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Caption: Figure 4.
Same as Figure 3, but for a different selection of models and showing a larger frequency range. The solid lines represent median GWB spectra for a subset of new-physics models (see Appendix B for more details), the gray violins correspond to the posteriors of an HD-correlated free spectral reconstruction of the NANOGrav signal, and the shaded regions indicate the power-law-integrated sensitivity (Thrane & Romano 2013) of various existing and planned GW interferometer experiments: LISA (Amaro-Seoane et al. 2017), DECIGO (Kawamura et al. 2011), BBO (Crowder & Cornish 2005), Einstein Telescope (ET; Punturo et al. 2010), Cosmic Explorer (CE; Reitze et al. 2019), the HLVK detector network (consisting of aLIGO in Hanford and Livingston (Aasi et al. 2015), aVirgo (Acernese et al. 2015), and KAGRA (Akutsu et al. 2019)) at design sensitivity, and the HLV detector network during the third observing run (O3). All sensitivity curves are normalized to a signal-to-noise ratio of unity and, for planned experiments, an observing time of 1 yr. For the HLV detector network, we use the O3 observing time. Different signal-to-noise thresholds ρ thr and observing times t obs can be easily implemented by rescaling the sensitivity curves by a factor of ﹩{\rho }_{\mathrm{thr}}/\sqrt{{t}_{\mathrm{obs}}}﹩. More details on the construction of the sensitivity curves can be found in Schmitz (2021). We emphasize that models whose median GWB spectrum exceeds the sensitivity of existing experiments are not automatically ruled out. This applies, e.g., to cosmic superstrings (SUPER) and the O3 sensitivity of the HLV detector network. Typically, no single GWB spectrum in a given model will coincide with the median GWB spectrum, which is constructed from distributions of h 2ΩGW values at any given frequency. Therefore, if the median GWB spectrum is in conflict with existing bounds, typically only some regions in the model parameter space will be ruled out, while others remain viable (see, e.g., Figure 11 for the SUPER model). Finally, note that any primordial GWB signal is subject to the upper limit on the amount of dark radiation in Equation (23), which requires the total integrated GW energy density to remain smaller than ﹩{ \mathcal O }({10}^{-(5\cdots 6)})﹩ (see Section 5.1).
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© 2023. The Author(s). Published by the American Astronomical Society.
