A causal limit to communication within an expanding cosmological civilization

TL;DR

This paper explores the fundamental causal limits on communication within an expanding intergalactic civilization, showing that beyond a certain expansion speed, most regions become causally disconnected from the home galaxy.

Contribution

It introduces a causal horizon model for expanding civilizations, quantifying how high expansion speeds limit communication and observing the geometry of causally connected regions.

Findings

Communication becomes impossible beyond a 0.26c expansion speed.

Most of the civilization's volume cannot signal the home galaxy at high speeds.

Space settlements beyond the horizon are unobservable from the origin.

Abstract

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 .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.