Sensitivity forecasts for gravitational-wave detectors to dark matter decaying into gravitons

TL;DR

This work addresses the detectability of a stochastic gravitational‑wave background produced by ultralight dark matter decays into gravitons, using a model‑independent parameterization with $m_\phi$ and $\tau_\phi$. It splits the signal into extragalactic and local Galactic components and links the graviton flux to SGWB observables via $H_0^2 \Omega_{\rm GW}(f) = \frac{2\pi^2}{3} f^3 S_h(f)$ and related quantities, applying this to a suite of detectors including LVK, LISA, ET–CE, BBO, IPTA, and SKA. The forecasts show detectable regions across a wide mass range $m_\phi \sim 10^{-23}$–$10^{-10}$ eV and lifetimes up to $\tau_\phi \sim 10^{14}$–$10^{18}$ times the present age of the universe, with the extragalactic component dominating at higher masses and the local Galactic component at lower masses; the cross-term is subdominant. The results demonstrate the potential of next‑generation GW observatories to probe dark matter decay to gravitons and motivate complementary probes such as inverse Gertsenshtein conversions. The findings hinge on the frequency band and overlap reduction between detectors, with a consistent framework to update forecasts as detector networks evolve.

Abstract

Dark matter may not be perfectly stable, and its decay could generate distinctive gravitational-wave signatures. In this work, we present model-independent predictions for the stochastic gravitational-wave background arising from the decay of ultralight dark matter into gravitons. Within this framework, we forecast the sensitivity reach of current and forthcoming gravitational-wave detectors to such signals.

Figures (4)

• Figure 1: Normalized overlap reduction functions $\gamma_{I \! J}(f)$ as a function of frequency for the different pairs of detectors in the LVK and ET-CE networks.
• Figure 2: Effective noise PSD $S_{\text{eff}}(f)$ as a function of frequency for the detectors considered in this work.
• Figure 3: Breakdown of the different contributions to the total SNR for LISA (from left to right, top to bottom: extragalactic, local, cross-term and total SNR) in the $(m_\phi, \tau_\phi)$ parameter space. Solid lines lines show the detectability bound $\text{SNR} = 8$. Dashed and dot-dashed lines show the extragalactic and local contributions to total, respectively.
• Figure 4: Forecast for the detectability ($\text{SNR} = 8$, solid line) of a SGWB from DM decay into two gravitons, $\phi \rightarrow 2h$, in the $(m_\phi, \tau_\phi)$ parameter space for different current and future GW detectors. Dotted and dash-dotted lines show the local and extragalactic contributions, respectively. Hatched regions show constraints from previous works.