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Gravitational waves: a new way to explore the Universe

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Einstein TelescopeTowards Gravitational wave astronomy

Gravitational waves (GW) are ripples in the fabric of space-time produced by violent events in the universe, like collisions of black holes and supernova explosions. GWs are emitted by accelerating masses much in the same way as electromagnetic waves are produced by accelerating charges - such as electrons in an antenna. Predicted by Albert Einstein in 1916 as a consequence of his General Theory of Relativity, the direct detection of GWs will open a new astronomy, allowing totally new insights into the universe. With GWs, we could probe parts of the universe hidden by cosmic dusts, or give a completely different perspective to astronomical events both visible and invisible in the electromagnetic spectrum.


Artist view of the ET observatory, composed by three superposed interferometric detectors (red, blue and green), arranged underground in a triangle. Each interferometer has two 10 km arms, coinciding with two sides of the triangle. The depth will be at least 150 m (Credit: Nikhef).


What is the Einstein Telescope (ET)?

ET will be a 3rd generation GW observatory. The conceptual design is being developed in a European-wide effort, supported by the European Commission under the Seventh Framework Programme. GWs, being perturbations of the structure of space-time, produce minuscule changes in the relative distances of free masses. Each ET interferometer will monitor such distances shooting laser beams in two directions and studying the interference pattern after beam reflection on mirror test masses put at kilometric distances.

ET constitutes a natural evolution with respect to first generation interferometers (Virgo, LIGO, GEO600) and Einstein Telescopewith respect to the “advanced” ones (expected to be operative by around 2015). The sensitivity will be improved with an arm length of 10 km, instead of 3 km as in Virgo. Further improvements will come from cryogenic mirrors to fight against thermal noise and from being underground, to reduce seismic noise.

With a sensitivity 10 times better than advanced detectors, ET will be able to explore a Universe region with a radius of billions of light years, collecting thousands of events (GW bursts) per year of observation.

 Artist view of the laser beam propagating inside a vacuum pipe.


gravitational wavesTechniques and challenges

Attaining the goal sensitivity will imply pushing beyond present limits several components and techniques. The mirrors will have diameters above 0.5 m in order to allow large beam sizes and will be cooled down below 20 K to lower thermal noise. Both requirements push towards crystalline mirror materials (silicon, sapphire), with high thermal conductivity and low thermal expansion coefficient. Special coating materials and doping techniques will be demanded to cope with the new materials and cryogenics requirements. The laser power will be of the order of 1 kW to reduce the shot noise (photon number statistical fluctuations). Pushing seismic noise limit down to 1 Hz will require 20 m tall mirror suspensions together with sophisticated control strategies. Finally “quantum non demolition” methods will be used to enhance the sensitivity in frequency ranges where interesting GW sources are expected.

Representation of gravitational waves produced by two merging black holes (Credit: NASA).


Timeline and Funding

Second generation detectors activity will begin after 2015, with expected detection rates up to tens of events per year. Time coincidences among several detectors will disentangle genuine signals and localize the source by arrival time difference. Coincidences with other “Magnificent Seven” detectors will help understand the Einstein Telescope studynature of the original astrophysical phenomena. The production of second generation detectors will likely continue to the start of ET exploitation, in the second half of the 2020-2030 decade. Meanwhile LISA, the space detector, will extend the detectable frequency range below the mHz. The combination of LISA and ET may allow following the GW train emitted by an inspiralling binary star for days before coalescence. This will constitute an unprecedented laboratory to study matter in strong gravity, probing General Relativity to extreme limits. Funding ET after the Design Study will require a first phase for executive design and prototypes. The second phase, by far more challenging, will start with the site acquisition and will end with the commissioning of the first interferometer. The other two interferometers will follow, staged in turn by a few years. The full enterprise, with an overall cost slightly less than one billion Euros, will require a wide international cooperation.


The study will define requirements and select possible candidate sites. Seismic noise affects ET both by shaking the mirror suspension point and by moving directly the mirror through the Newtonian attraction by the soil mass. Going underground both effects are reduced also at low frequency
(2-3 Hz). The site selection has to take into account geological characteristics of the soil, human activities and environmental conditions (wind, ocean waves, ...).



European Gravitational Observatory - administrative manager              

Istituto Nazionale di Fisica Nucleare - Italy

Max-Planck-Institut für Gravitationsphysik - Germany

Centre National de la Recherche Scientifique - France

NIKHEF - The Netherlands

University of Birmingham - United Kingdom

University of Glasgow - United Kingdom

Cardiff University - United Kingdom

Contacts, web:

Michele Punturo ET Scientific Coordinator

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Harald Lueck ET Scientific Vice-Coordinator

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> http://www.et-gw.eu