TY - JOUR
AU - Cruise,, Mike
AB - Abstract Mike Cruise surveys the development of this new field of astronomy in his 2019 Presidential Address. The development of astronomy since the early 1800s has been a story of new technologies opening up parts of the electromagnetic spectrum and so allowing the study of classes of objects and emission processes never previously anticipated. The telescopes that William and Caroline Herschel made at their observatory in Slough led to larger and more sophisticated optical telescopes in many locations, but it took some while for the full understanding of electromagnetism to bring significant access to wavelengths outside the optical. In 1931, Karl Jansky made his pioneering measurements of radio noise from the Milky Way at frequencies around 20 MHz, and the electronic developments in the second world war then stimulated the construction of dishes and interferometers for use at a range of radio wavelengths. This resulted in the rich field of radio astronomy we know today. After the war, access to space started, first using V2 rockets and then modern sounding rockets such as the Aerobee and the Skylark, followed by many orbiting satellites. These space platforms revolutionized astronomy by taking telescopes above the absorbing layers in the Earth's atmosphere to locations where X-rays from black holes and neutron stars could be observed, where cooled mirrors and detectors could study stars and dust in the infrared and, finally, where space-borne detectors could discover gamma-ray bursts and signals from active galaxies. Satellites provided long-term access to space for astronomical measurements – not just the few minutes of data available from sounding rockets. Each new waveband in this process of development has brought new information and, frequently, significant surprises. Astronomy seems to be almost unique among sciences in making a seminal discovery almost every decade for the past 60 years: quasars, pulsars, the cosmic microwave background, black holes emitting X-rays, dark matter, dark energy and so on. Having now witnessed the birth of gravitational wave astronomy just three years ago, it is interesting to speculate how this new branch of astronomy will develop with time and whether it, too, will expand to cover many wavelength ranges and so enable many new discoveries. Although there are significant similarities between electromagnetic and gravitational waves, there are also profound differences. As the subject develops, astronomers will undoubtedly learn to exploit the unique properties of gravitational waves in the study of certain classes of objects. Both electromagnetic and gravitational waves are transverse waves and can propagate at all frequencies in vacuum. Perhaps surprisingly, given the technical difficulty of the first detections, the energy flux from the black hole mergers so far detected is about the same as that of optical observations of Jupiter. Gravitational waves are ripples of curvature and spacetime is extremely stiff; as a result, gravitational waves carry a great deal of energy as they propagate unabsorbed and unscattered through space. A significant difference in complexity exists between the cosmic sources of electromagnetic radiation, such as stars and galaxies with many temperature regimes and component parts, and the sources of the gravitational events we are currently observing where the emission comes from essentially two point masses. So, a feature of gravitational wave astronomy seems to be an opportunity to make very precise measurements, often with significant implications for fundamental physics, on some of the simplest systems in astronomy. By this means we are allowed a different, and complementary, approach to understanding the universe in all its aspects. Detection technologies During the design of space missions for gravitational astronomy, a list was compiled of all the previously proposed technologies for detecting the presence of a gravitational wave (table 1). The criterion used in compiling the list was simply that a peer-reviewed paper had been published on the topic, not that it had been tested in the laboratory, at even the conceptual level. There were more than 20 different methods of detection found in literature searches. Some of them have reached proof of concept levels while others remain just interesting ideas. However, one method that has been under development for more than 40 years seems to be well adapted to use both on the ground and in space for certain frequency ranges. But it may be that in the future the pressure to make measurements across a wider frequency range may bring other technologies on the list below into play. 1 Possible gravitational wave detector technologies Open in new tab 1 Possible gravitational wave detector technologies Open in new tab The instruments currently in use as gravitational wave detectors almost exclusively use the very sophisticated techniques of laser interferometry to detect these elusive disturbances in spacetime. The lasers measure changes in the separation between pairs of test masses suspended against the local force of the Earth's gravity by vertical suspension fibres that allow free motion in the horizontal direction in response to the tidal forces of the gravitational wave. As the test masses move apart and towards each other due to the tidal force of the gravitational wave from a distant cosmic source, perhaps millions of parsecs away, the phase of the laser beam reflected between the test masses signals the presence of the gravitational wave. The effect of a gravitational wave is an oscillating strain in spacetime, meaning that for a given wave amplitude, the actual movement of the test mass is proportional to the undisturbed separation between them for distances that are much less than one wavelength. This allows the detector design to maximize the sensitivity by placing the test masses as far apart as possible, several kilometres in the case of the Advanced LIGO (aLIGO) and VIRGO detectors. Because the strain amplitude (the fractional change in separation) is of order 10−22 ⁠, the requirement on the laser interferometry is still highly challenging. Over distances of 4 km, movements smaller than 10−18 m need to be measured in order to detect the cosmic signals from merging black holes. This is more than 1000 times smaller than an atomic nucleus; as a result, the design of the test masses and their suspension is as important as the design of the laser and the interferometer. Both aLIGO and VIRGO are sensitive in the frequency range from 20 Hz to about 2 kHz, the range predicted to be the most promising for stellar-mass black hole orbits. First detection Announced in February 2016, the first gravitational wave event detected was that of a merger of 29 M⊙ and 36 M⊙ black holes which took place about 400 Mpc from Earth. The event only lasted a few tenths of a second, but the analysis of the waveform revealed the masses of the two black holes before the merger and the remnant black hole as it shook off its non-spherical gravitational modes afterwards. Other parameters measured were the spins of the two black holes and the direction of their spin axes, together with the distance to the event. It is remarkable that one event lasting a few tenths of a second could allow the determination of so many precise parameters. However, at the present state of development, the ground-based detectors can provide only a very poor indication of where in the sky the signals originated. By the end of 2018, 10 such events had been published, marking the beginning of population studies of black holes in the mass range 10 to 100 M⊙ as indicated in figure 1. In August 2018, a rather different event was reported, an event which took about 10 seconds to develop from the barely detectable signal to the completed merger. The excellent communications links from the aLIGO observatory automatically transmit the event triggers to the astronomical community as a whole, allowing space-borne gamma-ray detectors to search for coincident signals. Both the Integral and FERMI observatories detected significant spikes of electromagnetic radiation 1–2 seconds after the merger. The extremely high energy of the gamma-ray signals hinted very strongly that they were the result of nuclear transformations, a route by which the creation of chemical elements such as gold, silver and uranium, that are significantly heavier than iron, can take place. From this one event involving neutron stars, a new technique for studying chemical element production has been made available, helping us to understand some of the processes by which our planets, and the animal and human population, were formed. 1 Open in new tabDownload slide The masses (in multiples of solar masses) of the 10 merger events detected by December 2018. (LIGO-Virgo/Frank Elavsky/Northwestern) 1 Open in new tabDownload slide The masses (in multiples of solar masses) of the 10 merger events detected by December 2018. (LIGO-Virgo/Frank Elavsky/Northwestern) As the sensitivities of aLIGO and VIRGO are increased in the future, more black hole–black hole and neutron star–neutron star events are to be expected, adding to our knowledge of how these compact objects are created and evolve. According to our theoretical understanding of black holes, as well as the indirect optical and X-ray measurements, there will be black holes of much greater mass residing in the centre of galaxies. Images taken from the Hubble Space Telescope show clear cusps in stellar brightness close to the centre of galaxies. Analysis of the images allows estimates of the mass of the central objects, giving values of millions, or thousands of millions, times the mass of the Sun. Objects of this size take minutes or tens of minutes to complete their orbits and the gravitational wave signals will therefore be in the millihertz band, and not detectable in the frequency range of aLIGO and VIRGO. The astrophysical importance of these objects is indicated by the fact that the estimated masses correlate extremely well with the overall brightness of the galaxy that hosts them. The implication seems to be that supermassive black holes play an important part in the evolution of galaxies, probably by successive mergers of galaxies and their central black holes that send out gravitational wave evidence of how this process works. Studying these signals should help in understanding how the galaxies we see today have come about. The low-frequency end of the aLIGO sensitivity is limited by seismic noise from ground movement, ocean waves on distant shores and local human movement. This noise can be avoided only by moving the detectors to space and a space mission called LISA has been designed by teams of European and US scientists to work at frequencies of 1 to 100 mHz. LISA (Laser Interferometer Space Antenna) works on essentially the same principles as aLIGO. It is a laser interferometer comprising three spacecraft in a triangular constellation several million kilometres across, in orbit around the Sun. The constellation will follow behind the Earth in its orbit. Each spacecraft will carry lasers to measure the separation of the test masses on board and a telescope to transmit and receive the laser beams. The test masses will be completely freely floating inside each of the three spacecraft. These spacecraft will be separated by such large distances so that the very small strain in spacetime will cause measurable movement of the test masses. New challenges face a space laser interferometer compared to one on the ground. The solar wind will provide a fluctuating force on the spacecraft, possibly driving the test mass housing to contact the test mass itself and interrupt the measurements. To avoid this problem a control system will measure the separation between each test mass and its housing and trigger very fine thrusters on the spacecraft to correct any displacement. New levels of laser performance will be needed to ensure that the laser frequency noise does not obscure the signals being sought. Techniques for removing laser noise from the system have been developed that send the laser phase signal around the satellite constellation in several different paths of precisely equal length, making the response independent of the laser frequency. The control and measurement of a freely floating test mass cannot be demonstrated on the ground and so the European Space Agency, with notable foresight, decided in 2003 to fund a mission called LISA Pathfinder to test the new technology in orbit. LISA Pathfinder LISA Pathfinder had the task of demonstrating that a freely floating test mass could be controlled and have its position measured to less than 10 picometres at 1 mHz frequency. This is a precision never before achieved in space at these frequencies, and the LISA Pathfinder team faced many new challenges, not least developing precision instrumentation to work at such low frequencies. The mission was launched in November 2015 and worked to perfection, despite all the new technologies it required. The noise levels required in LISA Pathfinder to prove that the techniques were suitable for use in space were deliberately set at a level 10 times less demanding than those required for LISA but, in the event, the technology demonstration achieved the actual LISA requirements across almost the whole LISA frequency band, opening the way for the selection of LISA as a cornerstone mission in the European Space Agency future science programme (figure 2). 2 Open in new tabDownload slide The noise curve from LISA Pathfinder. The experiment achieved far better performance than required, at the levels needed for LISA for much of the frequency range. (ESA/LISA Pathfinder Collaboration) 2 Open in new tabDownload slide The noise curve from LISA Pathfinder. The experiment achieved far better performance than required, at the levels needed for LISA for much of the frequency range. (ESA/LISA Pathfinder Collaboration) Now selected as one of the next large missions in the ESA science programme and enhanced by NASA cooperation and funding, LISA will provide access to a new frequency band. The sensitivity and frequency range of LISA, shown in figure 3, will permit the study of supermassive black holes, white dwarfs in our galaxy and the inspiral of stellar-mass black holes into massive black hole systems. LISA will be able to detect signals from supermassive black hole mergers out to redshifts of z = 20, if such events happened at such early times. This impressive sensitivity will bring extragalactic astronomy and cosmology within the range of gravitational wave detectors such as LISA. Relic cosmological signals from the period of inflation could provide detectable signatures in this new frequency band, providing new tools to test theories of the Big Bang. The current plan is for the launch of LISA to take place in 2034. It requires careful and well-planned technical development, but undoubtedly an observatory working in this fertile frequency range will be a further important milestone in astronomy. 3 Open in new tabDownload slide The expected sensitivity of LISA and the potential astrophysical sources. They lie in between the bands for ground-based detectors like Advanced LIGO (aLIGO) and pulsar timing arrays such as the International Pulsar Timing Array (IPTA). To be detectable, the characteristic strain of a signal must be above the noise curve. (C Moore, R Cole and C Berry) 3 Open in new tabDownload slide The expected sensitivity of LISA and the potential astrophysical sources. They lie in between the bands for ground-based detectors like Advanced LIGO (aLIGO) and pulsar timing arrays such as the International Pulsar Timing Array (IPTA). To be detectable, the characteristic strain of a signal must be above the noise curve. (C Moore, R Cole and C Berry) Different forms of inflation in the early universe will inevitably generate different signatures in the gravitational wave spectrum. Some of these may be detectable by LISA, but various theories predict gravitational radiation at much lower frequencies than LISA can detect, at frequencies of 10−9 Hz or below. Arrays of radio telescopes are currently observing groups of radio pulsars in order to measure the phase changes in the pulsar signals as a result of very-low-frequency gravitational waves passing through the intervening space between us and the pulsars. In this case the two “test masses” can be thought of as the pulsar and the Earth. If many pulsar signals in a region of the sky exhibit the same changes in phase, then the presence of gravitational waves passing between the Earth and the pulsars can be detected. This technique is opening a new frequency band in the range 10−8– 10−9 Hz for the study of supermassive black hole mergers and possibly cosmological signals from the early universe. Several teams of scientists (the European Pulsar Timing Array, The Nanograv collaboration and International Pulsar Timing Array) are actively observing the selected pulsars. The future development of the technique will require the identification of many more, stable, millisecond pulsars and regular observations of these signals over many years. Gravitational waves from the beginning of the universe will also be imprinted on the cosmic microwave background (CMB) as patterns of correlated polarization referred to as B modes. These characteristic patterns have been searched for, but not yet found, in the data from Planck and ground-based CMB experiments such as BICEP2. Space missions specifically designed for polarization measurements at the frequency of the microwave background are under study in Japan and elsewhere. It seems entirely possible that gravitational wave energy at frequencies of 10−18 Hz could be discovered using these techniques of microwave imaging, the signatures of gravitational waves emanating from the very earliest moments after the Big Bang. The fact that gravitational waves have travelled unabsorbed or unscattered from the time of the Big Bang itself would make this new branch of astronomy especially useful in understanding the origins of the universe. The signals from inflation potentially span a much wider range of frequencies than either LIGO, LISA, pulsar timing arrays or CMB polarization can detect. The measurements that may discriminate between different forms of inflation may be at very much higher frequencies, in the microwave or even optical bands (figure 4). These frequencies are too high for laser interferometry because the interferometer essentially runs out of photons when trying to detect phase changes lasting only nanoseconds or less. Another technology will be required and fortunately some past theoretical developments provide a possible solution from the list of detection technologies in table 1. 4 Open in new tabDownload slide Various predictions of gravitational waves arising from inflation. Black lines (LIGO–Virgo) indicate upper limits on how large the gravitational wave background could be. The plot also shows upper limits from some other measurements – “CMB & Matter Spectra”, “BBN” and “Pulsar Limit”. Measurements were made with a variety of techniques and study different frequency ranges. For comparison, “AdvDet” shows the expected sensitivity of the advanced versions of LIGO and Virgo. The other lines show predictions from various models, some of which may be testable with the advanced LIGO and Virgo detectors and some of which will require building new detectors. (LIGO Scientific Collaboration & Virgo Collaboration 2014 Phys. Rev. Lett.113 231101) 4 Open in new tabDownload slide Various predictions of gravitational waves arising from inflation. Black lines (LIGO–Virgo) indicate upper limits on how large the gravitational wave background could be. The plot also shows upper limits from some other measurements – “CMB & Matter Spectra”, “BBN” and “Pulsar Limit”. Measurements were made with a variety of techniques and study different frequency ranges. For comparison, “AdvDet” shows the expected sensitivity of the advanced versions of LIGO and Virgo. The other lines show predictions from various models, some of which may be testable with the advanced LIGO and Virgo detectors and some of which will require building new detectors. (LIGO Scientific Collaboration & Virgo Collaboration 2014 Phys. Rev. Lett.113 231101) GW and EMW interactions Following a paper by Gertsenshtein in 1970, it became clear that the interaction of gravitational waves with electromagnetic fields offered interesting detector opportunities. Gertsenshtein analysed the effect of a strong beam of electromagnetic waves passing through a volume threaded by a static magnetic field and showed that the oscillating components of the local stress-energy tensor generated gravitational waves. The same sort of effect happened when strong, static electric fields replaced the static magnetic field. More importantly, the inverse transitions were identified, in which a gravitational wave passing through a strong, transverse static magnetic field generated an electromagnetic wave having the same frequency and direction as the incoming gravitational wave. This “inverse Gertsenshtein effect” is not difficult to understand. The incoming gravitational wave squeezes and stretches the static magnetic field and causes oscillations in the field strength at the frequency of the gravitational wave. These oscillating magnetic field components then generate oscillating electric field components through the normal, flat space, Maxwell equations and an electromagnetic wave propagates onwards from the interaction. In cavities many wavelengths long the process repeats many times adding to the strength of the converted electromagnetic wave in proportion to the length of the cavity. The interactions have to take place in a good vacuum so that the phase velocities of the two waves are precisely the same. Other, laboratory physics, experiments use long vacuum cavities with transverse magnetic fields to search for axions or weakly interacting particles and results from such facilities have published upper limits on the production of these exotic particles. The results are now being reinterpreted in terms of the maximum gravitational wave flux that could have been present, consistent with the detector noise. Upper limits to that flux can now be estimated at frequencies of 1014 or even 1018 Hz at optical and X-ray wavelengths. Candidate sources for such detections include stochastic background radiation from the era of the Big Bang. Mechanisms have been proposed for gravitational noise during these, very earliest, moments of time. As the universe cools down from the Big Bang itself, various phases of the vacuum may be passed through. If this doesn't happen uniformly, bubbles of different phases may coexist for a time and experience collisions generating huge amounts of energy from the boundary walls of the bubbles. In other models of the very early universe, topological defects such as strings may have decayed, releasing their tension as gravitational waves at very high frequencies. Even more exotic astronomical scenarios may be the source of very-high-frequency gravitational waves needing these new techniques for detection. The search for an overarching theory that incorporates both gravity and the standard model of particle physics has led to the idea that we may live in a world having five dimensions instead of the more obvious four (three spatial dimensions and time). A feature of some of these theories is that they hint at the answer to a significant puzzle: why is the force of gravity so much weaker than the other forces in physics? A possible answer is that gravity is indeed strong, but in the fifth dimension that we do not normally have contact with in our measurements. Only a shadow of that gravitational force is present in the four dimensions in which our current experiments operate. However, if spacetime is distorted by violent events such as black hole mergers and that distortion propagates into the fifth dimension it will access a much more significant force of gravity and the timescale on which the separations in the fifth dimension oscillate will be something like the curvature in that dimension divided by the velocity of light. Current limits on the curvature of any fifth dimension push the expected frequencies into the GHz range or above. Observing when black hole mergers take place may indeed make it possible to test, or even exclude, some theories of higher dimensional spaces using such very-high-frequency detectors. The particular feature of this inverse Gertsenshtein technique is that the efficiency of conversion of gravitational waves to electromagnetic waves is proportional to the square of the observing frequency and hence its application is extremely effective at such high frequencies. The conversion volumes involved will be of laboratory scale, offering the possibility of specially designed arrays of such units working together as the separate telescopes in a radio interferometer do, synthesizing an imaging aperture for gravitational waves. Magnetic conversion The technology of magnetic conversion of gravitational to electromagnetic waves using powerful magnets may also provide some preliminary data in a much lower frequency range, at frequencies as low as 1 mHz. There is no chance of such a detector approaching the sensitivity available from the LISA laser interferometer but, with 15 years before launch, some simple experiments are being undertaken to see if magnetic conversion could have any chance of detecting very bright sources in the LISA band. The magnetic conversion efficiency depends also on the square of the magnetic moment of the object encountered by the gravitational wave and so a detector working at the LISA frequencies would need a very large magnet to compensate for the frequency effect. Luckily we live on such an object – the Earth! With a magnetic moment of 7 × 1022 in SI units (A m2 ⁠) a huge sensitivity increase is available from this, normally very inefficient, process. There are two serious problems that need to be tackled before any credible detector based on magnetic conversion could be designed. First of all, we do not know enough about the geometry and electrical properties of the Earth's dynamo or the overlying layers of mantle and crust to be able to predict how efficient the conversion and transmission of electromagnetic waves would be. This may be an insuperable problem and make this technique quite unusable at low frequencies. The second issue is currently being addressed and that is the level of ambient electromagnetic noise at the very low frequencies in the LISA band. There seem to be few, if any, previous measurements of this noise and so a small project has been funded by the Leverhulme Trust to make such measurements for the purpose of exploring whether magnetic conversion is a credible technique at low frequencies. All that is needed is a suitable electromagnetic antenna and low-noise amplifiers. Electromagnetic antennas only operate efficiently when they are roughly the same size as the wavelength of the radiation being detected. In the case of electromagnetic waves at 1 mHz, this suggests that an antenna 1011 m long is needed – a structure that is clearly impractical. Another kind of antenna has been developed to overcome this difficulty and field tests have now started to measure the ambient radio noise at these unusual frequencies. The current detections are limited to a small range of frequencies, from 20 Hz to 2 kHz, a range which focuses on emission from the mergers of stellar-mass compact objects such as black holes and neutron stars. But, as outlined in figure 5, we can already chart the technological path to different frequency bands allowing the study of supermassive black holes, white dwarfs in our galaxy and possible cosmological signals. From this vantage point in time, less than four years since the first detections of gravitational waves, it is already apparent that a range of new detector technologies can be foreseen which could extend the search for new sources to frequency bands from 10−18 Hz to 1018 Hz. That is from one oscillation in the lifetime of the universe to X-ray frequencies. With this array of opportunities it seems likely that gravitational wave astronomy will follow the path pioneered using electromagnetic waves, with successive wavebands opening up as each new technology is optimized for the source characteristics being studied. There should be many more seminal discoveries in the decades ahead. 5 Open in new tabDownload slide The possible development of gravitational wave astronomy from 2015. (aLIGO: Caltech/MIT/LIGO Lab. Pulsar: MPIfR, Nancay, Arecibo, Parkes, Jodrell Bank, ASTRON, Green Bank. LISA: AEI/Milde Marketing/Exozet. LiteBIRD. Einstein Telescope. Earth: ESA/ATG Medialab) 5 Open in new tabDownload slide The possible development of gravitational wave astronomy from 2015. (aLIGO: Caltech/MIT/LIGO Lab. Pulsar: MPIfR, Nancay, Arecibo, Parkes, Jodrell Bank, ASTRON, Green Bank. LISA: AEI/Milde Marketing/Exozet. LiteBIRD. Einstein Telescope. Earth: ESA/ATG Medialab) PRESIDENTIAL ADDRESS The address was given at the RAS Ordinary Meeting on 10 May 2019. FURTHER READING The paper announcing the discovery of gravitational waves from a binary black hole merger: Find out more about LISA at the ESA site: http://sci.esa.int/lisa The principles of the effects of gravitational waves on electromagnetic fields: Abbott B P et al. ( LIGO and Virgo Collaborations ) 2016 Phys. Rev. Lett. 116 061102 Crossref Search ADS PubMed Gertsenshtein M E 1962 J. Exper. Theoret. Phys. 14 ( 1 ) 84 Cruise A M 2012 Classical and Quantum Gravity 29 095003 Crossref Search ADS © 2019 Royal Astronomical Society This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)
TI - The gravitational wave spectrumPresidential Address
JF - Astronomy & Geophysics
DO - 10.1093/astrogeo/atz161
DA - 2019-08-01
UR - https://www.deepdyve.com/lp/oxford-university-press/the-gravitational-wave-spectrumpresidential-address-1sx29W0T06
SP - 4.15
VL - 60
IS - 4
DP - DeepDyve
ER -