First Virgo Science Run

Press Conference - May 22, 2007 Cascina, Pisa, Italy

PRESS INFORMATION

Introduction

On May 18th, the Virgo interferometer started its first science run. This is a major milestone in the hunt for Gravitational Waves (GW) that started decades ago with the so called resonant bar detectors. Virgo, the largest European detector, is joining the operating LIGO detectors in the US and the GEO interferometer in Germany. This best ever network of instruments will have, among other things, the capability to observe the coalescence of binary black holes in distant galaxies and provide direction information.

In this document we report on the present status of Virgo and of GW research in general, and on the plans for the near future. More details on Virgo physics and technology can be found on the appended EGO/Virgo leaflet and in the DVD “Virgo, whispers from a violent universe”.

The Physics Goals of the Search for GW

Gravitational Waves are deformations of space time produced by violent processes involving the acceleration of huge masses. Typical sources are supernovae explosions or two compact objects, such as black holes or neutron stars, tightly orbiting towards their final collapse. GW emission is a prediction of Albert Einstein’s general theory of relativity. Up to now only the effects of GW emissions have been observed. This indirect evidence for GW - the observation of a system of two pulsars by Joseph Taylor and Russel Hulse - was awarded the Nobel Prize for Physics in 1993. The first direct observation of GW would be of prime importance, opening up the field of GW astronomy and providing brand new data to further our understanding of gravitation and general relativity.

The prediction of an event rate is difficult because the strength of the GW is rather unknown, as in the supernova case, or because the population remains difficult to estimate (binary black holes systems for instance). These uncertainties are a direct consequence of the fact that the search for GW is a new field where a lot has still to be discovered and understood. The Initial Virgo detector will be looking for a very few rare events in the best case, while the advanced detectors must observe at least a handful of events.

GWs are expected to produce very weak effects on earth. The signals that Virgo may observe will be manifested in miniscule distortions of the 3km arms. These changes will be of the order of about one billion times smaller than the diameter of an atom. This explains why the challenges to be faced by GW detectors have pushed them to the cutting edge in many fields: with frontier technological developments spanning from metallurgy to super-mirrors, to the application of sophisticated control system, advanced computing and data analysis techniques.

Press Conference May 22, 2007 1/6 The Virgo detector

The Virgo detector for Gravitational Waves consists mainly of a Michelson laser interferometer made of two orthogonal arms, each being 3 kilometres in length. Multiple reflections between mirrors located at the extremities of each arm extend the effective optical length of each arm up to a length of 120 kilometres. Virgo is located within the site of the European Gravitational Observatory (EGO).

The frequency range of Virgo extends from 10 to several kHz. This range, as well as the very high sensitivity, should allow for the detection of gravitational radiation produced by supernovae and the coalescence of binary systems in the Milky Way and in outer galaxies, for instance from the Virgo cluster.

In order to reach the extreme sensitivity required to be sensitive to Gravitational Waves, the whole interferometer attains optical perfection and is extremely well isolated from the rest of the world. To achieve this, Italian and French scientists involved in the project, have developed the most advanced techniques in the field of high power ultra-stable lasers, high reflectivity mirrors, seismic isolation and position and alignment control.

In the field of optics, Virgo uses a new generation of ultra-stable lasers, and the most stable oscillator ever built. A specific optical coating facility has been built to produce extremely high quality mirrors combining the highest reflectivity (over 99,999 %), with nanometer surface control.

To avoid spurious motions of the optical components due to seismic noise; each one of them is isolated by a very elaborate 10m high system of compound pendulums. Because the presence of a residual gas would slightly perturb the measurements, the light beam must propagate under ultra high vacuum. The two tubes, each 3km long and 1.2m in diameter, are actually the largest ultra high vacuum vessels in Europe and the second largest in the world. The environment of the Virgo interferometer is quieter than that of a spacecraft orbiting the earth.

VIRGO: The Virgo Collaboration and EGO

The Virgo collaboration is the group of engineers and scientists from the different laboratories that designed, built and run the Virgo interferometer and analyze its data. Initially formed by French and Italian groups, the collaboration has recently welcomed a Dutch group and now consists of about 150 people from 12 different laboratories, and continues to expand.

EGO, the European Gravitational Observatory, is the laboratory created in 2001 to host Virgo and to allow its commissioning and operation. EGO has the status of a consortium, funded on an equal basis by CNRS and INFN in order to take care of all the infrastructure needed for the exploitation of Virgo and to foster research and collaboration in Europe in the field of Gravitational Waves research.

A total of about 180 scientists are working on Virgo today.

Virgo Today

The construction of Virgo, approved by CNRS and INFN in 1993, started at the Cascina site in 1996. The construction was completed in June 2003 and has been followed by a commissioning

Press Conference May 22, 2007 2/6 phase to activate and adjust all of the control systems of this unique detector and to bring the noise level of the detector to its design value.

After steady progress in the commissioning activity of this frontier research instrument, the Virgo detector is near to its design sensitivity. The sensitivity of Virgo is especially attractive at low frequency, where it is the best detector worldwide, whereas for the other frequencies Virgo has capabilities close to the LIGO detectors. This achievement opens the road to a four month long period of data taking, in coincidence with LIGO.

The First Virgo Science run

The first science run started on May 18th and will run for four months. Once started, Virgo will run day and night listening to all gravitational signals, which may arrive at any time and from any part of the Universe. A crew of operators and scientists is running and monitoring the instrument on a 24 hours a day, 7 days a week basis. The signals are detected, recorded and pre- analysed through an on-line computing system. These data are then made available to the scientific community for further analysis.

LIGO

The Laser Interferometer Gravitational-Wave Observatory (LIGO) is the other large facility dedicated to the detection of Gravitational Waves. It consists of two widely separated installations within the United States, operating in unison as a single observatory. The arm lengths of the three LIGO interferometers are 2 and 4 km.

LIGO is funded by the National Science Foundation (NSF) and was designed and constructed by a team of scientists from the California Institute of Technology, the Massachusetts Institute of Technology, and by industrial contractors. Construction of the facilities was completed in 1999. After years of construction and commissioning, the LIGO interferometers started their first long science run in fall 2005. This data taking is expected to be completed in late September 2007, at which point the LIGO instruments will be enhanced and upgraded, as will those of Virgo.

LIGO and VIRGO collaboration

LIGO and Virgo are pursuing a common goal: the detection of Gravitational Waves produced by astrophysical events. Such events will reach all the detectors, wherever their location, since GW are not stopped by the Earth. However, the response of the different detectors and the observed arrival times will depend on the source localisation. Consequently, the combined data will provide more information about the location of the source than either project alone could provide. Moreover, combining the data will give a much better chance of finding the first Gravitational Waves, and will allow for greater confidence in any detections.

This is why earlier this year (see previous press release), the LSC (LIGO Science Collaboration, including GEO) and VIRGO managements signed an agreement. Besides formalizing a deep collaboration lasting from the design phase of the various interferometers, the agreement signature started a new way of life for LIGO (intended as the whole LSC, including the British- German collaboration GEO) and VIRGO (written all in capitals, to indicate together the Virgo Collaboration and EGO). The full text can be read at https://pub3.ego-gw.it/codifier/includes/showTmpFile.php?doc=1632&calledFile=VIR-PLA-DIR-1000-223.pdf.

Press Conference May 22, 2007 3/6 After a transition period, full data sharing between the two projects began on May 18th, with the beginning of the first Virgo Science Run. The data collected together will be analyzed coherently as if coming from a single detector, made of several sensors distributed on the two sides of the Atlantic and on the East coast of the Pacific. This will bring to the collected data a statistical significance improvement and a great increase in confidence.

Another visible effect of the MoU is to have many common collaboration meetings; the first LIGO/VIRGO meeting took place in Baton Rouge (Louisiana), March 17-22; the second common meeting, here in Cascina, is occupying this whole week. The main subjects addressed in the meeting are: achievements and plans for the commissioning activity, technical discussions and plans on detector upgrades, results of the several data analyses performed on previous Weekend Runs, plans and results of data analysis.

The Future of Virgo

The first generation of interferometric detectors has a chance of detecting GWs for the first time ever. Nevertheless, the rate of detectable events expected on the basis of the astrophysical population models will be very low: in the most optimistic scenario a few events per year of compact-star binary coalescences are expected. Thus, even in the case of a successful first detection, such detectors will not really start GW astronomy.

After some additional commissioning at the end of the year, the future of Virgo consists of two main steps: “Virgo+” and “Advanced Virgo”, leading to enhanced detector sensitivity, and increasing GW detection probability by up to 1000 times and more. Similar upgrades are foreseen for LIGO and the detailed shutdown strategy for the LIGO, Virgo and GEO upgrades is jointly discussed to optimize the scientific results.

Virgo+

This is the first step of the Virgo upgrades planned for 2008. The laser power will be increased from 20 to 50W. New mirrors held by silica fibres and a thermal compensation system will be installed. The electronics will be upgraded. The interferometer optical configuration will not be changed and since the foreseen improvements are based on a well-established R&D program, they should require a limited commissioning time in 2009. These upgrades are modest enough to be funded as part of the operational costs.

The expected sensitivity improvement factor is about 3, corresponding to an event rate (or detection probability) 30 times that of the present. This intervention will be performed in parallel to a similar one to be undertaken on the LIGO interferometers (denominated “Enhanced LIGO”) and will be followed by a long period of data taking.

Advanced Virgo

Advanced Virgo corresponds to a set of major upgrades to reach a sensitivity that is 10 times better than the Virgo nominal sensitivity, corresponding to an event rate increase of about 1000 times. This requires significant changes such as increasing the laser power to 200W, changing the optical geometry and interferometer topology. The Advanced Virgo configuration is still under study, with a target year for starting the installation of 2011.

A similar plan, called “Advanced LIGO”, to be realized within the same time schedule is in progress. In both cases, significant changes to the optical set-ups are foreseen. In addition, the

Press Conference May 22, 2007 4/6 LIGO seismic isolation system will be completely rebuilt to extend the observation band down to about 10 Hz, a value that will essentially match the performance of the currently installed Virgo seismic isolation. The advanced interferometers are expected to be in operation by around 2013- 14.

Third generation interferometers

The concept of a third generation detector is already discussed among European scientists. Preliminary activities are being carried out under the ILIAS European program, with the participation of GEO and Virgo scientists. As part of this effort a design study has been proposed in the recent FP7 call for research proposals issued by the European Union. The study will aim to identify the technologies and design characteristics that could lead to a sensitivity more than a hundred times better than that of the existing first generation devices and to cover the complete frequency range that is potentially observable from the ground: from about 1 Hz to 10 kHz. This will increase the observable volume of the universe and, correspondingly, the event rate by a factor of more than one million.

This task will be achieved by combining all presently known scientific breakthroughs of measurement science in a new, probably underground, observatory. In fact, to create such a third generation Gravitational Wave detector is a formidable, but possible, task. To achieve this aim, progress is required in many fields, including: advanced lasers, emitting hundreds of Watts of continuous power; novel signal enhancing techniques; interferometry using diffractive optics; cryogenic cooling of critical optical components; vibration isolation techniques; and monolithic suspensions for the optical components.

The resonant bar detectors

In Europe there is a long tradition and excellent know-how in developing detectors for Gravitational Waves. Indeed, until about 10 to 15 years ago, Europe occupied the leading position in this field. Currently, several first generation Gravitational Waves detectors are still active in Europe. There are three cryogenic resonant bar detectors (AURIGA, EXPLORER and NAUTILUS) that are managed by INFN in Italy (Frascati (Rome) and Legnaro (Padova)) and at CERN. There sensitivity remains limited however, compared to the recent performance of the new interferometer detectors like Virgo.

New ideas are being developed in the AURIGA group, to study a method for signal enhancement by two coupled resonant detectors. This technique may bring resonant bar detectors at the level of present interferometers.

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GLOSSARY

CNRS The French Centre National de la Recherche Scientifique, one of the two Virgo funding agencies. EGO EGO, the European Gravitational Observatory, a consortium, funded by CNRS and INFN, which hosts the Virgo detector. FP7 Seventh "Framework programme" which is the main financial tool through which the European Union supports research and development activities. GEO The German-British interferometer. GEO is funded by the Max Planck Society (Germany) and the Science and Technology Facilities Council (UK) GW Gravitational Waves ILIAS Is an Integrated Infrastructure Initiative funded by the European Union to produce a focused, coherent and integrated project to improve the existing Astroparticle infrastructures in Europe INFN The Italian Istituto Nazionale di Fisica Nucleare, one of the two Virgo funding agencies LIGO The Laser Interferometer Gravitational-Wave Observatory. LSC The LIGO Scientific Collaboration, a group of 500 scientists at universities around the United States and in eight foreign countries that carry out LIGO research. It includes GEO members. NSF The U.S. National Science Foundation. The prime LIGO funding agency. Virgo The Virgo detector located near Pisa. VIRGO The Virgo collaboration and EGO.

Contact:

Carlo Bradaschia (Virgo Media contact) +39 050 752 340; [email protected] Severine Perus (EGO Secretariat) +39 050 752 325; [email protected]

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