Stochastic Gravitational-Wave Backgrounds: Current Detection Efforts and Future Prospects

Abstract

The collection of individually resolvable gravitational wave (GW) events makes up a tiny fraction of all GW signals that reach our detectors, while most lie below the confusion limit and are undetected. Similarly to voices in a crowded room, the collection of unresolved signals gives rise to a background that is well-described via stochastic variables and, hence, referred to as the stochastic GW background (SGWB). In this review, we provide an overview of stochastic GW signals and characterise them based on features of interest such as generation processes and observational properties. We then review the current detection strategies for stochastic backgrounds, offering a ready-to-use manual for stochastic GW searches in real data. In the process, we distinguish between interferometric measurements of GWs, either by ground-based or space-based laser interferometers, and timing-residuals analyses with pulsar timing arrays (PTAs). These detection methods have been applied to real data both by large GW collaborations and smaller research groups, and the most recent and instructive results are reported here. We close this review with an outlook on future observations with third generation detectors, space-based interferometers, and potential noninterferometric detection methods proposed in the literature.

Consistency of the Parkes Pulsar Timing Array Signal with a Nanohertz Gravitational-wave Background

“…Noise processes with similar and not common noise spectra could arise from, e.g., spin noise, stochastic irregularities in the rotation of the pulsars (Shannon & Cordes 2010). In fact, the spin noise model discussed in Meyers et al (2021) suggests a spectral index of timing residuals to be γ = 4 (where the power spectral density is modeled as P( f ) ∝ f −γ ), which would be difficult to distinguish from the nanohertz gravitational-wave background expected from binary supermassive black holes, γ = 13/3, with current PTAs (see e.g., Figure 13 in Renzini et al 2022, to inspect measurement uncertainties). Empirical models for spin noise in millisecond pulsars predict timing noise having spectral indices and amplitudes similar to that expected of the gravitational-wave background.…”

Utilizing the null stream of the Einstein Telescope

“…It is demonstrated for compact binaries in [38] when assuming the same noise power spectra in ET components and employing Equation 11for the noise PSD. The resulting average signal PSD had a characteristic spectral slope of the stochastic GW background, P (f ) ∝ f −7/3 [93]. Using Equation 8for the noise PSD allows us to extend the application to the case with different instrumental noise levels in constituent ET interferometers.…”

Targeted search for the kinematic dipole of the gravitational-wave background

“…The right ascension and declination of the solar dipole in equatorial coordinates (chosen to match the spherical harmonic decomposition in ( 23)), as inferred by Planck [36], are (α sol , δ sol ) = (167.942 ± 0.007, −6.944 ± 0.007) deg. (9) Neglecting the orbital dipole for now, this allows us to derive the spherical harmonics associated with the solar dipole,…”

“…The direct detection of gravitational waves (GWs) from compact binary coalescences by the LIGO/Virgo/KAGRA Collaboration (LVK) has initiated the era of GW astronomy [1][2][3][4]. As well as transient, individually-resolvable signals such as these, one also expects a gravitational-wave background (GWB) due to the superposition of GWs produced by many weak, independent and unresolved sources of cosmological or astrophysical origin [5][6][7][8][9]. Once detected, this background will provide interesting astrophysical information about the formation of black holes and neutron stars throughout cosmic time [10,11], and will potentially shed light on early universe cosmology and particle physics beyond the Standard Model [12][13][14][15][16][17].…”