I was wondering how to calculate either the present comoving distance or the light travel distance when the source material only gives one value?

Is there and an online calculator or simple equation using Hubble constant that someone like myself who is not mathematically advanced can use to calculate these values?

The reason for this question is that I wanted to know the light travel time for the LQG's and other large distant cosmic structures so that I know the age at which we are looking at them but the sources only give their present comoving distance. Other sources also only give the light traveled time for distant objects and not present co-moving distance.

Edit- For the time being I only need to know the light travel distance for objects at a 9 billion light year comoving distance as this is the rough distance for the Giant GRB Ring, Huge-LQG, The Giant Arc, U1. 11 LQG and Clowes-Campusano LQG. Or if anyone knows a source that states the light travel distance for any mentioned.

I could post that as a separate question if the community thinks it should be?


1 Answer 1


Ned Wright's cosmology calculator will calculate all of the distances, and the age of the universe at the time the light was emitted, given a redshift $z$ and some cosmological parameters. Your best bet is to look up the redshift of the objects you're interested in, rather than trying to start from a comoving distance (which was originally calculated from the redshift anyway).

Click the "Flat" or "General" button to calculate results. "Open" will give wrong answers.

I wouldn't trust the results to more than a digit or two, because $H_0$ is not known very accurately.

  • $\begingroup$ Brilliant, thank you. Do I leave the other boxes alone and just change z? For a few of the large structures their redshift value in the calculator gives their stated comoving distance but some are pretty far off. $\endgroup$ Commented Oct 12, 2021 at 20:17
  • $\begingroup$ @JimDaniels Try $H_0=67.74$, $\text{Omega}_\text{M}=0.3089$ (and click "Flat", or also set $\text{Omega}_\text{vac}=0.6911$ and click "General"). Those are Planck 2015 values via Wikipedia. I don't know that they are necessarily better values, though. Ned Wright chose the default values, and he knows more than I do. The calculator was updated fairly recently (2018). $\endgroup$
    – benrg
    Commented Oct 12, 2021 at 20:26
  • $\begingroup$ Great, thanks again. It seems Wikipedia and many sources have incorrect comoving distances for the LGQ's as their redshifts around 1.2 giving a comoving distance around 12 Gly. $\endgroup$ Commented Oct 12, 2021 at 20:40

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