If we observe two galaxies that are diametrically opposed from the Earth, and each $1000\,{\rm ly}$ away from the Earth, is the separation distance between the galaxies $2000\,{\rm ly}$? Really the question is: if the galaxies were separated from Earth by $10\,{\rm Gly}$ each, then would their separation be $20\,{\rm Gly}$?

• (continuing ...) But it's certainly true that there are some objects so far away that we can't see them now. And it's possible to find a pair of galaxies, in opposite directions on the sky, that are visible from Earth but which are too far from each other to be visible to each other at the present time. To explore the relation between ages and distances, you might want to play around with Ned Wright's cosmology calculator, astro.ucla.edu/%7Ewright/CosmoCalc.html . To get the maximum distance you can see, plug in very large values of z (the maximum distance is really $z=\infty$). Aug 26, 2011 at 17:19