Plans shape up for a revolutionary new observatory to explore black holes and the Big Bang

Scientists present their design for Einstein Telescope – Europe’s next-generation detector that will ‘see’ the Universe in gravitational waves

A new era in astronomy will come a step closer when scientists from across Europe present their design study today for an advanced observatory capable of making precision measurements of gravitational waves – minute ripples in the fabric of spacetime – predicted to emanate from cosmic catastrophes such as merging black holes and collapsing stars and supernovae. It also offers the potential to probe the earliest moments of the Universe just after the Big Bang, which are currently inaccessible.

The Einstein Observatory (ET) is a so-called third-generation gravitational-wave (GW) detector, which will be 100 times more sensitive than current instruments. Like the first two generations of GW detectors, it is based on the measurement of tiny changes (far less than the size of an atomic nucleus) in the lengths of two connected arms several kilometres long, caused by a passing gravity wave. Laser beams passing down the arms record their periodic stretching and shrinking as interference patterns in a central photo-detector.

The first generation of these interferometric detectors built a few years ago (GEO600, LIGO, Virgo and TAMA) successfully demonstrated the proof-of-principle and constrained the gravitational wave emission from several sources. The next generation (Advanced LIGO and Advanced Virgo), which are being constructed now, should make the first direct detection of gravitational waves – for example, from a pair of orbiting black holes or neutron stars spiralling into each other. Such a discovery would herald the new field of GW astronomy. However, these detectors will not be sensitive enough for precise astronomical studies of the GW sources.

“The community of scientists interested in exploring GW phenomena therefore decided to investigate building a new generation of even more sensitive observatories. After a three-year study, involving more than 200 scientists in Europe and across the world, we are pleased to present the design study for the Einstein Telescope, which paves the way for unveiling a hidden side of the Universe,” says Harald Lück, deputy scientific coordinator of the ET Design Study.

The design study, which will be presented at the European Gravitational Observatory site in Pisa, Italy, outlines ET’s scientific targets, the detector layout and technology, as well as the timescale and estimated costs. I A superb sensitivity will be achieved by building ET underground, at a depth of about 100 to 200 metres, to reduce the effect of the residual seismic motion. This will enable higher sensitivities to be achieved at low frequencies, between 1 and 100 hertz (Hz). With ET, the entire range of GW frequencies that can be measured on Earth – between about 1 Hz and 10 kHz – should be detected. “An observatory achieving that level of sensitivity will turn GW detection into a routine astronomical tool. ET will lead a scientific revolution”, says Michele Punturo, the scientific coordinator of the design study. An important aim is to provide GW information that complements observational data from telescopes detecting electromagnetic radiation (from radio waves through to gamma-rays) and other instruments detecting high-energy particles from space (astroparticle physics).

A multi-detector
The strategy behind the ET project is to build an observatory that overcomes the limitations of current detector sites by hosting more than one GW detector. It will consist of three nested detectors, each composed of two interferometers with arms 10 kilometres long. One interferometer will detect low-frequency gravitational wave signals (2 to 40 Hz), while the other will detect the high-frequency components. The configuration is designed to allow the observatory to evolve by accommodating successive upgrades or replacement components that can take advantage of future developments in interferometry and also respond to a variety of science objectives.

The European dimension
The European Commission supported the design study within the Seventh Framework Program (FP7-Capacities) by allocating three million Euro.
“With this grant, the European Commission recognized the importance of gravitational wave science as developed in Europe, its value for fundamental and technological research, provided a common framework for the European scientists involved in the gravitational wave search and allowed for a significant step towards the exploration of the Universe with a completely new enquiry instrument”, says Federico Ferrini, director of the European Gravitational Observatory (EGO) and project coordinator of the design study for the Einstein Telescope.

ET is one of the 'Magnificent Seven' European projects recommended by the ASPERA network for the future development of astroparticle physics in Europe. It would be a crucial European research infrastructure and a fundamental cornerstone in the realisation of the European Research Area.

List of experts

France
Raffaele Flaminio 
Benoit Mours

Germany 
Karsten Danzmann
Harald Lück

Italy
Federico Ferrini 
Michele Punturo 
Fulvio Ricci

The Netherlands 
Jo van den Brand
Chris Van Den Broeck 
David Rabeling

UK
Andreas Freise 
Stefan Hild 
Sheila Rowan 
B. Sathyaprakash

Background notes

• The Einstein Telescope Project (ET) is a joint project of eight European research institutes, under the direction of the European Gravitational Observatory (EGO). The participants are EGO, an Italian French consortium located near Pisa (Italy), Istituto Nazionale di Fisica Nucleare (INFN) in Italy, the French Centre National de la Recherche Scientifique (CNRS), the German Max Planck Institute for Gravitational Physics (Albert Einstein Institute) in Hannover, the Universities of Birmingham, Cardiff and Glasgow in the UK, and the Dutch Nikhef in Amsterdam. Scientists belonging to other institutions in Europe, as well as the US and Japan, actively collaborated in the realisation of this design study.
• The direct detection of gravitational waves – predicted by Einstein’s theory of gravity, the General Theory of Relativity – is one of the most important fundamental research areas in modern science. Apart from verifying General Relativity, especially for extreme gravitational fields found in the vicinity of a black hole, GW detection could allow us, for the first time, to look back at the earliest moments of the Universe just after its birth. Cosmological observations are currently limited to those using electromagnetic waves and cosmic-rays (high-energy particles such as protons). This information can reach us from the past, but from a time no earlier than 380,000 years after the Big Bang. Before then, light and matter continually interacted, so that the Universe was rendered opaque. The Universe became transparent only when matter and light separated during this epoch. Cosmological epochs dating further back have thus far remained hidden, so it has not been possible to verify from observations the various theories about their nature. The direct measurement of gravitational waves may allow us “to listen” back as far as the first trillionth of a second after the Big Bang. This would give us totally new information about our Universe.
• GW research is a global effort because the full information about many GW sources can be obtained only with several interferometers working simultaneously in different places. Therefore, the US (LIGO), German-UK (GEO600), Italian-French and Dutch (Virgo) scientific communities have been working together closely for a long time. They share technology R&D and theoretical advances, as well as data-analysis methods and tools. The joint European project ET will help to improve further this worldwide collaboration.

The current observatories:

• GEO600, is a German-UK detector located near Hannover, Germany, and is operated by researchers at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute/AEI) in Hannover, and at the Universities of Glasgow, Cardiff and Birmingham in the UK. It is funded by the Max Planck Society, the state of Lower Saxony, the Volkswagen Foundation and the UK Science and Technologies Facilities Council (STFC). GEO works in close cooperation with the cluster of excellence, QUEST (Centre for Quantum Engineering and Space-Time Research) at the Leibniz Universität Hannover.

• Virgo is a 3-kilometre arm interferometer at Cascina, near Pisa, Italy. This project accomplished the additional goal of making low-frequency measurements at around 10Hz. Initially, Virgo was funded by the CNRS (Centre National de la Recherche Scientifique) and the INFN (Istituto Nazionale di Fisica Nucleare) but has now expanded to include Dutch, Polish and Hungarian research groups.
• The US LIGO detectors consist of 2-kilometre and 4-kilometre instruments at Hanford, Washington, and a 4-kilometre instrument at Livingston, Louisiana. The LIGO project has been developed and is operated by the California Institute of Technology (CalTech) and the Massachusetts Institute of Technology (MIT), and funded by the National Science Foundation (NSF).

German and UK participants in ET

New technologies at the limits of the laws of nature
Germany and the UK have a broad and vigorous programme of gravitational wave research and contributed substantially to the ET conceptual design study. The German Max Planck Institute for Gravitational Physics (Albert Einstein Institute/AEI) and the UK Universities of Birmingham, Cardiff and Glasgow cooperate since many years in the GEO Collaboration and worked within the ET study on several work packages outlining ET’s scientific targets, the detector layout and technology, as well as the timescale and estimated costs. Further below you will find more information and comments from the leading German and UK scientists.

Germany and the United Kingdom have been involved in gravitational wave research from its very beginning: Researchers from the University of Glasgow and the Max Planck Society have been working on the concept of a laser interferometric gravitational wave detector since the 1970s. In the 1980s colleagues from Cardiff University and a few years later from the University of Birmingham joined and in 1994 the project received funding to build the GEO600 detector near Hanover, Germany. The successful design strategy for GEO600 was to build a low-cost, high-technology detector with a sensitivity comparable to the much longer US LIGO and French-Italian-Dutch Virgo detectors. GEO600 took its first test data in 2002 and has been working as part of the global network, searching for gravitational wave signals pioneering new technologies.

Today GEO600 and the GEO project has firmly established itself as an international think tank for experimental gravitational wave research. The GEO project with its members from Germany and UK is a full partner in the US Advanced LIGO project with a central role in data analysis and the instrumental upgrade (supported by a capital investment from PPARC/STFC and MPG). The German-UK gravitational wave community also plays a major role in the space-based projects LISA Pathfinder and LISA (Laser Interferometer Space Antenna).

The close collaboration of German and UK scientists with colleagues from other European countries is one of the critical factors for success of the ET design study. This grouping of gravitational wave research scientists working together is opening the road for further collaboration towards a third generation gravitational wave observatory on the European level.

Cardiff University: School of Physics and Astronomy
Scientists of the School of Physics and Astronomy at Cardiff University coordinated the ET Work Package 4 “Astrophysics issues”. The group, led by Professor B.S. Sathyaprakash, coordinated with the entire Science Team and evaluated inputs on the science potential, prioritized the list of scientific benefits and gauged the scope of alternative proposals for different configurations of the detector.

Prof. Sathyprakash summarizes the results as follows: “Einstein Telescope will truly revolutionize our understanding of the Universe by impacting fundamental physics, cosmology and astrophysics. ET will be an astronomical observatory to unveil the secret and hidden lives of neutron stars and black holes – the most compact objects in the Universe.
ET will observe gravitational radiation arising from their collisions in binary systems when the Universe was still in its infancy, assembling the first galaxies and the large scale structure. ET will detect their formation when mature stars collapse and explode in violent supernovae and hypernovae. It will be sensitive to quakes in neutron stars and ripples on black holes caused by a colliding star or a black hole, providing us new insights into complex physical processes.”

The Gravitational Physics Group at Cardiff University works on the ultimate goal in gravitational wave research: the detection of gravitational waves in the data from the international gravitational wave detector network. The group's effort is predominantly spent in searching for what is believed to be the most promising class of astronomical sources, namely the coalescence of binary neutron stars and black holes. The group also researches into the theoretical aspects of gravitational waves sources, specializing in the production of a stochastic background of gravitational waves in the primordial Universe and modelling the late time dynamics of binary black holes.

Max-Planck-Gesellschaft zur Förderung der Wissenschaften (MPG) e.V., acting through Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI)
The AEI scientists under the leadership of Prof. Karsten Danzmann and Prof. Bruce Allen, directors of the Albert Einstein Institute, and Dr. Harald Lück who co-chaired the ET design study, are contributing to the following tasks in the ET project:

Work Package 1 (Site identification): 
The main objectives of this work package are the definition of the seismic requirements of an experimental site, selection between underground or surface site, definition of the requirements of the infrastructures, conceptual design of the main infrastructures and evaluation of their costs, and a site candidates listing. The AEI contributes to the tasks of site selection and seismic isolation technologies.

Work Package 3 (Topology identification): 
The main objectives of work package 3 are the definition of the requirements in terms of quantum noise, identification of the detector topology, selection of the detector geometry. Experience within the AEI with high power lasers, the corresponding high power handling optics, diffractive optics, squeezed light, numerical simulations of various interferometer topologies is being used in all of the tasks of Work Package 3, i.e. the optical design work.

Work Package 4 (Astrophysics issues): 
The AEI is developing data analysis techniques for searches for all types of signals. The close connection to the LISA science team will ease investigating complementarities and cooperation possibilities between the 3rd generation observatory and LISA.

The Max Planck Institute for Gravitational Physics (Albert Einstein Institute/AEI) currently comprises five divisions located on two sites, one in Potsdam-Golm and one in Hannover. In its five divisions, the AEI research program covers the entire spectrum of gravitational physics: experimental, observational, and theoretical.

The unique high power lasers used in GEO600 and Virgo and the lasers foreseen for the upgrades of the detectors have been developed by the Laser Zentrum Hannover (LZH) and the AEI. Additionally the AEI is investigating the use of squeezed light and diffractive optics in ground-based gravitational-wave detectors. It is also leading the design of the LISA Pathfinder mission, which paves the way for LISA itself. The AEI has built, commissioned and is running and improving the gravitational wave detector GEO600 together with its colleagues from UK. The GEO project works in close cooperation with the cluster of excellence QUEST (Centre for Quantum Engineering and Space-Time Research) at the Leibniz Universität Hannover.

On the data analysis and astrophysics side, the AEI has developed the core data analysis software and algorithms used to search LSC data for continuous wave signals. Its staff develops and manages the public distributed computing project Einstein@Home, which is the second largest public distributed computing project in the world. Since its inception, the numerical relativity group at the AEI has been at the forefront of all breakthroughs in numerical solutions of Einstein's equations and gravitational wave source modelling. Proximity between the data analysis and numerical relativity groups provides extraordinary synergistic opportunities. The AEI is running a large computer cluster (ATLAS) with thousands of computing nodes dedicated to searching the vast amount of collected data for gravitational waves.

“The ET methodology is ingenious and the technological requirements are jaw-droppingly demanding. We researchers are so excited about this project because with ET we will be able to obtain information about the most distant regions of the universe. We can expect exciting discoveries in the field of gravitational wave astronomy”, says Karsten Danzmann.

University of Birmingham: School of Physics and Astronomy
The Gravitational Wave group within the School of Physics and Astronomy at the University of Birmingham has been involved in the Einstein Telescope from its very beginning and is now contributing to work packages 3 (chaired by Andreas Freise) and 4.

Dr. Andreas Freise from the University of Birmingham's School of Physics and Astronomy who leads the optical design of the Einstein Telescope says: "The Einstein Telescope is an amazing instrument, it combines many new ideas and technologies to create the most sensitive instrument listening to the faint echos in the fabric of space and time". He knows several complex gravitational wave detectors from first hand experience: "I helped building the GEO600 and Virgo detectors and later had the chance to contribute to the optical design for Advanced Virgo. Now, leading the optical design of the Einstein Telescope has been a great opportunity for me, using hands-on experience to design something completely new."

Prof. Alberto Vecchio who leads the LIGO Scientific Collaboration activities at the University of Birmingham, said: "We expect to achieve the first direct detection of gravitational waves with Advanced LIGO, to which our group has made direct contributions. As new ideas mature, the Einstein Telescope becomes the next natural step in the quest for observing the universe with new "eyes", and a bold step beyond Advanced LIGO: with its exquisite sensitivity the Einstein Telescope will enable us to produce precise maps of black holes, unveiling many of their mysteries, and possibly peek into the first moments of cosmic history after the Big Bang."

The research programme of the Gravitational Wave Group at the University of Birmingham is centred on the observation of the universe in the gravitational wave band, and on testing gravity at new scales. The group's expertise in theory and data analysis complements the experimental activities, and underpins the wide- ranging theoretical and observational programme. This is focused on studies of gravitational wave sources and relevant astrophysical scenarios, the development of efficient analysis techniques and the search for gravitational waves in the data of LIGO, Virgo and GEO600.

University of Glasgow: Institute for Gravitational Research (IGR)
The Institute for Gravitational Research (IGR) at the University of Glasgow has 40 years experience of pioneering efforts in the gravitational wave field and now contributes to ET under the leadership of Prof. Sheila Rowan; Director of the Institute for Gravitational Research on the following tasks:

Work Package 2 (Suspension requirements): 
The suspensions of ET ́s optical elements must provide the necessary attenuation from seismic and acoustic noise and must implement the control strategy necessary to keep the interferometer at its working point. The IGR has a high level of expertise in the study of mechanical losses of materials at room and low temperature losses and thus contributes strongly in this area. IGR scientists applied this expertise to the conceptual design of ET ́s overall cryogenic suspension. As the group has also unparalleled experience in the design and construction of monolithic final stage suspension so also contributed strongly to the last stage suspension design tasks of ET.

Work Package 3 (Topology identification): 
Within the German-British gravitational wave project GEO the IGR is developing several aspects of advanced interferometry. In particular, expertise in all-reflective interferometers, and in radiation pressure effects in high power coupled cavity systems, informed the preliminary design of ET ́s interferometer topology and options for signal readout.

Work Package 4 (Astrophysics issues): 
Glasgow has a high level of expertise and experience in determining the astrophysical implications of detector data. Hence the IGR scientists study the cosmological implications of z~1 neutron star coalescences, the requirements for joint (trigger mode) observations with the Square Kilometer Array and optimisation of the detector for galactic pulsar observations.

The IGR has a substantial research programme centred on the detection and analysis of gravitational wave signals. The IGR members are co-founders of the GEO collaboration and the LIGO Scientific Collaboration, with significant investment from UK funding councils (STFC and SFC) and the University of Glasgow underpinning their state of the art experimental laboratories. The experimental expertise in the areas of low thermal noise suspensions for interferometric detectors and advanced optical topologies for interferometers has helped shape the worldwide field of gravitational wave detection with the novel suspension technologies they developed for the GEO instrument being adopted for the US ‘Advanced LIGO’ detector system upgrade, supported by a capital investment from STFC (on which Glasgow is UK lead institution) and a variant adopted for use in upgrades to the Virgo detectors, along with developments of our advanced interferometric techniques.

"Today, European researchers are presenting an exciting programme of gravitational wave detection involving instruments that are unbelievably sensitive able to sense changes in distances, resulting from the effects of periodic space-time distortions on a mass, much smaller than an atomic nucleus. UK scientists, supported by STFC and SFC, are not only pioneers in developing a significant part of the novel ET technology, but also in formulating detailed theoretical predictions about gravitational wave events across the Universe and the associated patterns of frequencies we can expect to see", says Sheila Rowan.