“A Causal Limit to Communication within an Expanding Cosmological Civilization”, S. Jay Olson2022-08-16 (science)⁠:

If a civilization embarks on high-speed intergalactic expansion, growing to a cosmological scale over time, communication between remote galaxies in the civilization will incur an extreme time delay, due to the distance involved. Indeed, if the net expansion speed v is more than 0.26c, most of the final volume of such a civilization will not be able to signal the home galaxy at all, due to the presence of a causal horizon.

We illustrate the regions of such a civilization according to the degree of “conversation” that is possible with the home galaxy, and describe how the geometry depends on expansion speed.

We conclude by reflecting on the value of space settlement beyond the horizon, where colonies can never be observed by the initiating home galaxy.