Gravitational-Wave Research Seminar

Decade-long timing observations of arrays of millisecond pulsars have placed highly constraining upper limits (∼ 10^-15   strain at f = 1/yr) on the amplitude of the nanohertz gravitational-wave (GW) stochastic signal from the mergers of supermassive black-hole binaries (SMBHBs). Data from the North American Nanohertz Observatory for Gravitational-waves (NANOGrav) has recently been used to probe the M-sigma relation, investigate environmental factors influencing the final-parsec problem, and to place limits on cosmic-string tension which are several times better than Planck + high-l CMB data. Given that SMBHBs may be significantly coupled to their local nuclear environment (thereby diminishing the GW signal at our lowest, most-sensitive frequencies) this prompts the question whether nanohertz GWs are likely to be detected in the near future. We have answered this question quantitatively using simple statistical estimates, deriving the range of true signal amplitudes that are compatible with current upper limits, and computing expected detection probabilities as a function of observation time. We comment on the detection prospects for small arrays of pulsars with the lowest timing noise (which have provided the tightest upper limits) versus large and expanding pulsar arrays (which can map the correlation signature induced by the background signal), and discuss the influence of various environmental influences on our near-future chances.