Caltech/JPL Association for Gravitational-Wave Research Seminar
In 1980 Zimmerman and Thorne published a paper entitled "The gravitational waves that bathe the Earth: upper limits based on theorists' cherished beliefs." Their idea was to imagine that there is some previously unknown type of gravitational-wave source out there, and consider what limits we can already place on the maximum strength of such a source, based on simple energetic and statistical arguments. Their emphasis on was possible LIGO detections of sources at z < 5. The recent activity in GW searches with pulsar timing arrays (PTAs) made it timely to re-visit this question in the context of PTAs, whose frequency band is 10^10 times lower. The results of this exercise turned out to be rather interesting. We find that sources from the very early universe, and especially memory-effect signals from early universe bursts, offer the largest potential discovery space (in the sense that they have the weakest current constraints). We explain these and related results.