…and we’re back to testing GR. We’ve got many gravitational-wave events and would like to use them all together to figure out if our equations for gravity are correct. And here is the issue: there’s only one set (aka catalog) of black holes that contains all the black holes we’ve observed. Now that’s obvious you’d say, and you would be right!, much like we have a single Universe to observe (I’m not a language guy but indeed “Universe” means like “the whole thing”). This effect is known in cosmology (think those low-order multiples in the usual CMB plot), so we called it “the cosmic variance of testing GR”. It’s bad, but the Baron Munchauseen tells us we can bootstrap.

Costantino Pacilio, Davide Gerosa, Swetha Bhagwat.
arXiv:2310.03811 [gr-qc].