I have a question about why the duration of the gravitational wave signal detected by LIGO was 0.25 seconds or so. I think I understand correctly that as the two binary black holes rotate around each other and move closer together there is a loss of angular momentum that is converted to gravitational waves of a certain frequency. The generation of gravitational waves would then end upon the merging of the two black holes. My questions are: is the frequency of the gravitation wave constant throughout the entire history of the black hole merger? Is this frequency red-shifted in the gravitational wave’s journey from its source to the LIGO instrument due to the expansion of the universe? Why is the duration of the signal 0.25 seconds?