Pre-DECIGO can get the smoking gun to decide the astrophysical or cosmological origin of GW150914-like binary black holes

Pre-DECIGO (DECihertz laser Interferometer Gravitational wave Observatory) consists of three spacecraft arranged in an equilateral triangle with 100 km arm lengths orbiting 2000 km above the surface of the earth. It is hoped that the launch date will be in the late 2020s.Pre-DECIGO has one clear target: binary black holes (BBHs) like GW150914 and GW151226. Pre-DECIGO can detect ~30M⊙–30M⊙ BBH mergers like GW150914 up to redshift z~30. The cumulative event rate is ~1.8×105 events yr−1 in the Pop III origin model of BBHs like GW150914, and it saturates at z~10, while in the primordial BBH (PBBH) model, the cumulative event rate is ~3×104 events yr−1 at z=30 even if only 0.1% of the dark matter consists of PBHs, and it is still increasing at z=30. In the Pop I/II model of GW150914-like BBHs, the cumulative event rate is (3–10) × 105 events yr−1 and it saturates at z~6. We present the requirements on orbit accuracy, drag-free techniques, laser power, frequency stability, and interferometer test mass. For BBHs like GW150914 at 1 Gpc (z~0.2), SNR~90 is achieved with the definition of Pre-DECIGO in the 0.01–100 Hz band. Since for z≫1 the characteristic strain amplitude hc for a fixed frequency band weakly depends on z as z−1/6, ∼ 10% of BBHs near face-on have SNR > 5 (7) even at z~30 (10). Pre-DECIGO can measure the mass spectrum and the z-dependence of the merger rate to distinguish various models of BBHs like GW150914, such as Pop III BBH, Pop II BBH, and PBBH scenarios.Pre-DECIGO can also predict the direction of BBHs at z=0.1 with an accuracy of ~0.3 deg2 and a merging time accuracy of ~1 s at about a day before the merger so that ground-based GW detectors further developed at that time as well as electromagnetic follow-up observations can prepare for the detection of merger in advance, like a solar eclipse. For intermediate mass BBHs such as ~640M⊙–640M⊙ at a large redshift z>10, the quasinormal mode frequency after the merger can be within the Pre-DECIGO band so that the ringing tail can also be detectable to confirm the Einstein theory of general relativity with SNR~35.