A new paper accepted by ApJL, arXiv:2001.04471 "Nonstationarity of AGN variability: The only way to go is down!" by Caplar et al, concludes
that visible quasar luminosity is linearly decreasing due to
cosmological effects, and that this change is visible over a 15-year timeline. It seems to me that a selection effect is a more likely
But what is omitted is that
SDSS-DR7 quasars needed to be of a certain minimum brightness for a
spectrum to be obtained by them.
This is a version of "Eddington bias," and it would have been my
first thought to explain the effect too. ...
Bias questions are difficult, and I'm not sure the effect isn't an >observational bias, but it's not obvious to me it is.
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