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This paper presents a complete description of Virgo, the French-Italian gravitational wave detector. The detector, built at Cascina, near Pisa (Italy), is a very large Michelson interferometer, with 3 km-long arms. JINST 7 P03012In this paper, following a presentation of the physics requirements, leading to the specifications for the construction of the detector, a detailed description of all its different elements is given. These include civil engineering infrastructures, a huge ultra-high vacuum (UHV) chamber (about 6000 cubic metres), all of the optical components, including high quality mirrors and their seismic isolating suspensions, all of the electronics required to control the interferometer and for signal detection. The expected performances of these different elements are given, leading to an overall sensitivity curve as a function of the incoming gravitational wave frequency.This description represents the detector as built and used in the first data-taking runs. Improvements in different parts have been and continue to be performed, leading to better sensitivities. These will be detailed in a forthcoming paper.

A stochastic background of gravitational waves is expected to arise from a superposition of a large number of unresolved gravitational-wave sources of astrophysical and cosmological origin. It should carry unique signatures from the earliest epochs in the evolution of the Universe, inaccessible to standard astrophysical observations(1). Direct measurements of the amplitude of this background are therefore of fundamental importance for understanding the evolution of the Universe when it was younger than one minute. Here we report limits on the amplitude of the stochastic gravitational-wave background using the data from a two-year science run of the Laser Interferometer Gravitational-wave Observatory(2) (LIGO). Our result constrains the energy density of the stochastic gravitational-wave background normalized by the critical energy density of the Universe, in the frequency band around 100 Hz, to be <6.9 X 10(-6) at 95% confidence. The data rule out models of early Universe evolution with relatively large equation-of-state parameter(3), as well as cosmic (super) string models with relatively small string tension(4) that are favoured in some string theory models(5). This search for the stochastic background improves on the indirect limits from Big Bang nucleosynthesis(1,6) and cosmic microwave background(7) at 100Hz

The Virgo collaboration has just concluded its first long science run (VSR1). In these four months the detector achieved a good duty cycle, larger than 80%, and an average horizon distance for binary neutron star system sources of about 4 Mpc. An intense commissioning activity was resumed after the run was complete to further increase the performances of the detector and to prepare the Virgo+ upgrades. The detector performances during the first science run and the last commissioning achievements are briefly discussed here.

We present the results of a search for gravitational-wave bursts (GWBs) associated with 137 gamma-ray bursts (GRBs) that were detected by satellite-based gamma-ray experiments during the fifth LIGO science run and first Virgo science run. The data used in this analysis were collected from 2005 November 4 to 2007 October 1, and most of the GRB triggers were from the Swift satellite. The search uses a coherent network analysis method that takes into account the different locations and orientations of the interferometers at the three LIGO-Virgo sites. We find no evidence for GWB signals associated with this sample of GRBs. Using simulated short-duration No. 2, 2010 SEARCH FOR GWBs ASSOCIATED WITH GRBs USING LIGO AND VIRGO 1441 (<1 s) waveforms, we set upper limits on the amplitude of gravitational waves associated with each GRB. We also place lower bounds on the distance to each GRB under the assumption of a fixed energy emission in gravitational waves, with a median limit of D ∼ 12 Mpc(E iso GW /0.01 M c 2 ) 1/2 for emission at frequencies around 150 Hz, where the LIGO-Virgo detector network has best sensitivity. We present astrophysical interpretations and implications of these results, and prospects for corresponding searches during future LIGO-Virgo runs.