Gregory's ServerNow things get spicy. Over the past couple years, folks have measured the **cosmic gravitational wave background**, a low background hum in spacetime itself coming from all directions. This hum is thought to be due to the mergers of supermassive black holes across the universe, which each ring out with their own characteristic amplitude and frequency. Those billions of mergers add incoherently and produce something that sounds like static on Earth.
The amplitude of that static is a **huge** open question -- last year there were a number of groups which measured something that looked a lot like that static, with a characteristic strain amplitude of 2.4 parts in 1000000000000000.
If this signal is due to supermassive black holes, what does our new model say about it? Well, with some reasonable assumptions you can compute it! And we find their number!
In the figure we see our measured strain along with several other prior measurements.
7/8
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
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...