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We model the settlement of the galaxy by space-faring civilizations in orderto address issues related to the Fermi Paradox. We explore the problem in a waythat avoids assumptions about the intent and motivation of any exo-civilizationseeking to settle other planetary systems. We first consider the speed of anadvancing settlement via probes of finite velocity and range to determine ifthe galaxy can become inhabited with space-faring civilizations on timescalesshorter than its age. We also include the effect of stellar motions on the longterm behavior of the settlement front which adds a diffusive component to itsadvance. The results of these models demonstrate that the Milky Way can bereadily 'filled-in' with settled stellar systems under conservative assumptionsabout interstellar spacecraft velocities and launch rates. We then consider thequestion of the galactic steady-state achieved in terms of the fraction ofsettled planets. We do this by considering the effect of finite settlementcivilization lifetimes on the steady states. We find a range of parameters forwhich the galaxy supports a population of interstellar space-faringcivilizations even though some settleable systems are uninhabited. Both resultspoint to ways in which Earth might remain unvisited in the midst of aninhabited galaxy. Finally we consider how our results can be combined with thefinite horizon for evidence of previous settlements in Earth's geologic record.Our steady-state model can constrain the probabilities for an Earth visit by asettling civilization before a given time horizon. These results break the linkbetween Hart's famous "Fact A" (no interstellar visitors on Earth now) and theconclusion that humans must, therefore, be the only technological civilizationin the galaxy.

The Fermi Paradox and the Aurora Effect: Exo-civilization Settlement, Expansion and Steady States