Impact of facility timing and coordination for next-generation gravitational-wave detectors

TL;DR

The paper investigates how timing and coordination between next-generation gravitational-wave detectors (ET and CE) affect scientific capabilities, focusing on observation-times $T_{ m obs}$ needed to reach thresholds for multiple metrics across BBH, BNS, and PBH populations. Using Fisher-information forecasts via gwbench with bootstrapped populations and a fiducial merger rate, it shows that SNR-based metrics are robust to delays, while localization metrics are highly sensitive to network size and timing, making delays effectively network-wide interruptions for localization-driven science. The study finds that two XG detectors together (e.g., ET-2L+CE) dramatically shorten $T_{ m obs}$ for localization and multi-messenger targets, and that adding LIGO-India substantially mitigates localization delays for single-XG configurations. For PBH discernability and stochastic-background measurements, at least two XG detectors are essential, with stochastic sensitivity benefiting most from early, paired XG operation; these results guide the prioritization and planning of international GW infrastructure to maximize science return.

Abstract

While the Einstein Telescope and Cosmic Explorer proposals for next-generation, ground-based detectors promise vastly improved sensitivities to gravitational-wave signals, only joint observations are expected to enable the full scientific potential of these facilities, making timing and coordination between the efforts crucial to avoid missed opportunities. This study investigates the impact of long-term delays on the scientific capabilities of next-generation detector networks. We use the Fisher information formalism to simulate the performance of a set of detector networks for large, fiducial populations of binary black holes, binary neutron stars, and primordial black-hole binaries. Bootstrapping the simulated populations, we map the expected observation times required to reach a number of observations fulfilling scientific targets for key sensitivity and localization metrics across various network configurations. We also investigate the sensitivity to stochastic backgrounds. We find that purely sensitivity-driven metrics such as the signal-to-noise ratio are not strongly affected by delays between facilities. This is contrasted by the localization metrics, which are very sensitive to the number of detectors in the network and, by extension, to delayed observation campaigns for a detector. Effectively, delays in one detector behave like network-wide interruptions for the localization metrics for networks consisting of two next-generation facilities. We examine the impact of a supporting, current-generation detector such as LIGO India operating concurrently with next-generation facilities and find such an addition will greatly mitigate the negative effects of delays for localization metrics, with important consequences on multi-messenger science and stochastic searches.

Figures (8)

• Figure 1: Detector sensitivity curves used in this study, see Sec. \ref{['sec:networks']}. The A+ and A# curves are used for the LIGO-India detector I, ET-10 and ET-15 for ET configurations $\texttt{ET-}\triangle$ and $\texttt{ET-2L}$, respectively, and CE-40 for the CE facility $\texttt{CE}$.
• Figure 2: Comparison of the expected observation times $T_{{\rm obs}}$ to reach a target of $N_{\rm th}\xspace = 10$ BBH (top) and BNS (bottom) signals satisfying three different thresholds for SNR $\rho_{\rm th}$, sky area $\Omega_{\rm th}$, relative luminosity distance error $(\Delta D_{L} / D_L)_{\rm th}$, comoving error volume $V_{\rm th}$, post-merger SNR $\rho_{\rm pm,th}$, early-warning SNR $\rho_{\rm ew,th}$, and sky area $\Omega_{\rm ew,th}$. For each of these, three thresholds are considered and indicated with different marker shapes. The solid, horizontal lines represent the timelines for the base detectors (CE atop and ET beneath) and the combined network (center) to reach the targets; in addition to the impact of potential observation delays of 1 (dashed), 3 (dashed-dotted), 6 (dotted), and 9 (loosely dotted) months. Targets that are either completed within $0.01\,{\rm yr}$ or that cannot be reached within $5\,{\rm yr}$ are places in grey boxes. For example, a single L-shaped $\texttt{CE}$ detector would not be able to detect $N_{\rm th}\xspace=10$ BBHs to within $\Omega_{\rm th}\xspace=10\,{\rm deg^2}$ ($\blacktriangleleft$, top left panel) within $5\,{\rm yr}$, while $\texttt{ET-2L}$ and the joint network $\texttt{ET-2L} + \texttt{CE}$ would reach this target within two weeks or less than $0.01\,{\rm yr}$, respectively.
• Figure 3: Impact of the addition of LIGO-India, either in $\texttt{I+}$ or $\texttt{I\#}$ configuration, to five XG detectors and networks, $\texttt{ET-}\triangle$, $\texttt{ET-2L}$, $\texttt{CE}$, $\texttt{ET-}\triangle + \texttt{CE}$, $\texttt{ET-2L} + \texttt{CE}$, on the expected observation times $T_{{\rm obs}}$ to reach a target of $N_{\rm th} = 10$ BBH (left) and BNS (right) signals satisfying three different thresholds for SNR $\rho_{\rm th}$, sky area $\Omega_{\rm th}$, relative luminosity distance error $(\Delta D_{L} / D_L)_{\rm th}$, or comoving error volume $V_{\rm th}$ in addition to post-merger SNR $\rho_{\rm pm,th}$ for the BBHs and early-warning SNR $\rho_{\rm ew,th}$ and sky area $\Omega_{\rm ew,th}$ for the BNSs.
• Figure 4: Comparison of the expected observation times $T_{{\rm obs}}$ to reach a target of $N_{\rm th}\xspace = 1$ PBH signal satisfying three different thresholds for the lower redshift error bound $\Delta z_{\rm\vee,th}\xspace\in\{30,40,50\}$ at two values for the PBH-formation suppression factor $f_{\rm sup}\in\{10^{-3},1\}$ when observed by the three base XG facility configurations, $\texttt{ET-}\triangle$ (right), $\texttt{ET-2L}$ (left), $\texttt{CE}$ (both), and two networks, $\texttt{ET-}\triangle + \texttt{CE}$, $\texttt{ET-2L} + \texttt{CE}$, combining the ET configurations with CE. The solid, horizontal lines represent the timelines for the base detectors (CE atop and ET beneath) and the combined network (center) to reach the targets. The non-solid lines take potential observation delays of 1 (dashed), 3 (dashed-dotted), 6 (dotted), and 9 (loosely dotted) months in either base detector into account, with the respective non-solid, vertical lines indicating the end of the delay. Grey boxes indicate---for each base detector---the targets that either are completed within $0.01\,{\rm yr}$ or that cannot be reached within $5\,{\rm yr}$.
• Figure 5: Power-law integrated (PI) curves for different XG networks, always considering a first set of detectors are online followed by a second detector with a delay of $3N$ months: the $\texttt{ET-2L}$ detectors, followed by the $\texttt{CE}$ detector (top left); the $\texttt{ET-}\triangle$ detector, followed by the $\texttt{CE}$ detector (top right); a single $\texttt{ET-L}$ detector and the $\texttt{I+}$ detector, followed by a second $\texttt{ET-L}$ (bottom left); the $\texttt{CE}$ detector and the $\texttt{I+}$ detector, followed by the $\texttt{ET-2L}$ detectors. In all cases, each curve represents a 3 year observation where the first set of detectors are online for the first $3N$ months, while the full network is online for $3(12-N)$ months. The dotted lines represent the GWB spectral density $\Omega_{\rm GWB}^{\rm CBC}(f)$ calculated for each CBC population considered here.