The study, published in The Astrophysical Journal, is based on a mathematical model that considers how civilizations evolve and interact with their surroundings. The model also incorporates the Fermi paradox, which asks why we have not detected any signs of extraterrestrial intelligence (ETI) yet.
The authors of the study, Jonathan Carroll-Nellenback from the University of Rochester and his colleagues, argue that the Fermi paradox is not a paradox at all, but rather a consequence of the vastness of space and the finite speed of light.
They claim that ETIs may be very common in the galaxy, but their signals and probes take a long time to reach us, and vice versa. Moreover, they suggest that ETIs may have different motivations and strategies for exploring and colonizing the galaxy, which affect how visible they are to us.
The study builds on previous work by Carl Sagan and William Newman, who proposed in 1981 that civilizations expand outward from their home planets in spherical shells that grow at a constant rate. However, Carroll-Nellenback and his team modified this idea by introducing two new factors: the settlement front and the exploration front.
The settlement front is the boundary of the region where a civilization has established permanent colonies. The exploration front is the boundary of the region where a civilization has sent probes or ships to investigate potential habitable worlds.
The settlement front always lags behind the exploration front, because it takes time and resources to establish a colony. The distance between the two fronts depends on how fast a civilization can explore and colonize new worlds, and how long it takes for them to communicate with each other.
The study also considers different scenarios for how civilizations interact with each other. For example, some civilizations may cooperate and share information, while others may compete and hide their presence.
Some civilizations may stay close to their home planets, while others may venture far away. Some civilizations may last for a long time, while others may go extinct or transcend to a different state of existence.
The authors used computer simulations to test how these factors affect the distribution and detectability of ETIs in the galaxy. They found that under most scenarios, the galaxy becomes filled with overlapping spheres of influence that grow over time.
However, these spheres are not necessarily visible to us, because they are too far away or too faint to detect. The authors estimate that it would take at least 3 billion years for a civilization to colonize the entire galaxy, and even longer for us to notice them.
The study concludes that the Fermi paradox is not a strong argument against the existence of ETIs, but rather a reflection of our limited knowledge and perspective.
The authors suggest that we should keep searching for signs of ETIs, using both existing and future technologies, such as radio telescopes, optical telescopes, infrared telescopes, and interstellar probes.
They also recommend that we should be prepared for the possibility that we may encounter ETIs that are very different from us, both in terms of their biology and their culture.
This article is based on the original source: Carroll-Nellenback, J., Frank, A., Wright, J. T., & Scharf, C. (2021). A Model of Habitability Within the Milky Way Galaxy. The Astrophysical Journal, 907(2), 91.