Colonizing the Galaxy

It's a trope of modern science fiction that you need faster-than-light travel of some sort to establish an interstellar empire. That may not be the case, according to a recent study that used a computer simulation to model how a civilization could expand through an entire galaxy. 

The researchers found that an entire galaxy could be colonized in a billion years given expansion at very conservative sub-light speeds. That may sound like an immense length of time, but it's only a few percent of the age of the galaxy.

A simulation produced by the team shows the process at work, as a lone technological civilization, living in a hypothetical Milky Way-like galaxy, begins the process of galactic expansion. Grey dots in the visualization represent unsettled stars, magenta spheres represent settled stars, and the white cubes are starships in transit. The computer code and the mathematical analysis for this was project were written at the University of Rochester by Jonathan Carroll-Nellenback. Astronomer Adam Frank from the University of Rochester also participated in the study.

Things start off slow in the simulation, but the civilization’s rate of spread really picks up once the power of exponential growth kicks in. But that’s only part of the story; the expansion rate is heavily influenced by the increased density of stars near the galactic center and a patient policy, in which the settlers wait for the stars to come to them, a result of the galaxy spinning on its axis.

The whole process, in which the entire inner galaxy is settled, takes one billion years. That sounds like a long time, but it’s only somewhere between 7% and 9% the total age of the Milky Way galaxy.

Given that the parameters used in the simulation were extremely conservative, it's likely that it wouldn't take anywhere near a billion years. I can point to several science fiction stories that postulate slower-than-light interstellar civilizations with starships operating at speeds anywhere from .1c to near relativisitic velocities. For example, take a look at Alastair Reynolds' Revelation Space series or Vernor Vinge's A Deepness on the Sky.  

I think the key point raised by the study is "Where are they?". Given that a technological civilization could expand into a significant portion of the galaxy in as little as a few million years, the galaxy could be littered with their remains, if not with active, thriving civilizations.