This is really important -- if using the previous models for the numbers of black holes we find a much smaller strain than folks actually measure. This has led folks to suggest that either the gravitational wave background is coming from something else or that there are missing black holes.

We can also count up the total mass density in black holes. When we do this we get **much** larger values than folks have found in the past. These prior measurements use the brightness of active galactic nuclei to infer the accretion history of present-day black holes. The fact that we find a much larger value could suggest a couple things! This likely suggests that the link between observed AGN luminosity and the mass accretion rate is a bit fuzzier than folks have assumed, and perhaps the overall corrections folks make for the presence of dust or the efficiency of accretion is a bit off.

Anyway, feel free to check out the paper: https://arxiv.org/abs/2407.14595 It's a pretty accessible and fun read!

8/8