That blog post is certainly very useful and informative, but as a critique of the paper, it's actually quite thin. It does not contribute any evidence as to the validity or invalidity of the observational technique (presumably because his area is theory), it just points out something that the authors already point out in their paper: the rise in black hole mass could be either from normal accretion, or from cosmological coupling. Then he says he thinks it's more likely the former than the latter-- without giving any reason at all for that belief of his! If I'm an author, and I'm looking at Ethan's post, my reaction would be "everything you said I already knew before I analyzed the data, but he data convinced me of the latter possibility. I guess everyone is entitled to their opinion." And that's it, the post gives no reason to believe one or the other hypothesis, it is pure opinion without citing any new evidence at all.
Indeed, Siegel is even a bit disingenuous here. He lists three reasons why no one was thinking about black holes as part of the overall expansion, and they were:
- For one, we can quantify how much gravitational binding energy there is in black holes, and it’s only about 0.01% of the needed amount of energy to explain dark energy.
- For another, the dark energy density needs to remain constant over time, but the number density and mass density of black holes decreases over time, especially at very late times.
- And for yet another, individual black holes actually grow over time and new black holes continuously form, but this growth occurs much more slowly than the rate at which the Universe expands.
So yes, those are good reasons for people to rule out black holes as being cosmologically important--
unless there's cosmological coupling. He kind of leaves out that crucial caveat! If I'm an author, and reading that, I'm actually a bit miffed at this point, because he makes it sound like those are three reasons to rule out cosmological coupling, when in fact those three reasons are the
whole point of why cosmological coupling could make black holes cosmologically important! It's easy to see how cosmological coupling provides an alternative to all three of those objections to the importance of black holes. And here's the disingenuous part: if Siegel's hunch is right and the black-hole mass increase is due to normal accretion, then that already invalidates the first bullet above, because k=3 mass increases in black holes make their mass way more important cosmologically-- you just don't get the acceleration if it isn't dark energy (as per what
PeterDonis has been telling us). The black holes would have to be eating up a
significant chunk of the mass in the universe to grow that fast by accretion!
Now, none of that makes Siegel wrong, but also, none it makes the authors wrong. It's an observational issue now, to corroborate or refute their conclusions, and it is a theoretical issue to seek the missing GR solution they have all mentioned. I just don't see the point in putting odds on how all that will turn out-- if you have nothing to say about potential observational flaws, or potential sticking points in the hoped-for GR solution, then you don't really have much to add to the question. That said, I do appreciate all the useful information he presented, and he has every right to include his opinion, it's just not a critique of the paper.
I feel he would have been on stronger ground had he simply said their claim that the absence of cosmological coupling is ruled out to 99.98% confidence overlooks the significant possibility that their observed effect is due to something else (like normal accretion). I think we can all agree that chance is much much larger than 0.02% ! But when Siegel put odds on them being right, that was just personal opinion, not really a valid critique. It's "argument by authority" rather than "argument by evidence."
And here's the thing no one seems to be noticing: If the BH mass rise is due to normal accretion processes, then why k=3? Coincidence, apparently? My guess is, this "why 3" issue is what convinced the authors.