Implications for the Stochastic Gravitational Wave Background from a Massive Quasar-binary The collective inspirals of very close-separation supermassive black holes (SMBHs) are expected to produce a stochastic gravitational wave background (GWB) at nHz frequencies, which is accessible to pulsar timing arrays. However, we have yet to detect this background or find any SMBH binaries that are sufficiently massive or at small enough separations to contribute appreciably to the GWB. Using Hubble Space Telescope imaging, we have identified a pair of rapidly-growing supermassive black holes (M>4.e8 solar masses) at z=0.2 that are separated by only 400 parsecs. While this pair is not currently in the GW-emission regime, it points to a population of SMBH pairs that should have merged by today, unless there is a final parsec problem. Using estimates for the number density of such sources, we place limits on the expected GWB amplitude. I'll discuss the issues surrounding these calculations and how we can expect to learn more about the GWB and SMBH binary dynamics as we search for more of these SMBH pairs.
Sign up for lunch
Lunch will be available outside the auditorium starting at 12pm on Thursday. If you would prefer to eat during the talk, you are welcome to do so, but we ask that you clean up after yourselves even more carefully than you normally would, the Physics colloquium happening in this same room a few hours later.