What will the size of the observable universe be in 4 billion years?
At present day, light would only reach us if it came from a galaxy within 13.5 billion1 light years, with anything outside that receding at a velocity higher than the speed of light (so never reaching us), what I understand as the Hubble volume.
In exactly 4 billion years, from the Milky Way-Andromeda galaxy, how far would an object be before it recedes at beyond the speed of light?
I imagine it's terribly simple to work out, but from what I understood, the Hubble constant changes (decreasing, it seems) so I'd need to work out what it is in 4 billion years time to accurately work out the size of the Hubble volume at said point in time.
I'm writing a hard sci-fi book about a human empire limited to the speed of light, so it's important (for me) that I work out the maximum size of the empire before it begins to lose contact with its periphery.
I checked out some of the duplicate/similar threads but they don't seem to answer the question. I understand if it can't be answered, but could anyone give a guesstimate? We won't be alive anyway when the time comes and it turns out to be wrong :)