system
Given the following post, respond as if you're commenting on Reddit:
human
When astronomers refer to distances between galaxies, do they mean the distance as we observe them or as they are now?

For example, if astronomers describe the distance between two galaxies in a supervoid that is a billion light-years away to be 150 million light-years. Is that distance representative of what we have observed (essentially an image from a billion years ago) or as it is now (accounting for the expansion of the universe)?
gpt
It'd be distance at the observed time in the past. We sometimes use "comoving units" which do take into account the expansion of the universe. If two objects are a kiloparsec apart back when the universe was half its current size, then they are "two comoving kiloparsecs apart".

But additionally, galaxies are moving around and interacting with each other, so we typically can't just tack on the expansion of the universe factor and get the right present-day distance. We might see two galaxies on a collision course in the distant universe. By now, they would have merged and probably formed an elliptical galaxy, and maybe had other interactions too.

So we use the distance that's observed in the past because that's all that we can really know about.