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According to this article about the gravitational wave observation, GW190521, the estimated luminosity distance is appx 5+/-2 Gigaparsecs, which is appx 16 billion light years +/- 6:

https://dcc.ligo.org/public/0165/P2000020/012/LS17910.pdf

While I understand that this is a crude measurement, is it not true that the distance cannot be greater than the appx 13.7 billion light years of the universe? If so, should not there be another constraint on the distance? Or is it just more accurate to report the calculated luminosity distance and leave it at that in the report?

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    $\begingroup$ A luminosity distance of 5 Gpc corresponds to a redshift of only about 0.8, or a loopback time of about 7 Gyr. $\endgroup$
    – user26287
    Sep 3, 2020 at 17:11

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Luminosity distance is simply the distance that the object would have to be at in a static special-relativistic (or Newtonian) universe to have that luminosity. It's not the distance in the expanding general-relativistic universe that we actually inhabit.

Out to distances of roughly 1 billion light years, spacetime is roughly flat, the Hubble speed is not too large, and the luminosity distance is not too inaccurate. At substantially larger distances, it's way off.

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  • $\begingroup$ To "expand" on this... Since the universe is expanding, the light it kind of going "uphill". Thus it hasn't traveled as far as you would expect it to have. $\endgroup$
    – IronEagle
    Sep 3, 2020 at 21:16

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