Digging Deeper: Observing Primordial Gravitational Waves below the Binary-Black-Hole-Produced Stochastic Background

The merger rate of black hole binaries inferred from the detections in the first Advanced LIGO science run implies that a stochastic background produced by a cosmological population of mergers will likely mask the primordial gravitational wave background. Here we demonstrate that the next generation of ground-based detectors, such as the Einstein Telescope and Cosmic Explorer, will be able to observe binary black hole mergers throughout the Universe with sufficient efficiency that the confusion background can potentially be subtracted to observe the primordial background at the level of ${\mathrm{\Omega }}_{\mathrm{GW}}\simeq {10}^{-13}$ after 5 years of observation.

Figure 1

Design power spectral density of second generation detectors, Advanced LIGO (aLIGO), Advanced Virgo (AdV), and KAGRA, and proposed sensitivity of third generation detectors Einstein Telescope (ET) and Cosmic Explorer (CE). Expected intermediate sensitivities such as Advanced LIGO Plus ($\mathrm{A}+$) and Voyager are also shown.

Figure 2

Energy density spectrum ${\mathrm{\Omega }}_{\mathrm{GW}}$ in GWs from undetected BBHs (${\rho }_T<12$) within advanced (top plot), $\mathrm{A}+$ (middle plot), and third generation (bottom plot) detectors. Solid (green) curves are the total backgrounds for models A (thick lines) and B (thin lines), respectively, when detected BBH signals are not removed from the data, so they are the same in each plot. We see that in the tens of Hz region one obtains the characteristic $f^{2/3}$ slope. The cosmological background from inflation assuming a tensor-to-scalar ratio of $r=0.1$ is shown for comparison, and confusion background from unresolved binary neutron stars, assuming an average local rate of $60{\mathrm{Gpc}}^{-3}{\mathrm{yr}}^{-1}$ [63]. The horizontal solid line is the minimal flat spectrum that can be detected with $\rho =3$ with a five-detector network after 5 years.