In article ,
Eric Flesch writes:
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
cause.

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. The source selection
details are at
https://iopscience.iop.org/article/1...256/139/6/2360

Having only glanced through the paper -- and I realize I'm late
responding to this -- I don't see how Eddington bias can give the
redshift dependence in Fig 1. Also, Fig 3 shows the effect is
greatest for the brightest objects, not the faintest ones.

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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