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.
We report on a high precision measurement of gravitational acceleration using ultracold strontium atoms trapped in a vertical optical lattice. Using amplitude modulation of the lattice intensity, an uncertainty ∆g/g ≈ 10 −7 was reached by measuring at the 5 th harmonic of the Bloch oscillation frequency. After a careful analysis of systematic effects, the value obtained with this microscopic quantum system is consistent with the one we measured with a classical absolute gravimeter at the same location. This result is of relevance for the recent interpretation of related experiments as tests of gravitational redshift and opens the way to new tests of gravity at micrometer scale.PACS numbers: 91.10. Pp, 03.75.Dg, 37.25.+k, 37.10.Jk Atom interferometry, and in general methods based on quantum interference of ultracold atoms, were largely used in recent years for gravitational physics experiments and new exciting prospects can be envisioned in the near future [1]. For example, Raman interferometry was used for precise measurements of Earth's gravitational acceleration g [2] and its gradient [3], for determining the value of the gravitational constant [4,5], for a possible redefinition of the kg [6], and for geophysical applications [7]. Schemes based on Bloch oscillations of atoms trapped in vertical optical lattices were also used to measure gravity with the possibility of combining high sensitivity and micrometric spatial resolution [8][9][10]. The results of atom interferometry experiments were interpreted as tests of the isotropy of post-Newtonian gravity [11], of quantum gravity [12], and of gravitational redshift [13]. Prospects include high precision tests of the weak equivalence principle [14,15], the detection of gravitational waves [16,17], and future experiments in space [18].So far, however, Bloch oscillation measurements had limited accuracy compared to Raman atom interferometers. Here, we present a precision measurement of gravitational acceleration g with a new method based on ultracold 88 Sr atoms trapped in an amplitude-modulated vertical optical lattice [19] and compare the result with the value obtained with a classical absolute gravimeter based on a Michelson interferometer with one arm including a freely-falling corner-cube. We also improved our previous observation of long-lived Bloch oscillations [9] and discuss the precision of the two methods for the determination of g. In addition to demonstrating the sensitivity and accuracy of this new method, our data can be interpreted as a measurement of the gravitational redshift to the Compton frequency of Sr matter waves, as suggested by H. Müller et al. [13]. Our data surpasses previous Bloch oscillation measurements by one order of magnitude, making it the most precise test of the gravitational redshift based on Bloch oscillations at micrometric spatial scales. The interpretation of atom interferometer redshift tests is complicated by special relativistic time dilation since the atoms are moving [20,21], but Bloch oscillations experiments with ...
We report on an all-sky search with the LIGO detectors for periodic gravitational waves in the frequency range 50 -1000 Hz and with the frequency's time derivative in the range ÿ1 10 ÿ8 Hz s ÿ1 to zero. Data from the fourth LIGO science run (S4) have been used in this search. Three different semicoherent methods of transforming and summing strain power from short Fourier transforms (SFTs) of the calibrated data have been used. The first, known as StackSlide, averages normalized power from each SFT. A ''weighted Hough'' scheme is also developed and used, which also allows for a multiinterferometer search. The third method, known as PowerFlux, is a variant of the StackSlide method in which the power is weighted before summing. In both the weighted Hough and PowerFlux methods, the weights are chosen according to the noise and detector antenna-pattern to maximize the signal-to-noise ratio. The respective advantages and disadvantages of these methods are discussed. Observing no evidence of periodic gravitational radiation, we report upper limits; we interpret these as limits on this radiation from isolated rotating neutron stars. The best population-based upper limit with 95% confidence on the gravitational-wave strain amplitude, found for simulated sources distributed isotropically across the sky and with isotropically distributed spin axes, is 4:28 10 ÿ24 (near 140 Hz). Strict upper limits are also obtained for small patches on the sky for best-case and worst-case inclinations of the spin axes.
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