Gravitational-wave observatory
The present-day generation of laser interferometers has reached the necessary sensitivity to detect gravitational waves from astronomical sources, thus forming the primary tool of gravitational-wave astronomy.
In June 2023, four pulsar timing array collaborations presented the first strong evidence for a gravitational wave background of wavelengths spanning light years, most likely from many binaries of supermassive black holes.
Thus, even waves from extreme systems such as merging binary black holes die out to a very small amplitude by the time they reach the Earth.
Astrophysicists predicted that some gravitational waves passing the Earth might produce differential motion on the order 10−18 m in a LIGO-size instrument.
[2] A simple device to detect the expected wave motion is called a resonant mass antenna – a large, solid body of metal isolated from outside vibrations.
Strains in space due to an incident gravitational wave excite the body's resonant frequency and could thus be amplified to detectable levels.
While there were several cases of unexplained deviations from the background signal, there were no confirmed instances of the observation of gravitational waves with these detectors.
These modern cryogenic forms of the Weber bar operated with superconducting quantum interference devices to detect vibration (ALLEGRO, for example).
Some of them continued in operation after the interferometric antennas started to reach astrophysical sensitivity, such as AURIGA, an ultracryogenic resonant cylindrical bar gravitational wave detector based at INFN in Italy.
MiniGRAIL is based at Leiden University, and consists of an exactingly machined 1,150 kg (2,540 lb) sphere cryogenically cooled to 20 mK (−273.1300 °C; −459.6340 °F).
[5] The spherical configuration allows for equal sensitivity in all directions, and is somewhat experimentally simpler than larger linear devices requiring high vacuum.
MiniGRAIL is highly sensitive in the 2–4 kHz range, suitable for detecting gravitational waves from rotating neutron star instabilities or small black hole mergers.
[7] This allows the masses to be separated by large distances (increasing the signal size); a further advantage is that it is sensitive to a wide range of frequencies (not just those near a resonance as is the case for Weber bars).
These are at 90 degree angles to each other, with the light passing through 1 m (3 ft 3 in) diameter vacuum tubes running the entire 4 kilometres (2.5 mi).
Interferometric detectors are limited at high frequencies by shot noise, which occurs because the lasers produce photons randomly.
Supernovae and neutron star or black hole mergers should have larger amplitudes and be more interesting, but the waves generated will be more complicated.
By studying a fixed set of pulsars across the sky, these arrays should be able to detect gravitational waves in the nanohertz range.
On 17 March 2014, astronomers at the Harvard-Smithsonian Center for Astrophysics announced the apparent detection of the imprint gravitational waves in the cosmic microwave background, which, if confirmed, would provide strong evidence for inflation and the Big Bang.
[28] There are currently two detectors focusing on detections at the higher end of the gravitational-wave spectrum (10−7 to 105 Hz)[citation needed]: one at University of Birmingham, England, and the other at INFN Genoa, Italy.
The Birmingham detector measures changes in the polarization state of a microwave beam circulating in a closed loop about one meter across.
The INFN Genoa detector is a resonant antenna consisting of two coupled spherical superconducting harmonic oscillators a few centimeters in diameter.
The Chongqing University detector is planned to detect relic high-frequency gravitational waves with the predicted typical parameters ~ 1010 Hz (10 GHz) and h ~ 10−30 to 10−31.
[30] A torsion-bar antenna (TOBA) is a proposed design composed of two, long, thin bars, suspended as torsion pendula in a cross-like fashion, in which the differential angle is sensitive to tidal gravitational wave forces.
[33] Atom interferometry is proposed to extend the detection bandwidth in the infrasound band (10 mHz – 10 Hz),[34][35] where current ground based detectors are limited by low frequency gravity noise.
[36] A demonstrator project called Matter wave laser based Interferometer Gravitation Antenna (MIGA) started construction in 2018 in the underground environment of LSBB (Rustrel, France).
[40][41] The interferometric detectors deployed in the 1990s and 2000s were proving grounds for many of the foundational technologies necessary for initial detection and are commonly referred to as the first generation.
[41][40] The second generation of detectors operating in the 2010s, mostly at the same facilities like LIGO and Virgo, improved on these designs with sophisticated techniques such as cryogenic mirrors and the injection of squeezed vacuum.
