Present and Future of Gravitational Wave Astronomy

Gravitational waves (GW) are propagating perturbations of the space-time metric, generated by time-varying mass distributions. Their existence was predicted more than 100 years ago, in 1916, as a consequence of the General Relativity theory of gravitation [1]. The most intense sources of gravitational waves are astrophysical objects such as neutron stars and black holes. Despite the large amount of energy emitted in the form of gravitational waves by the coalescence of compact binary systems, the radiation has to travel tens or thousands of Mpc before reaching earth, where it can be detected. This, and the weakness of the coupling to matter, are the reasons why the direct observation of gravitational waves has been such a challenging scientific endeavor, and why the signal from the first binary black hole coalescence, detected in 2015, produced a relative change in the local earth space-time metric of only about ${10}^{-21}$ [2], see Figure 1. Many sources of noises had to be understood and overcome [3], so that the two Advanced LIGO observatories [4,5] were sensitive enough to detect this event with high signal-to-noise ratio, opening the era of gravitational-wave astronomy and astrophysics. This event was the first demonstration of the existence of binary black holes, and the first confirmation that General Relativity is still valid in the intense-field regime involved in the collision of two compact objects such as black holes.

The first detection, dubbed GW150914, was only the first step of the newly-born gravitational-wave astronomy. The second major milestone was the detection of the first binary neutron star merger, GW170817 [6], only two years after the first event, see Figure 2. By then, the LIGO and Virgo [7,8] detectors had joined forces, and had already detected several more binary black hole events. However, the importance of the detection of two neutron stars colliding was on par with the first discovery. Not only we could detect gravitational waves from the two objects, proving without a doubt that they had masses compatible with neutron stars, but we also observed a short gamma-ray burst in coincidence with the event, proving the connection between the two phenomena. The sky localization provided by the gravitational wave observation by multiple detectors, LIGO and Virgo, allowed optical telescopes to identify the aftermath of the collision, and subsequently follow up in the entire electromagnetic spectrum. The promise of multi-messenger astronomy was indeed fulfilled.

The network of instruments actively observing the gravitational-wave universe is going to grow with the KAGRA detector [9,10]. As a first step, KAGRA operated in conjunction with the GEO 600 observatory [11] after the LIGO and Virgo O3 run ended [12]. In the three main observing runs carried out so far, the LIGO and Virgo collaborations have reported a total of 90 events with high probability of being of astrophysical origin [13], see Figure 3, an average of one event every five days in the last run.

Signals from coalescing binary systems [14] are just the beginning of the discoveries that gravitational-wave detectors will bring. Short-duration signals are expected to be produced by other sources, the most important being core-collapse supernovae [15]: both modeled and unmodeled searches are ongoing for such signals. Additionally, continuous-wave signals are expected to be produced by rotating pulsars [16]. Although they have not been detected yet, the upper limits obtained so far are already more stringent that the spin-down limits for many systems. With improved sensitivities, ground-based gravitational-wave observatories might be finally able to detect stochastic backgrounds [17] either from the confusion noise of coalescing binary systems, of from a cosmological origin.

The increasingly large number of gravitational wave detections made it possible to study the astrophysics of the sources [18] and the behavior of gravity in the intense field regime [19]. To be able to infer precise information on those fundamental topics, it is crucial to continuously increase the detector sensitivity, to improve the signal-to-noise ratio of the events, but it is also of the utmost importance to calibrate the detector output with high precision [20] and to remove any additional spurious disturbance that might mimic astrophysical signals [21].

The instruments that allowed all those scientific discoveries are laser Michelson interferometers [22]. The phase of two coherent laser beams propagating along orthogonal directions is changed by the passage of gravitational waves, modifying the interference condition at the beam recombination. The phase change is proportional to the differential fluctuation in the length of the two arms, due to the strain induced by gravitational-wave signals. Therefore, the detector sensitivity is directly proportional to the baseline length of the Michelson interferometer: this is the main reason to build km-scale detectors. Even with such large scale Michelson interferometers, the fundamental limitation due to laser quantum noise would not have allowed the sensitivity needed to detect gravitational waves. Over the years, the initial design of the detectors evolved to include more and more complex techniques aimed at increasing the sensitivity. The Michelson interferometer arms were replaced by resonant Fabry–Perot cavities to increase the phase accumulation by the laser beam in response to gravitational-wave signals. Power-recycling cavities were added to increase the circulating laser power. Signal-recycling cavities were added to shape the detector bandwidth. Quantum-measurement techniques such as squeezed vacuum injections were implemented [23] to beat the laser shot-noise limit. Sophisticated seismic isolation and suspension systems were developed to reduce noise from ground motion [24]. Cryogenic operation of the interferometer test masses was implemented [10] to reduce thermal noise. The result of all those efforts are the currently operating Advanced Virgo [8], Advanced LIGO [5] and KAGRA observatories [10].

At the time this special issue is being published, the major ground-based detectors are undergoing an upgrade period. They are expected to be back in observation in 2023, with improved sensitivity to all kind of gravitational-wave sources. Further improvements are planned in the coming years for those detectors [25], and design studies are well underway for the next generation observatories. The next decades will see the European project Einstein Telescope [26] and the US project Cosmic Explorer [27] move from the drawing board to construction and operation. The currently existing detector will undergo upgrades, to move to silicon test masses operated at cryogenic temperatures [25], providing not only a significant improvement in sensitivity, but also serving as a crucial research and development platform for Einstein Telescope and Cosmic Explorer. Together with the planned space-borne gravitational-wave detectors LISA [28] and DECIGO [29], they hold the promise of detecting signals from the coalescence of compact binary systems from the entire observable universe, many of those events with very large signal-to-noise ratio, opening the era of high precision gravitational-wave physics, astrophyisics and cosmology.

Figure 3. Collection of time-frequency maps of all the LIGO and Virgo detections in the first three observation runs. Graphics reproduced from [30].

Figure 3. Collection of time-frequency maps of all the LIGO and Virgo detections in the first three observation runs. Graphics reproduced from [30].