Assembly of the radiative cryostat.
Cryogenic inertial sensors.
A platform for detecting gravitational waves Gravitational waves were first directly observed in 2015 and over 100 more have since been detected. The next generation of detectors are now under development, with researchers in the SILENT project developing a platform to isolate the Einstein telescope from background disturbance and increase sensitivity at low frequencies, as Professor Christophe Collette explains. The Einstein telescope is designed to detect gravitational waves at a higher level of sensitivity than currently possible, helping scientists identify the merger of black holes far out in space and opening up new insights into the origins of the universe. The telescope itself is set to be located deep underground when it enters operation, to minimise disturbance from ground motion of the earth. “Because of the seismic activity of the Earth some disturbances or waves are generated at the surface, which leads to a degree of motion, around the order of a micron,” explains Professor Christophe Collette, head of the Precision Mechatronics Laboratory at the University of Liege. A second major factor behind the decision to locate the instrument underground is the goal of minimising disturbance from Newtonian noise, which is caused by fluctuations in the Earth’s gravitational field. “If the distance between the mirror in the Einstein telescope and the
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mass surrounding it changes, then the force which is applied by this body on the mirror is also going to change. This in turn is going to induce some motion of the mirror,” says Professor Collette.
SILENT project. “In the project we aim to develop instrumentation and control strategies to boost the performance of the Einstein telescope at low frequencies,” he outlines. The SILENT team is developing a
“In the project we aim to develop instrumentation and control strategies to boost the performance of the Einstein telescope at low frequencies.” SILENT project This represents a powerful set of reasons for putting the telescope underground, with two main sites currently being considered, one on the island of Sardinia and another in the area around the border between Germany, the Netherlands and Belgium. Controlling the movement and vibration of the telescope will help enhance its sensitivity to gravitational waves, a topic central to Professor Collette’s work as Principal Investigator of the ERC-backed
platform to essentially isolate the telescope from these sources of disturbance and so increase measurement sensitivity. “We have basically developed two platforms in the project. One is very much modular, designed with certain technical challenges in mind,” says Professor Collette. “Then, based on the lessons learned with this platform, we have developed and built a second. This platform is now in operation, it’s performing pretty effectively, and we are currently working to improve the performance.”
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The platform itself is designed to support the mirrors within the Einstein telescope, which are a key part of the instrument as a whole. In current gravitational wave detectors like LIGO, Virgo and KAGRA, light is split using a beam splitter, which then travels down two perpendicular arms in a vacuum tube to mirrors, before the light bounces back and recombines. “The interference of these two reflected beams can then be analysed to detect gravitational waves,” explains Professor Collette. While the exact design of the Einstein telescope is still to be finalised, Professor Collette says the key role of the SILENT platform will be to isolate the mirrors in the instrument. “There is nothing to isolate inbetween these mirrors, in the vacuum tube. However, the mirrors have to be extremely stable, and to achieve that we need to develop an effective suspension system,” he continues. “The SILENT platform will provide an environment which is much quieter than the ground, down to frequencies around 10 mHz. Below that, we want the mirror to move together with the ground.” This platform is now being used in a prototype mirror called E-TEST, which will give scientists the opportunity to test and refine different technologies. The E-TEST prototype is a single, full-scale mirror, cooled down to cryogenic temperatures and isolated from seismic motion at low frequencies. “One key feature of the next
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The (not so) SILENT team at PML.
generation of gravitational wave detectors like the Einstein telescope is that they will use cryogenic mirrors which work at very low temperatures, close to absolute zero. These mirrors will be much larger and the internal noise of the instrument will be pushed down, so the sensitivity will be increased,” outlines Professor Collette. It is hoped this increased sensitivity will enable the next generation of detectors to detect more gravitational waves than currently possible. “About 100 gravitational waves have been detected with existing instruments since 2015, when the first was detected by LIGO,” continues Professor Collette. “We expect to measure something like 100,000 gravitational waves a year with the Einstein telescope.”
Gravitational waves The main priority with the Einstein telescope is to detect the merger of black holes, which
Installation of the SILENT prototype at PML.
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