SC1/ GRAVITATIONAL WAVE ASTRONOMY
Since the dawn of civilization astronomy has been a driving force for progress and development in science and society. Light, or more generally electromagnetic radiation, is our most important window to the universe, since it can traverse the vast expanses of space . However, many astrophysical processes, and even entire regions of space, are entirely inaccessible to us, because none of their electromagnetic signals reaches us, or they are too weak to be measured. In contrast to photons, gravitational waves, as predicted by Einstein , can propagate virtually without interaction with any type of matter. They develop, when extremely large masses are accelerated . It is for this reason, that gravitational wave astronomy will open another, entirely new window to the cosmos.
After 50 years of international scientific efforts, 2015 a team of scientists from Advanced LIGO , the currently most sensitive gravitational wave interferometer, succeeded: the measured the first gravitational wave [5, 6], a feat which not expected before 2017 [7, 8]. This milestone will lay the foundations for harnessing gravitational waves as tool for astronomy.
By virtue of their fundamentally different mechanism of interacting with matter, gravitational waves can reach us from regions of the universe that are opaque to electromagnetic radiation and of which our knowledge of the fundamental physics is minimal. The results of their observation have the potential to dramatically increase our understanding of the formation, current state, and ultimate fate of the universe. Many fields of science will profit, among them :
- Fundamental physics: As binary systems undergo a spiraling collapse and merge to a black hole, particularly intense emissions of gravitational waves can be expected. Observations of this kind will allow for a direct test of Einstein's theory of general relativity in strong gravitational fields.
- Elementary particle physics: Likewise, the gravitational-wave observations of spiraling binary systems may carry information about the fundamental properties of matter as well as the equations of state in extreme conditions and environments such as the interior of neutron stars.
- Cosmology: Directly detecting the signatures of black hole formation will yield a new perspective on the evolution of the universe and allow for an accurate census of black holes between 10 and 1000 sun masses.
- Cosmography: Intensity-resolved gravitational wave observations provide an alternative approach for determining the absolute brightness of celestial objects, and in turn an independent degree of freedom to measure Hubble's constant and other cosmological parameters.
- Astrophysics: Gravitational waves originating from supernova explosions carry information about the dynamics of collapsing stars.
In order to address these questions, the sensitivity of gravitational wave detectors has to be increased. A design study for a third-generation detector ("Einstein telescope" ET, the first genuine gravitational wave observatory from the beginning of 2028) is envisioned in a joint developmental road map  between 57 research institutions from Europe, the US and Asia.
At its heart lies a cryogenic interferometer (ET-LF) comprised of high-performance photonic components that will enable the telescope to perform its unique scientific role . The requirements to the optic componenents of ET-LF are extremely high, so they cannot be implemented with the current state of the art. Some desired properties are outlined in the design study below.
ENABLER: LOW-NOISE CRYOGENIC NANOSTRUCTURES
On the fundamental level, the performance and sensitivity of gravitational wave detectors are limited by the thermal noise of the involved optical components, in particular their amorphous functional coatings . Whereas certain incremental improvements in their low-temperature noise characteristics  have to be made.
Currently, two alternative approaches exist towards addressing these challenges in the construction of functional gravitational wave observatories:
- Crystalline AlxGa1-xAs layer systems 
- Resonant waveguide gratings in silicon 
The advantages of resonant waveguide gratings are rooted in their monolithic character. By dispensing with additional materials, deformations and stress can be avoided even under cryogenic conditions . This is a considerable advantage over crystalline layer systems whose different thermal coefficient of expansion causes mechanical stresses in the cryogenic application scenario and can lead to a destruction of . For these reasons, NPL will leverage its strong capabilities in nanostructure technology, focus its efforts on resonant waveguide gratings, and collaborate closely with users to bring this field to technological maturity.
For the manufacturing of the optics, only sequential processing methods (electron beam lithography, scanning beam interference lithography) combined with ultra-precise positioning systems can fulfill the extreme demands in precision. However, the high cost of the massive silicon mirror substrates imposes high risks on any direct-writing approach. Consequently, replicative methods could be the method of choice if they can be scaled to the necessary size of the imprint mold. In either case, the key technological challenges are:
- Fabrication of the imprint mold (electron beam lithography or scanning-beam interference lithography as well as etching- and coating techniques)
- Replication on the mirror substrate (nanoimprint lithography as well as etching and coating techniques
Currently, none of the hardware and process steps are ready for use. The development of technological solutions for this science case is being secured by the establishment of a junior research group at the project partner PtB.
The listed investments for equipment and facilities relate to the final configuration for the fabrication of optics according to the current state of knowledge in the gravitational wave astronomy. In addition, it will be advisable to also include a development phase for the fabrication of imprint molds and masks and test the imprint and etching technologies on smaller substrates with relaxed demands on precision.
IMPORTANT PROJECTS OF THE NPL-PARTNERS
- Electrode Housing Box Metrology for Lisa Pathfinder
at European Space Agency (ESA) in cooperation with Airbus Defence and Space link.
- High resolution interferometer based on reflective optical components for SFB/Transregio 7 Gravitational Wave Astronomy
at Deutsche Forschungsgesellschaft (DFG) in cooperation with Leibniz Universität Hannover, Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut), Eberhard Karls Universität Tübingen, Max-Planck-Institut für Astrophysik (MPIA) link.
- High-reflection waveguide coatings of detector test masses for SFB/Transregio 7 Gravitational Wave Astronomy at Deutsche Forschungsgesellschaft (DFG) in Zusammenarbeit mit Leibniz Universität Hannover, Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut), Eberhard Karls Universität Tübingen, Max-Planck-Institut für Astrophysik (MPIA) link.
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 B.P. Abbott, R.Abbott, T.D. Abbott etal. Properties of the binary black hole merger gw150914. Phys. Rev. Lett., 116:241102 (2016). doi: 10.1103/PhysRevLett.116.241102.
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 G.D. Cole, W.Zhang, M.J. Martin etal. Tenfold reduction of brownian noise in high-reflectivity optical coatings. Nat. Photonics, 7:644 (2013). doi: 10.1038/nphoton.2013.174.
 F.Brückner, D.Friedrich, T.Clausnitzer etal. Realization of a monolithic high-reflectivity cavity mirror from a single silicon crystal. Phys. Rev. Lett., 104:163903 (2010). doi: 10.1103/PhysRevLett.104.163903.
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Gravitational wave detectors and observatories
- Basic Physics
- Nuclear Physics
- Elementary Particle Physics
- Low-noise cryogenic Nano optics
Gravitational waves […] will open a new window to probe fundamental physics processes in regions and at energy scales hitherto not accessible.
(aspera – Astroparticle physics for Europe)
A dominant contribution [to the overall thermal noise] is […] caused by mechanical loss of the high reflective […] coatings.
(Einstein Telescope Design Study)