Gravitational waves carry information about regions of our universe which are otherwise obscured by interstellar matter or are 'invisible' due to the lack of emitting electro-magnetic radiation. Despite their prediction almost 90 years ago and 4 decades of experimental effort (summarized in this article) gravitational waves still await their direct detection. This article gives an introduction into the field of gravitational wave detection and points to more detailed papers within this proceedings issue.

The Laser Interferometer Gravitational-Wave Observatory (LIGO) consists of detector facilities in Hanford, WA and Livingston, LA, USA, separated by 3000 km. Laser interferometry is used to monitor displacements of freely suspended mirrors, separated by 2 - 4 km, along perpendicular arms at each facility. The initial LIGO detector design sensitivity corresponds to measuring gravitational-wave induced differential displacements of order 1 millifermi over the 4-km arms. Progress in the commissioning and operation of these detectors will be reviewed.

The GEO 600 laser interferometer with 600m armlength is part of a worldwide network of gravitational wave detectors. GEO 600 is unique in having advanced multiple pendulum suspensions with a monolithic last stage and in employing a signal recycled optical design. This paper describes the recent commissioning of the interferometer and its operation in signal recycled mode.

The experimental search for the direct interaction of exotic new particles in underground detectors is entering a new phase in which the experimental sensitivities are beginning to probe into the favoured parameter space for the neutralinos expected within supersymmetric extensions to the standard particle physics model (SUSY). This is happening at a time when the evidence for cold dark matter is stronger than ever. This review of the field gives a summary of current status, both the evidence and the underground searches, and looks forward to expected progress in the coming years.

Recent advances in gravitational wave detectors mean that we can start to make astrophysically important statements about the physics of neutron stars based on observed upper limits to their gravitational luminosity. Here we consider statements we can already make about a selection of known radio pulsars, based on data from the LIGO and GEO600 detectors, and look forward to what could be learned from the first detections.

To meet the overall isolation and alignment requirements for the optics in Advanced LIGO, the planned upgrade to LIGO, the US laser interferometric gravitational wave observatory, we are developing three sub-systems: a hydraulic external pre-isolator for low frequency alignment and control, a two-stage active isolation platform designed to give a factor of ~1000 attenuation at 10 Hz, and a multiple pendulum suspension system that provides passive isolation above a few hertz. The hydraulic stage uses laminar-flow quiet hydraulic actuators with millimeter range, and provides isolation and alignment for the optics payload below 10 Hz, including correction for measured Earth tides and the microseism. This stage supports the in-vacuum two-stage active isolation platform, which reduces vibration using force feedback from inertial sensor signals in six degrees of freedom. The platform provides a quiet, controlled structure to mount the suspension system. This latter system has been developed from the triple pendulum suspension used in GEO 600, the German/UK gravitational wave detector. To meet the more stringent noise levels required in Advanced LIGO, the baseline design for the most sensitive optics calls for a quadruple pendulum, whose final stage consists of a 40 kg sapphire mirror suspended on fused silica ribbons to reduce suspension thermal noise.

The LIGO Laboratory 40m prototype interferometer at Caltech is being commissioned to prototype an optical configuration for Advanced LIGO. This optical configuration has to control five length degrees of freedom, and its control topology will be significantly more complicated than any other present interferometers. This paper explains the method of sensing, controls and lock acquisition.

In a "Dual" gravitational wave (GW) detector a wide band sensitivity is obtained by measuring the differential displacement, driven by the GW, of the facing surfaces of two nested massive bodies mechanically resonating at different frequencies. A "selective readout" scheme, capable of specifically selecting the signal contributed by the vibrational modes sensitive to the gravitational waves, could then reduce the thermal noise contribution from the not sensitive modes. In a dual detector the sensitivity improvement in the displacement transduction could be pursued by means of mechanical amplification systems. This solution is innovative for the resonant GW detectors and we report about preliminary theoretical and experimental study.

The joint ESA/NASA mission LISA is a space-borne interferometer to detect and observe gravitational wave. The scientific goals of LISA are to determine the role of massive black holes (MBH) in galaxy evolution, to perform precision tests of General Relativity, determine the poupulation of ultra-compact obejcts in the galaxy and to probe the physics of the early Universe. The LISA mission was selected as an ESA Cornerstone and is included in NASA's 'Beyond Einstein' initiative, with a launch in the 2012/2013 time frame.

We discuss a polarisation based homodyne interferometer that demonstrates a promising sensitivity of approximately 3x10^-12 m/Hz^1/2. This performance figure is limited above 10Hz by the resolution of the current analogue-to-digital converter (ADC). Sensitivity below 10Hz is influenced by environmental factors and / or noise inherent in the laser. We then describe the development of a compact interferometric sensor, undertaken at The University of Birmingham, discussing its application as a zero-stiffness sensor for drag-free satellites and suggest a geometry of electrostatic actuator also with zero-stiffness.

Radio pulses emitted in the Atmosphere during the air shower development of high-energy primary cosmic rays were measured during the late 1960ies in the frequency range from 2 MHz to 520 MHz. Mainly due to difficulties with radio interference these measurements ceased in the late 1970ies. LOFAR (Low Frequency Array) is a new digital radio interferometer under development. Using high bandwidth ADCs and fast data processing it it will be able to filter out most of the interference. By storing the whole waveform information in digital form one can analyze transient events like air showers even after they have been recorded. To test this new technology and to demonstrate its ability to measure air showers a "LOFAR Prototype Station" (LOPES) is set up to operate in conjunction with an existing air shower array (KASCADE-Grande). The first phase consisting of 10 antennas is already running. It operates in the frequency range of 40 to 80 MHz, using simple short dipole antennas and direct 2nd Nyquist sampling of the incoming wave. It has proven to be able to do simple astronomical measurements, like imaging of a solar burst. It has also demonstrated how digital interference suppression and beamforming can overcome the problem of radio interference and pick out air shower events.

The Cern Axion Solar Telescope - CAST - uses a prototype 9 Tesla LHC superconducting dipole magnet to search for a hypothetical pseudoscalar particle, the axion, which was proposed by theory in the 1980s to solve the strong CP problem and which could be a dark matter candidate. In CAST a strong magnetic field is used to convert the solar axions to detectable photons via inverse Primakoff effect. The resulting X-rays are thermally distributed in the energy range of 1-7 keV and can be observed with conventional X-ray detectors. The most sensitive detector system of CAST is a pn-CCD detector originally developed for XMM-Newton combined with a Wolter I type X-ray mirror system. The combination of a focusing X-ray optics and a state of the art pn-CCD detector which combines high quantum efficiency, good spacial and energy resolution, and low background improves the sensitivity of the CAST experiment such that for the first time the axion photon coupling constant can be probed beyond the best astrophysical constraints. In this paper we report on the performance and status of the X-ray telescope and pn-CCD detector of CAST.

The algorithms for the detection of gravitational waves are usually very complex due to the low signal to noise ratio. In particular the search for signals coming from coalescing binary systems can be very demanding in terms of computing power, as in the case of the well known Standard Matched Filter Technique. To overcame this problem, we tested a Dynamic Matched Filter Technique, still based on Matched Filters, whose main advantage is the requirement of a lower computing power. In this work this technique is described, together with its possible application as a pre-data analysis algorithm. A parallel computing implementation of the algorithms is also described and tested. Finally the results on simulated data are reported.

In this paper we propose a new strategy for gravitational waves detection from coalescing binaries, using IIR Adaptive Line Enhancer (ALE) filters. This strategy is a classical hierarchical strategy in which the ALE filters have the role of triggers, used to select data chunks which may contain gravitational events, to be further analyzed with more refined optimal techniques, like the the classical Matched Filter Technique. After a direct comparison of the performances of ALE filters with the Wiener-Komolgoroff optimum filters (matched filters), necessary to discuss their performance and to evaluate the statistical limitation in their use as triggers, we performed a series of tests, demonstrating that these filters are quite promising both for the relatively small computational power needed and for the robustness of the algorithms used. The performed tests have shown a weak point of ALE filters, that we fixed by introducing a further strategy, based on a dynamic bank of ALE filters, running simultaneously, but started after fixed delay times. The results of this global trigger strategy seems to be very promising, and can be already used in the present interferometers, since it has the great advantage of requiring a quite small computational power and can easily run in real-time, in parallel with other data analysis algorithms.

The LISA Technology Package (LTP) aboard of LISA pathfinder mission is dedicated to demonstrate and verify key technologies for LISA, in particular drag free control, ultra-precise laser interferometry and gravitational sensor. Two inertial sensor, the optical interferometry in between combined with the dimensional stable Glass ceramic Zerodur structure are setting up the LTP. The validation of drag free operation of the spacecraft is planned by measuring laser interferometrically the relative displacement and tilt between two test masses (and the optical bench) with a noise levels of 10pm/√Hz and 10 nrad/√Hz between 3mHz and 30mHz. This performance and additionally overall environmental tests was currently verified on EM level. The OB structure is able to support two inertial sensors (≈17kg each) and to withstand 25 g design loads as well as 0...40°C temperature range. Optical functionality was verified successfully after environmental tests. The engineering model development and manufacturing of the optical bench and interferometry hardware and their verification tests will be presented.

The accretion of electrostatic charge in the isolated LISA test masses due to energetic particles in the space environment hinders the drag-free operation of the gravitational inertial sensors. Robust predictions of charging rates and associated stochastic fluctuations are therefore required for the exposure scenarios expected throughout the mission. We report on detailed charging simulations with the Geant4 toolkit, using comprehensive geometry and physics models, for galactic cosmic-ray protons and helium nuclei. These predict net charging rates of up to +100 elementary charges per second during the solar minimum period, decreasing by half at solar maximum. Charging from sporadic solar events involving energetic protons was also investigated. Other physical processes hitherto overlooked as potential charging mechanisms have been assessed. Significantly, the kinetic emission of very low-energy secondary electrons due to bombardment of the inertial sensors by primary cosmic rays and their secondaries can produce charging currents comparable with the Monte Carlo rates.

We discuss LISA observations in the frequency range 10-100 mHz and the implications for data analysis and parameter estimation in the context of solar-mass to intermediate mass black hole binaries.

LIGO is dedicated to the detection of gravitational waves. To achieve the design sensitivity of the proposed Advanced LIGO detectors, the seismic isolation system is required to isolate the interferometer mirrors from ground motion above 0.1 Hz. The dominant source of motion above 0.1 Hz is the microseismic peaks near 0.15 Hz. The system needs to isolate the payload from this motion by at least a factor of five in all three translational degrees of freedom. Tilt-horizontal coupling is the most challenging problem for seismic isolation below 1 Hz. Tilt-horizontal coupling results from the principle of equivalence: inertial horizontal sensors cannot distinguish horizontal acceleration from tilt motion. Tilt-horizontal coupling rises dramatically at low frequencies, which makes low frequency isolation difficult. Several techniques are used to address the tilt-horizontal coupling problem. The isolation platform is designed to separate horizontal motions from tilt motions. Feedback control to displacement sensors is used to command the platform in all degrees of freedom. These sensors are "corrected" by ground seismometers, using an optimal FIR filtering technique to separate tilt noise from horizontal acceleration. With these techniques, we obtained isolation factors of 10 to 20 simultaneously in all three degrees of freedom at 0.15 Hz.

One of the main requirements of a digital system for the control of interferometric detectors of gravitational waves is the computing power, that is a direct consequence of the increasing complexity of the digital algorithms necessary for the control signals generation. For this specific task many specialized non standard real-time architectures have been developed, often very expensive and difficult to upgrade. On the other hand, such computing power is generally fully available for off-line applications on standard Pc based systems. Therefore, a possible and obvious solution may be provided by the integration of both the real-time and off-line architecture resulting in a hybrid control system architecture based on standards available components, trying to get both the advantages of the perfect data synchronization provided by the real-time systems and by the large computing power available on Pc based systems. Such integration may be provided by the implementation of the link between the two different architectures through the standard Ethernet network, whose data transfer speed is largely increasing in these years, using the TCP/IP, UDP and raw Ethernet protocols. In this paper we describe the architecture of an hybrid Ethernet based real-time control system prototype we implemented in Napoli, discussing its characteristics and performances. Finally we discuss a possible application to the real-time control of a suspended mass of the mode cleaner of the 3m prototype optical interferometer for gravitational wave detection (IDGW-3P) operational in Napoli.

The French-Italian interferometric gravitational wave detector VIRGO is currently being commissioned. Its principal instrument is a Michelson interferometer with 3 km long optical cavities in the arms and a power-recycling mirror. This paper gives an overview of the present status of the system. We report on the presently attained sensitivity and the system’s performance during the recent commissioning runs.

The Australian Consortium for Gravitational Astronomy has built a High Optical Power Test Facility north of Perth, Western Australia. Current experiments in collaboration with LIGO are testing thermal lensing compensation, and suspension control on an 80m baseline suspended optical cavity. Future experiments will test radiation pressure instabilities and optical spring in a high power optical cavity with ~200kW circulating power. Once issues of operation and control have been resolved, the facility will go on to assess the noise performance of the high optical power technology through operation of an advanced interferometer with sapphire tests masses, and high performance suspension and isolation systems. The facility combines research and development undertaken by all consortium members, which latest results are presented.