The Galactic Club, or Galactic Cliques? Exploring the limits of   interstellar hegemony and the Zoo Hypothesis
  The Zoo solution to Fermi's Paradox proposes that extraterrestrial intelligences (ETIs) have agreed to not contact the Earth. The strength of this solution depends on the ability for ETIs to come to agreement, and establish/police treaties as part of a so-called "Galactic Club". These activities are principally limited by the causal connectivity of a civilisation to its neighbours at its inception, i.e. whether it comes to prominence being aware of other ETIs and any treaties or agreements in place.   If even one civilisation is not causally connected to the other members of a treaty, then they are free to operate beyond it and contact the Earth if wished, which makes the Zoo solution "soft". We should therefore consider how likely this scenario is, as this will give us a sense of the Zoo solution's softness, or general validity.   We implement a simple toy model of ETIs arising in a Galactic Habitable Zone, and calculate the properties of the groups of culturally connected civilisations established therein. We show that for most choices of civilisation parameters, the number of culturally connected groups is greater than 1, meaning that the Galaxy is composed of multiple Galactic Cliques rather than a single Galactic Club. We find in our models for a single Galactic Club to establish interstellar hegemony, the number of civilisations must be relatively large, the mean civilisation lifetime must be several millions of years, and the inter-arrival time between civilisations must be a few million years or less. 
Duncan H Forgan
physics.pop-ph