An Overview of Gravitational-Wave Sources

TL;DR

This article surveys the landscape of gravitational-wave sources across ground-based HF and space-based LF observatories, detailing detector sensitivities, source populations, and the information encoded in GW signals. It highlights the complementary science returns from advanced LIGO-class detectors and LISA, including binary inspirals, BH mergers, NS tides, GRBs, supernovae, spinning NSs, EMRIs, and SMBH mergers, along with the stochastic background probes of the early universe. The work emphasizes precise parameter estimation, the potential for tests of strong-field gravity, EOS constraints from tidal disruption, and the prospects for multi-messenger astronomy with GRBs and neutrinos, while acknowledging significant uncertainties in rates and waveform modeling. Overall, it frames a roadmap where near-term detections will test GR in new regimes and long-term observations will illuminate black-hole astrophysics, galaxy formation, and fundamental physics through gravitational-wave cosmology.

Abstract

We review current best estimates of the strength and detectability of the gravitational waves from a variety of sources, for both ground-based and space-based detectors, and we describe the information carried by the waves.

Figures (5)

• Figure 1: The noise $\tilde{h}(f)$ in several planned LIGO interferometers plotted as a function of gravity-wave frequency $f$, and compared with the estimated signal strengths $\tilde{h}_s(f)$ from various sources. The signal strength $\tilde{h}_s(f)$ is defined in such a way that, wherever a signal point or curve lies above the interferometer's noise curve, the signal, coming from a random direction on the sky and with a random orientation, is detectable with a false alarm probability of less than one per cent; see the text for greater detail and discussion.
• Figure 2: The inspiral, merger, and ringdown waves from equal-mass black-hole binaries as observed by LIGO's initial interferometers: The distance to which the waves are detectable (right axis) and the signal-to-noise ratio for a binary at 1Gpc (left axis), as functions of the binary's total mass (bottom axis). (Figure adapted from Flanagan and Hughes hughes_flanagan.)
• Figure 3: (a) The signal-to-noise ratio for the inspiral, merger and ringdown waves from fast-spinning, equal-mass black-hole binaries at a fiducial distance of 1Gpc, as observed by advanced (LIGO-II) interferometers. (b) The cosmological redshift out to which these waves can be seen, assuming a cosmology with Hubble expansion rate $H_o = 65$ km/s/Mpc, cold-matter density 0.4 of that required to close the universe $\Omega_M = 0.4$, and vacuum energy density (cosmological constant) 0.6 of closure, $\Omega_\Lambda = 0.6$. (Figure from Ref. thorne_LIGOII, based on the LIGO-II wide-band noise curve in Fig. \ref{['fig:SourceSensitivity']} and the analysis of Flanagan and Hughes hughes_flanagan.)
• Figure 4: LISA's sky- and polarization-averaged rms noise $h_n(f)$ in a bandwidth equal to frequency, lisa_noise_curve compared with the strengths of the waves from several low-frequency sources. The wave strengths are plotted so the height above the noise curve is the $S/N$ for a wave search using optimal signal processing; see text for further detail. For frequency-sweeping waves (arrowed curves) the dots are, from left to right, the signal strength and frequency 1 year, 1 month, and one day before the end. The thick-dashed curve is a background of waves from WD-WD binaries that are so numerous they cannot be removed from the data, and so constitute a noise when searching for other waves.
• Figure 5: The waves from equal-mass, supermassive black-hole binaries as observed by LISA in one year of integration time. Plotted horizontally is the binary's total mass $M$ multiplied by 1 plus the binary's cosmological redshift $z$. The solid, dashed, and tight-dotted curves refer to the left axis, which shows the estimated S/N for LISA's observations when the binary is 1 Gpc from earth (redshift $z\simeq 0.2$). The wide-spaced dots are curves of constant binary mass $M$, for use with the right axis, which shows the luminosity distance and redshift to which the binary can be detected, with S/N = 5 (look at the intersection of a wide-dotted curve with a solid, dashed, or tight-dotted curve and then project horizontally onto the right axis). (The assumed cosmology is $H_o=65$ km/s/Mpc, $\Omega_\Lambda = 0.6$, $\Omega_M = 0.4$.) The bottom-most curves are the signal strengths after one year of signal integration, for BH/BH binaries 10 years and 100 years before their merger. (Figure adapted from Flanagan and Hughes hughes_flanagan.)