Beyond the detection of gravitational wave radiation is its potential as a tool of astronomical discovery. It is in the nature of gravitational wave emission mechanisms that the wave frequency is directly proportional to the mass of the systems that are responsible for the waves that can be observed. Thus, 100 Hz gravitational waves are associated with stellar mass binary systems, 100 mHz waves with intermediate mass black hole systems, and 0.1 mHz waves with binary black hole systems of million solar mass scale. To observe the gravitational waves from binary systems of supermassive black holes, such as arise following galactic major mergers, requires the ability to observe the presence of waves with frequencies less than or of order 100 nHz : i.e., periods of a year or more. With such observations, however, comes the prospect of tracking the evolution of their co-evolving galactic hosts and probing the environment of merged galaxies at the sub-parsec scale. In this colloquium we will explore how pulsar timing can be used to detect gravitational waves at these very low frequencies, the variety of sources that we might expect to observe, what gravitational wave observations can tell us about about conditions in the cores of merged galaxies, and the prospects for making this exciting story of discovery a reality.
Followed by wine and cheese in Pupin 1402