If advanced civilizations appear in the universe with an ability and desire to expand, the entire universe can become saturated with life on a short timescale, even if such expanders appear rarely. Our presence in an apparently untouched Milky Way thus constrains the appearance rate of galaxyspanning Kardashev type III (K3) civilizations, if it is assumed that some fraction of K3 civilizations will continue their expansion at intergalactic distances. We use this constraint to estimate the appearance rate of K3 civilizations for 81 cosmological scenarios by specifying the extent to which humanity is a statistical outlier. We find that in nearly all plausible scenarios, the distance to the nearest visible K3 is cosmological. In searches for K3 galaxies where the observable range is limited, we also find that the most likely detections tend to be expanding civilizations who have entered the observable range from farther away. An observation of K3 clusters is thus more likely than isolated K3 galaxies.
This is an author-produced, peer-reviewed version of this article. The final, definitive version of this document can be found online at International Journal of Astrobiology, published by Cambridge University Press. Copyright restrictions may apply. doi: 10.1017/S1473550416000082
Olson, S. Jay. (2017). "Estimates for the Number of Visible Galaxy-Spanning Civilizations and the Cosmological Expansion of Life". International Journal of Astrobiology, 16(2), 176-184.