Prospects for future binary black hole gravitational wave studies in light of PTA measurements

Abstract

NANOGrav and other Pulsar Timing Arrays (PTAs) have discovered a common-spectrum process in the nHz range that may be due to gravitational waves (GWs): if so, they are likely to have been generated by black hole (BH) binaries with total masses > 109 M⊙. Using the Extended Press-Schechter formalism to model the galactic halo mass function and a simple relation between the halo and BH masses suggests that these binaries have redshifts z = 𝒪(1) and mass ratios ≳10, and that the GW signal at frequencies above 𝒪(10) nHz may be dominated by relatively few binaries that could be distinguished experimentally and would yield observable circular polarization. Extrapolating the model to higher frequencies indicates that future GW detectors such as LISA and AEDGE could extend the PTA observations to lower BH masses ≳103 M⊙.

Probing the Dark Matter density with gravitational waves from super-massive binary black holes

“…intermediate mass BH [30][31][32][33][34], and binary solar-mass BH (two anomalies are interpreted as the possible observation of ρ DM ∼ 10 11-13 GeV/cm 3 in [35]). We find that a better probe is now offered by super-massive BH [18][19][20][21][22][23][24][25][26][27][28], as gravitational waves observe their dynamics while they are non-relativistic, in the relevant frequency range for this effect.…”

“…• Dynamical friction due to baryonic matter, such as stars and gas, that possibly formed a disk [18][19][20][21][22][23][24][25][26][27][28]. While the cosmological density of DM is about 5 times larger than the matter density, the situation around the center of merged galaxies is uncertain and possibly baryonic matter is more dense that DM.…”

“…However, despite hopes that these gravitational waves could be generated by new physics phenomena, the observed amplitude and spectral slope are approximately consistent with the astrophysical background. The astrophysical background is expected to be primarily generated by binary supermassive black holes (SMBHs) with masses M 1,2 in the range ∼ 10 8-9 M ⊙ [18][19][20][21][22][23][24][25][26][27][28]. The expected amplitude of the astrophysical background is subject to an order of magnitude uncertainty due to several factors.…”

Does NANOGrav observe a dark sector phase transition?

“…An even clearer signature would be a broken powerlaw, but to test for such a feature will require even higher signal-to-noise ratios and hence take correspondingly longer time to probe observationally. A valuable source of information to further disambiguate between SMBHBs and GWBs of primordial origin may lie in the anisotropies of the GWB [121][122][123], in particular via the polarization of the GWB [124][125][126][127][128][129][130] and possibly the detection of singular GW sources as already searched for [131,132].…”

Probing Supermassive Black Hole Seed Scenarios with Gravitational-wave Measurements

“…NANOGrav (Agazie et al 2023a) and other PTA experiments (Antoniadis et al 2023a;Xu et al 2023;Zic et al 2023) have recently reported the observation of a stochastic background of GWs at frequencies in the nHz range, for which the most conservative astrophysical interpretation is that binary systems of SMBHs are emitting them with masses ∼10 9 M e (Agazie et al 2023b(Agazie et al , 2023cAntoniadis et al 2023b;Ellis et al 2023). Naive extrapolation of binary merger models to lower BH masses suggests that GWs from IMBH binaries may be observable at higher frequencies between 0.1 mHz and 1 Hz, for example by the LISA spaceborne laser interferometer experiment or atom interferometer experiments (Ellis et al 2024).…”