The first detection on Earth of a gravitational wave signal from the coalescence of a binary black hole system in 2015 established a new era in astronomy, allowing the scientific community to observe ...the Universe with a new form of radiation for the first time. More than five years later, many more gravitational wave signals have been detected, including the first binary neutron star coalescence in coincidence with a gamma ray burst and a kilonova observation. The field of gravitational wave astronomy is rapidly evolving, making it difficult to keep up with the pace of new detector designs, discoveries, and astrophysical results. This Special Issue is, therefore, intended as a review of the current status and future directions of the field from the perspective of detector technology, data analysis, and the astrophysical implications of these discoveries. Rather than presenting new results, the articles collected in this issue will serve as a reference and an introduction to the field. This Special Issue will include reviews of the basic properties of gravitational wave signals; the detectors that are currently operating and the main sources of noise that limit their sensitivity; planned upgrades of the detectors in the short and long term; spaceborne detectors; a data analysis of the gravitational wave detector output focusing on the main classes of detected and expected signals; and implications of the current and future discoveries on our understanding of astrophysics and cosmology.
The sensitivity of current and planned gravitational wave interferometric detectors is limited, in the most critical frequency region around 100 Hz, by a combination of quantum noise and thermal ...noise. The latter is dominated by Brownian noise: thermal motion originating from the elastic energy dissipation in the dielectric coatings used in the interferometer mirrors. The energy dissipation is a material property characterized by the mechanical loss angle. We have identified mixtures of titanium dioxide ( TiO2 ) and germanium dioxide ( GeO2 ) that show internal dissipations at a level of 1 × 10−4, low enough to provide improvement of almost a factor of 2 on the level of Brownian noise with respect to the state-of-the-art materials. We show that by using a mixture of 44% TiO2 and 56% GeO2 in the high refractive index layers of the interferometer mirrors, it would be possible to achieve a thermal noise level in line with the design requirements. These results are a crucial step forward to produce the mirrors needed to meet the thermal noise requirements for the planned upgrades of the Advanced LIGO (Laser Interferometer Gravitational-Wave Observatory) and Virgo detectors.
Machine learning has emerged as a popular and powerful approach for solving problems in astrophysics. We review applications of machine learning techniques for the analysis of ground-based ...gravitational-wave (GW) detector data. Examples include techniques for improving the sensitivity of Advanced Laser Interferometer GW Observatory and Advanced Virgo GW searches, methods for fast measurements of the astrophysical parameters of GW sources, and algorithms for reduction and characterization of non-astrophysical detector noise. These applications demonstrate how machine learning techniques may be harnessed to enhance the science that is possible with current and future GW detectors.
Advanced gravitational wave interferometers are the second generation of high sensitivity detectors aiming at the direct observation of gravitational waves of astrophysical origin. To improve the ...sensitivity tenfold around the most sensitive frequency region at 100Hz with respect to first generation instruments, several new techniques are being implemented. This paper focuses on the output mode cleaner (OMC), which is a resonant cavity, placed at the main output port of the interferometer. The OMC plays the role of a passive spatial and frequency filter for the beam carrying the gravitational wave signal. Such a cavity is crucial to reach the design sensitivity of advanced detectors. So far, the proper resonance condition of the laser beam was ensured by actively controlling the optical length of the OMC. We propose a new scheme: in order to keep the OMC at resonance, the laser frequency is controlled instead of the OMC length. This approach no longer requires actuators on the OMC, allowing an improvement of the OMC in terms of filtering capabilities, noise performances and simplicity. We show how to implement this technique in the control acquisition sequence, and the sensing and control strategy of advanced detectors.
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