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News Article | February 15, 2017
Site: phys.org

The novel mode filter for laser beams in the LG33 mode, which was developed at the AEI. Top: mode filter in the laboratory. Bottom: schematic of the mode filter. Credit: Noack/Max Planck Institute for Gravitational Physics One year ago, the first direct detection of gravitational waves was announced. Laser experts from the Max Planck Institute for Gravitational Physics (Albert Einstein Institute; AEI), from the Leibniz Universität Hannover, and from the Laser Zentrum Hannover e.V. (LZH) played leading roles in this discovery, because their super-precise laser technology at the heart of the LIGO instruments in the USA enabled the detection of weak gravitational-wave signals. Now, AEI researchers have presented two new technologies capable of further increasing the sensitivity of future gravitational-wave detectors. The Max Planck Society now strengthens the development of laser systems for third-generation gravitational-wave detectors. The AEI, in collaboration with the LZH, receives over the next five years 3.75 million Euro research funding for the development of novel lasers Zentrum Hannover receives over the next five years 3.75 million Euro research funding for the development of novel lasers and stabilization methods. "We have made two important breakthroughs," says Apl. Prof. Benno Willke, leader of the laser development group at the AEI. "Our work is another step towards using a novel type of laser beam profile in interferometric gravitational-wave detectors. Furthermore, we have shown how to increase the power stability of the high-power lasers used in the detectors. These are important steps towards the future of gravitational-wave astronomy." The results were published in the renowned science journal Optics Letters and were highlighted by the editors. The beams of all laser systems currently used in gravitational-wave detectors have higher intensity at the centre than at the edges. This leads to an undesirable strong influence of mirror surface fluctuations on the measurement precision of gravitational-wave detectors. This so-called thermal noise can be reduced by a more homogeneous laser intensity distribution. In 2013 a team with AEI involvement showed how more homogeneous high-power laser beams in the so-called LG mode can be created. Now, Andreas Noack has studied in his MSc thesis in Benno Willke's team how these laser beams can be fed into future gravitational-wave detectors. The first step on the way into the detector is a device known as a pre-mode cleaner, which optimizes the beam profile and reduces beam jitter. Willke's team showed that the new LG beam is incompatible with the pre-mode cleaners currently in use. The researchers also showed how to solve this problem. They developed a new pre-mode cleaner, which is compatible with the LG laser beams. "The design of the next-generation gravitational wave detectors is not set," says Willke. "Therefore, we are testing different types of lasers to have as many options for new gravitational wave detectors as possible. We now have made a big step ahead with the promising LG beams." All interferometric gravitational-wave detectors like LIGO, Virgo, and GEO600 rely on laser systems that keep their high output power stable over years and that show very little short timescale power fluctuations. Benno Willke's research group plays a world-wide leading role in this research area. They constructed the laser systems for GEO600 and Advanced LIGO, without which the first direct detection of gravitational waves in September 2015 would not have been possible. Now, Jonas Junker has further refined the existing power stabilization system in his MSc thesis in Willke's team. A part of the laser light is picked off and distributed on multiple photodetectors to precisely determine the total laser power. If it varies, the main laser power is corrected accordingly. In their experiment, the scientists extended the current system by adding, among other things, another photodetector to also control and correct the pointing of the laser beam. The improved power stabilization scheme has been successfully applied to the 35 Watt laser system of the 10 meter prototype interferometer at the AEI. The prototype is used by researchers in Hannover for demonstrations and tests of technologies for the third generation of detectors and for research on quantum mechanical effects in these instruments. The level of power stability reached is five times higher than that in comparable experiments of other groups. This value agrees very well with results from isolated table-top experiments. "An experiment in the well isolated environment of an optical laboratory is completely different from a complex large-scale experiment like the 10 meter prototype. We have shown for the first time that it is possible to transfer the excellent stability level from a table-top experiment," says Willke. "We show that these photodiode arrays work as expected, meaning it should also be possible to achieve this high stability with the identical multi-photodetector arrays used in Advanced LIGO." Explore further: LIGO discovery named Science's 2016 Breakthrough of the Year More information: Andreas Noack et al. Higher-order Laguerre–Gauss modes in (non-) planar four-mirror cavities for future gravitational wave detectors, Optics Letters (2017). DOI: 10.1364/OL.42.000751

Schutz B.F.,Albert Einstein Institute | Schutz B.F.,University of Cardiff
Classical and Quantum Gravity | Year: 2011

This paper develops a general framework for studying the effectiveness of networks of interferometric gravitational wave detectors and then uses it to show that enlarging the existing LIGO-VIRGO network with one or more planned or proposed detectors in Japan (LCGT), Australia, and India brings major benefits, including much larger detection rate increase than previously thought. I focus on detecting bursts, i.e. short-duration signals, with optimal coherent data-analysis methods. I show that the polarization-averaged sensitivity of any network of identical detectors to any class of sources can be characterized by two numbers - the visibility distance of the expected source from a single detector and the minimum signal-to-noise ratio (SNR) for a confident detection - and one angular function, the antenna pattern of the network. I show that there is a universal probability distribution function (PDF) for detected SNR values, which implies that the most likely SNR value of the first detected event will be 1.26 times the search threshold. For binary systems, I also derive the universal PDF for detected values of the orbital inclination, taking into account the Malmquist bias; this implies that the number of gamma-ray bursts associated with detected binary coalescences should be 3.4 times larger than expected from just the beaming fraction of the gamma burst. Using network antenna patterns, I propose three figures of merit (f.o.m.'s) that characterize the relative performance of different networks. These measure (a) the expected rate of detection by the network and any sub-networks of three or more separated detectors, taking into account the duty cycle of the interferometers, (b) the isotropy of the network antenna pattern, and (c) the accuracy of the network at localizing the positions of events on the sky. I compare various likely and possible networks, based on these f.o.m.'s. Adding any new site to the planned LIGO-VIRGO network can dramatically increase, by factors of 2-4, the detected event rate by allowing coherent data analysis to reduce the spurious instrumental coincident background. Moving one of the LIGO detectors to Australia additionally improves direction finding by a factor of 4 or more. Adding LCGT to the original LIGO-VIRGO network not only improves direction finding but will further increase the detection rate over the extra-site gain by factors of almost 2, partly by improving the network duty cycle. Including LCGT, LIGO-Australia, and a detector in India gives a network with position error ellipses a factor of 7 smaller in area and boosts the detected event rate a further 2.4 times above the extra-site gain over the original LIGO-VIRGO network. Enlarged advanced networks could look forward to detecting 300-400 neutron star binary coalescences per year. © 2011 IOP Publishing Ltd.

Lundgren A.,Albert Einstein Institute | O'Shaughnessy R.,University of Wisconsin - Milwaukee
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2014

In coming years, gravitational-wave detectors should find black hole-neutron star (BH-NS) binaries, potentially coincident with astronomical phenomena like short gamma ray bursts. These binaries are expected to precess. Gravitational-wave science requires a tractable model for precessing binaries, to disentangle precession physics from other phenomena like modified strong field gravity, tidal deformability, or Hubble flow; and to measure compact object masses, spins, and alignments. Moreover, current searches for gravitational waves from compact binaries use templates where the binary does not precess and are ill-suited for detection of generic precessing sources. In this paper we provide a closed-form representation of the single-spin precessing waveform in the frequency domain by reorganizing the signal as a sum over harmonics, each of which resembles a nonprecessing waveform. This form enables simple analytic calculations of the Fisher matrix for use in template bank generation and coincidence metrics, and jump proposals to improve the efficiency of Markov chain Monte Carlo sampling. We have verified that for generic BH-NS binaries, our model agrees with the time-domain waveform to 2%. Straightforward extensions of the derivations outlined here (and provided in full online) allow higher accuracy and error estimates. © 2014 American Physical Society.

Dittrich B.,Albert Einstein Institute | Hohn P.A.,University Utrecht
Classical and Quantum Gravity | Year: 2012

A general canonical formalism for discrete systems is developed, which can handle varying phase space dimensions and constraints. The central ingredient is Hamilton's principal function that generates canonical time evolution and ensures that the canonical formalism reproduces the dynamics of the covariant formulation following directly from the action. We apply this formalism to simplicial gravity and (Euclidean) Regge calculus, in particular. A discrete forward/backward evolution is realized by gluing/removing single simplices step by step to/from a bulk triangulation and amounts to Pachner moves in the triangulated hypersurfaces. As a result, the hypersurfaces evolve in a discrete multi-fingered time through the full Regge solution. Pachner moves are an elementary and ergodic class of homeomorphisms and generically change the number of variables, but can be implemented as canonical transformations on naturally extended phase spaces. Some moves introduce a priori free data that, however, may become fixed a posteriori by constraints arising in subsequent moves. The end result is a general and fully consistent formulation of canonical Regge calculus, thereby removing a longstanding obstacle in connecting covariant simplicial gravity models to canonical frameworks. The presented scheme is, therefore, interesting in view of many approaches to quantum gravity, but may also prove useful for numerical implementations. © 2012 IOP Publishing Ltd.

Sindoni L.,Albert Einstein Institute
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2011

In this paper, we examine the possibility to implement some form of emergent Newtonian gravity in a generic multicomponent Bose-Einstein condensate. Parallel to what happens for the emergence of low-energy Lorentz invariance, strong requirements have to be imposed on the underlying condensed matter model. We will show, within a simplified model, that the presence of a global symmetry alleviates the problems associated with Lorentz violation, allowing the presence of a long range potential, to which the analog matter fields (the quasiparticles) are coupled following a weaker form of equivalence principle. © 2011 The American Physical Society.

Baratin A.,Albert Einstein Institute | Oriti D.,Albert Einstein Institute
Physical Review Letters | Year: 2010

We introduce a dual formulation of group field theories as a type of noncommutative field theories, making their simplicial geometry manifest. For Ooguri-type models, the Feynman amplitudes are simplicial path integrals for BF theories. We give a new definition of the Barrett-Crane model for gravity by imposing the simplicity constraints directly at the level of the group field theory action. © 2010 The American Physical Society.

Prix R.,Albert Einstein Institute | Shaltev M.,Albert Einstein Institute
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

Coherent wide parameter-space searches for continuous gravitational waves are typically limited in sensitivity by their prohibitive computing cost. Therefore, semicoherent methods (such as StackSlide) can often achieve a better sensitivity. We develop an analytical method for finding optimal StackSlide parameters at fixed computing cost under ideal conditions of gapless data with Gaussian stationary noise. This solution separates two regimes: an unbounded regime, where it is always optimal to use all the data, and a bounded regime with a finite optimal observation time. Our analysis of the sensitivity scaling reveals that both the fine- and coarse-grid mismatches contribute equally to the average StackSlide mismatch, an effect that had been overlooked in previous studies. We discuss various practical examples for the application of this optimization framework, illustrating the potential gains in sensitivity compared to previous searches. © 2012 American Physical Society.

Bahr B.,University of Cambridge | Dittrich B.,Albert Einstein Institute
New Journal of Physics | Year: 2010

In Regge calculus, space-time is usually approximated by a triangulation with flat simplices. We present a formulation using simplices with constant sectional curvature adjusted to the presence of a cosmological constant. As we will show, such a formulation allows us to replace the length variables by three- or four-dimensional dihedral angles as basic variables. Moreover, we will introduce a first-order formulation, which, in contrast to using flat simplices, does not require any constraints. These considerations could be useful for the construction of quantum gravity models with a cosmological constant. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

Babak S.,Albert Einstein Institute | Sesana A.,Albert Einstein Institute
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

We study the capability of a pulsar timing array to individually resolve and localize in the sky monochromatic gravitational wave (GW) sources. Given a cosmological population of inspiralling massive black hole binaries, their observable signal in the pulsar timing array domain is expected to be a superposition of several nearly-monochromatic GWs of different strength. In each frequency bin, the signal is neither a stochastic background nor perfectly resolvable in its individual components. In this context, it is crucial to explore how the information encoded in the spatial distribution of the array of pulsars might help recovering the origin of the GW signal, by resolving individually and locating in the sky the strongest sources. In this paper we develop a maximum-likelihood-based method finalized to this purpose. We test the algorithm against noiseless data showing that up to P/3 sources can be resolved and localized in the sky by an array of P pulsars. We validate the code by performing blind searches on both noiseless and noisy data containing an unknown number of signals with different strengths. Even without employing any proper search algorithm, our analysis procedure performs well, recovering and correctly locating in the sky almost all the injected sources. © 2012 American Physical Society.

Ryan J.P.,Albert Einstein Institute
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

Tensor models and, more generally, group field theories are candidates for higher-dimensional quantum gravity, just as matrix models are in the 2D setting. With the recent advent of a 1/N expansion for colored tensor models, more focus has been given to the study of the topological aspects of their Feynman graphs. Crucial to the aforementioned analysis were certain subgraphs known as bubbles and jackets. We demonstrate in the 3D case that these graphs are generated by matrix models embedded inside the tensor theory. Moreover, we show that the jacket graphs represent (Heegaard) splitting surfaces for the triangulation dual to the Feynman graph. With this in hand, we are able to reexpress the Boulatov model as a quantum field theory on these Riemann surfaces. © 2012 American Physical Society.

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