Gravity seminar - Hannah Middleton Seminar

Pulsar timing arrays are searching for the stochastic gravitational wave background from the merging population of massive black hole binaries. Upper limits on the background are beginning to reach astrophysically interesting sensitivities, however no detection has been reported so far. The recent upper limit from the Parkes Pulsar Timing Array has been interpreted as casting into doubt the standard model of binary assembly through galaxy mergers and hardening via stellar interactions, suggesting that their evolution must be accelerated or stalled. We use a Bayesian hierarchical analysis to consider the implications of the upper limit for a range of astrophysical scenarios. Weak constraints can be placed on the population parameters, however we find that these astrophysical scenarios are as yet fully consistent with the current observations.