The Galactic Club, or Galactic Cliques? Exploring the limits of interstellar hegemony and the Zoo Hypothesis

TL;DR

This paper models the formation of extraterrestrial civilizations in the galaxy to evaluate the plausibility of the Zoo Hypothesis, finding that multiple disconnected groups are more common than a single unified Galactic Club.

Contribution

It introduces a simple toy model to analyze the causal connectivity of ETIs and assesses the likelihood of a unified Galactic Club versus multiple cliques.

Findings

Most galaxy regions form multiple cultural groups of ETIs.

A single Galactic Club requires large civilization numbers and long lifetimes.

Causal disconnection makes the Zoo Hypothesis less robust.

Abstract

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.