a potential autologous cell source, mesenchymal stem cells (MSCs), into mineralized tissue in 3D. This could provide a new route for providing autologous bone grafts, which are currently in short supply, and are currently associated with pain and donor-site morbidity. In addition, other cell responses include the promotion of cell fusion in placental cell lines, in addition to modifying biofilm deposition from bacteria (P. aeruginosa), suggesting that the response to nanoscale vibration is applicable to a wider range of cell types in addition to potentially being established early in evolutionary history. The development of the current hardware for supplying nanoscale vibrations to cell cultures, and an overview of cell responses, is discussed.
The Institute for Gravitational Research at the University of Glasgow has led the development of the fused silica suspensions used to support the 40kg mirrors of the aLIGO detectors. These suspension systems required the development of ultralow mechanical loss glass structures and corresponding bespoke modelling techniques to provide the required analysis tools.
Over the past 4 years, Prof. Hammond has grown an applied research group in collaboration with the School of Engineering, focussing on precision MicroElectroMechanicalSensors (MEMS) for gravity imaging applications. The group has developed the first MEMS based gravity sensor with sufficient sensitivity and stability to detect the earth tides; elastic deformations in the solid earth caused by the tidal potential of the moon and sun. The unique combination of expertise on low noise opto-mechanical systems, combined with the fabrication expertise of the James Watt Nanofabrication Centre, has enabled the development of this new research theme, currently funded under the QuantIC hub, one of the 4 UK quantum technology hubs.
In this talk I will describe the translation of fundamental research from the field of gravitational wave detection into applied gravity sensing. I will detail the design of the MEMS gravimeter system including the novel low frequency flexure architecture and the construction of a high sensitivity, long term stable, optical shadow sensor. I will further provide an overview of the current activities being undertaken by the research team. These include the development of a field portable system that can operate outside of the laboratory, capable of measuring vertical gravity changes within the Physics building in Glasgow. I will also detail some of the ongoing industrial research projects to further improve the device and allow for new sensing methodologies in precision MEMS gravimeters. This includes the development of an on-chip silicon interferometer with Schlumberger, the development of MEMS gradiometer systems with Clydespace, and modelling of gravitational signatures with QinetiQ. Finally I will provide an outlook of the direction of the research group, the current efforts to commercialise this technology.
The first and second observational runs of the Advanced LIGO and Virgo detectors are seeing the first detections of gravitational waves (GWs) from binary black holes. Future observational runs by advanced gravitational-wave detectors should measure not only stellar-mass binary black hole mergers but other compact object mergers that comprise neutron stars. We expect such systems to emit electromagnetic (EM) emission in addition to gravitational radiation as a result of the complex merger. Such cosmic laboratories present us today with both a challenge and an opportunity. The challenge is to explain how and where these systems formed and the rich physics at play in high velocity, strongly-curved spacetime in Universe for the first time. The opportunity is to detect the joint EM and gravitational radiation with a suite of new telescopes and GW detectors. In this talk, I will first discuss how to infer and characterise the fundamental properties of the black hole binary systems with GWs. I will then summarise the EM follow-up campaigns of the first GW detections. With these GW observations in hand, I will then introduce EM counterparts of neutron star binary mergers and then discuss how to place compact object mergers in their full astrophysical context with joint gravitational-wave and EM observations. I will conclude with the unprecedented opportunities that are opening up in strong-field gravity astrophysics during the coming decades.
This is an extremely exciting time for pulsar astronomy. New facilities like LOFAR and Fermi are providing new ways to find the fastest spinning pulsars known, the millisecond pulsars. This is in conjunction with the large scale surveys with telescopes like Arecibo, Parkes, Effelsberg and the GBT, which have also contributed large numbers of new millisecond pulsars. In the very near future we have the prospect of FAST in China and MeerKAT in South Africa not only dramatically increasing the numbers of known sources, but also improving the precision to which we can measure these precise cosmic clocks. Innovative new approaches like the Large European Array for Pulsars are also helping push the limits of pulsar timing. On the near horizon we have the Square Kilometre Array (SKA), it will revolutionise pulsar searches and the precision timing of known pulsars. The improved number of sources will reveal new, more precise clocks, and more extreme binaries, such as the pulsar-black hole binary, which can provide the most stringent tests of strong field gravity. The improved precision will reveal new phenomena and also allow us to make a detection of grativational waves in the nanohertz frequency regime. It is here where we expect to see the signature of the binary blackholes that are formed as galaxies merge in throughout cosmological history. These new telescopes and their potential scientific return will be presented.
Sources which emit high-energy radiation include many types of compact objects, such as neutron stars, black holes and binary systems. These are also likely sources of gravitational waves, and hence the probability of high energy emission associated with gravitational wave sources appears reasonably high, although far from certain. Professor O’Brien will discuss the possibilities for a high-energy electromagnetic counterpart to gravitational wave sources and how these could relate to known types of object. Detecting such emission and identifying a new source is a significant challenge for current high-energy facilities. To search the high-energy sky usually requires space-based instrumentation, although at the very highest energies ground-based Cherenkov telescopes can also provide valuable constraints. Professor O’Brien will describe the current observational facilities and those planned for the future, several of which are specifically designed to efficiently search large sky areas.