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Gravitational Wave Astronomy

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Gravitational­wave astronomy is now a reality thanks to the first detection of a gravitational wave signal by the LIGO and Virgo Collaboration. This result was achieved during the first science run (O1) of the two Advanced LIGO detectors that lasted from mid September 2015 to mid January 2016. During 2016 also the Advanced Virgo interferometer will start observations, after some years
focused on subsystem upgrades. NEWS researchers have been directly involved in this discovery, as members of the LIGO­Virgo collaboration. Advanced LIGO and Advanced Virgo represent the second generation of gravitational wave detectors, with a design sensitivity about 10 times larger than the previous generation. This improvement in sensitivity corresponds to a volume 1000 times larger that can be probed, enhancing by the same factor the rate GW transient signals that will become detectable. The final jump in sensitivity, that will be reached in successive steps, is the result of different improvements in the detector.
The power of the laser beam is increased to reduce the high­frequency shot noise, requiring the introduction of a compensation system to avoid the negative effect of optical distorsions induced by the absorbed light.
The thermal noise is suppressed by using a sophisticate mirror suspension technology based on silica fibers, and by enhancing the mirror itself which is larger and with higher optical performances. Here a crucial issue is the optimization of the reflective coating of the mirror, which is the dominant source of thermal noise in the more sensitive region of the detectors, a problem which still needs to be investigated. More advanced improvements are foreseen in perspective, such as the use of non classical states of light (squeezed vacuum) to reduce quantum noise, and the implementation of subtraction techniques to reduce gravity gradient noise in the low frequency region. NEWS participants from Virgo will be directly committed to the design and
implementation of these updates.
The first detection of GW was related to the merger of two massive black holes (BH), but there are many other promising sources of GWs. First of all, the mergers of other compact objects binaries, e.g neutron star­neutron star (NS­NS), and neutron star­black holes (NS­BH), that are expected to produce a transient GW signal. Rotating neutron stars should produce a continuous, substantially periodic GW signal. A supernova event can generate a short burst of gravitational waves. Furthermore, we expect a continuous stochastic background that should be due to the sum of many unresolved sources, of astrophysical or cosmological origin. The first objective of the LIGO/Virgo collaboration is the direct detection of all these sources. However with the final design sensitivity, or with an enhanced one, it will become possible to study in a more and more accurate way the parameters of the source. This will open a new era where accurate tests of general relativity, nuclear physics and, in perspective, theories of fundamental interactions will be possible. Detecting a GW signal and extracting physical parameters from it requires sophisticated analysis
techniques. These are often quite demanding from a computational point of view and there is a large space for their improvement. In order to localize the source and to study the polarization of the signal in an accurate way at least three non aligned detectors are needed. Data analysis is a truly collaborative effort of the LIGO/Virgo collaboration, and frequent contacts between the members of the collaboration are required.
Very soon other detectors, such as KAGRA in Japan, will join this effort, allowing a further increase of the precision and sensitivity of the detector network. NEWS project is expected to give a significant improvement to these activities.

Useful Links

Virgo web site

LIGO web site at the California Institute of Technology

LIGO web site at the Massachussets Institute of Technology

Kagra web site at the National Astronomical Observatory of Japan (NAOJ)

Kagra web site at the Institute of Cosmic Rays Research (ICRR) of the University of Tokyo