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

Abstract

We model the settlement of the galaxy by space-faring civilizations in order to address issues related to the Fermi Paradox. We are motivated to explore the problem in a way that avoids assumptions about the 'agency' (i.e. questions of intent and motivation) of any exo-civilization seeking to settle other planetary systems. We begin by considering the speed of an advancing settlement front to determine if the galaxy can become inhabited with space-faring civilizations on timescales shorter than its age. Our models for the front speed include the directed settlement of nearby settleable systems through the launching of probes with a finite velocity and range. We also include the effect of stellar motions on the long term behavior of the settlement front which adds a diffusive component to its advance. As part of our model we also consider that only a fraction f of planets will have conditions amenable to settlement by the space-faring civilization. The results of these models demonstrate that the Milky Way can be readily 'filled-in' with settled stellar systems under conservative assumptions about interstellar spacecraft velocities and launch rates. We then move on to consider the question of the galactic steady-state achieved in terms of the fraction X of settled planets. We do this by considering the effect of finite settlement civilization lifetimes on the steady states. We find a range of parameters for which 0 < X < 1 i.e. the galaxy supports a population of interstellar space-faring civilizations even though some settleable systems are uninhabited. In addition we find that statistical fluctuations can produce local over-abundances of settleable worlds. These generate long-lived clusters of settled systems immersed in large regions which remain unsettled. Both results point to ways in which Earth might remain unvisited in the midst of an inhabited galaxy. Finally we consider how our results can be combined with the finite horizon for evidence of previous settlements in Earth's geologic record. Using our steady-state model we constrain the probabilities for an Earth visit by a settling civilization before a given time horizon. These results break the link between Hart's famous "Fact A" (no interstellar visitors on Earth now) and the conclusion that humans must, therefore, be the only technological civilization in the galaxy. Explicitly, our solutions admit situations where our current circumstances are consistent with an otherwise settled, steady-state galaxy.

A Statistical Estimation of the Occurrence of Extraterrestrial Intelligence in the Milky Way Galaxy

“…We designed a galactic spatial hash table with the same volume of the Milky Way, uniformly divided into three-dimensional cells with the volume of each cell at 0.01 kpc 3 . This galactic table enables the computation of large-scale objects at a real-time frame rate, and allows fast location and proximity detection queries [30].…”

“…For instance, a previous study [2] suggested that the Fermi Paradox results from a lack of evidence and proper estimates of ETI. However, many scholars challenge this stance, as it was based on the argument that SETI is extensive enough to exclude ETI elsewhere [3]; this is invalid, as we cannot a priori preclude the possibility of the existence of other intelligent life in the Galaxy. A recent statistical approach also demonstrated that the detection of a signal of extraterrestrial intelligence is extremely difficult due to technical and physical limitations [4].…”

Panspermia in a Milky Way-like Galaxy

“…The last category concerns statistical investigations of panspermic propagation through stellar systems or galaxies using analytical or simple numerical models (Adams & Spergel 2005;Lin & Loeb 2015;Lingam 2016;Ginsburg et al 2018;Ðošović et al 2019;Carroll-Nellenback et al 2019), or of the effect of a spatially variable probability of habitable planets (habitability; e.g., Gonzalez et al 2001;Lineweaver et al 2004;Gowanlock et al 2011) on the viability of panspermia.…”

Monte Carlo estimation of the probability of causal contacts between communicating civilizations

“…Optimistic estimates from the Drake equation contrast with the so-called Fermi paradox, which states the apparent contradiction between the expected abundance of life in the Galaxy and the lack of evidence for it (e.g. Hart 1975;Brin 1983;Barlow 2013;Anchordoqui et al 2017;Forgan 2017;Carroll-Nellenback et al 2019;Sotos 2019). There are many propositions aimed at solving this paradox, which make use of statistical (Horvat 2006;Maccone 2015;Solomonides et al 2016) or stochastic approaches (Forgan 2009;Forgan and Rice 2010;Glade et al 2012;Bloetscher 2019).…”