This Einstein ring

enter image description here

Image credit: NASA / ESA / Hubble / T. Treu / Judy Schmidt

shows multiple images of the quasar 2M1310-1714. Its distance is quoted at 17 billion light years and although the age of the universe is around 13.8 billion light years old, when the comoving distance is taken into account, the distance to the edge of the observable universe is 46.6 Gly.

However, I keep coming across a link to Ned Wright's "Why the Light Travel Time Distance should not be used in Press Releases". The link is dead but the summary states

Since public information offices in the US never want to mention the redshift of an object, distances are usually given as light travel time distances. This has one simple property: the distance in light years is never greater than the age of the Universe in years.

Is this the sense in which "17 billion light years" is being used here, and if so, what is the mechanism by which gravitational lensing can show objects beyond the 46.6 Gly boundary. The only papers I can find are about imaging the ring, nothing about the distance to it or how that distance was determined.

  • 2
    $\begingroup$ "quoted at 17 billion light years" That doesn't seem to be in the source. Presumably that is a co-moving distance, because if it were a light travel time distance we would be seeing it at a time before the start of the universe . . . which isn't possible. $\endgroup$
    – James K
    Aug 25, 2021 at 21:19

1 Answer 1


The redshift of the quasar is 1.975, so it is nowhere near the edge of the observable universe.

17 billion light years is the comoving distance (i.e. where it is now), as you can confirm with this cosmology calculator.

  • 1
    $\begingroup$ +1 That's what I was looking for! Where did you find the redshift? $\endgroup$ Aug 26, 2021 at 10:12
  • 2
    $\begingroup$ @DaveGremlin link added $\endgroup$
    – ProfRob
    Aug 26, 2021 at 12:15

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