TThe more we learn about planets inside and outside our solar system, the harder the question, “Where are the aliens?” seems to be.
In addition to finding a slew of planets in the habitable zones around their stars and locating new classes of exoplanets that could be good candidates for life, we have discovered life on Earth that can withstand extreme conditions we never thought possible. So where is everyone?
Among early attempts to answer this question, known as the Fermi Paradox, an astrophysicist attempted to investigate how long it would take for one (or more) civilizations to colonize the galaxy. In 1974, Michael Hart’s dramatically titled article appeared:An explanation for the absence of aliens on Earth” states that if a civilization tried, it would be able to colonize the Milky Way in a relatively short time.
“Suppose we end up sending expeditions to each of the 100 nearest stars. (These are all within 20 light-years of the Sun.) Each of these colonies has the potential to eventually send out its own expeditions, and their colonies can in turn colonize, and so on,” Hart wrote.
‘If there were no break between journeys, the limit of space exploration would lie roughly on the surface of a sphere whose radius increased at a rate of 0.10 degrees Celsius. At that rate, most of our Milky Way would be crossed within 650,000 years. If we assume that the time between trips is of the same order as the length of a single trip, then the time required to span the Milky Way will be roughly doubled. We see that if there were other advanced civilizations in our Galaxy, they would have had ample time to reach us, unless they began space exploration less than two million years ago.”
If this is even remotely true, and considering that our galaxy is about 13.6 billion years old, it’s puzzling that we haven’t found any alien civilizations yet. In fact, the paper argues, the absence of aliens on Earth could be seen as evidence that there is no alien life to be found at all. If so, Hart said, a sociological explanation would be needed for why they didn’t colonize the Milky Way when they had ample opportunity to do so. Unless a conclusive one can be found, and until we have more information about planetary formation and other factors, Hart said this is “strong evidence that we are the first civilization in the galaxy.”
So when we look at the stars, should we expect a colonized galaxy? According to a team in a pre-print article that tried to investigate how quickly a civilization can occupy the planets within their reach, the expansion is not that uniform.
They created a simplified model of the universe, consisting of cells representing habitable planets. A spherical 3D portion of this model was then isolated, with a “civilization” in the center. This civilization moves at a constant speed to the other cells and uses the resources of the new planet to spread further throughout the galaxy.
To complicate matters further, the team used three different models of the universe: a static universe, a matter-dominated universe, and a dark matter-dominated universe. Until about 9.8 billion years ago, the Standard Model of cosmology assumed that the universe was dominated by matter. In both matter- and dark-matter dominated universes, there are planets that are beyond the reach of any civilization because they are receding (from their point of view) faster than the speed of light.
We can only see light that has reached our vantage point, which means there is a limit to how much of the universe we can see, also called the observable universe, since the light has not yet reached us.
As the universe expands (as in a dark matter-dominated universe) the distance between us and all other stars increases, and our observable universe becomes smaller. In a dark matter-dominated universe, there is a limit to how much civilizations can expand, and that limit is shrinking as the universe continues to expand.
“For a matter-dominated universe, however, the advancing Hubble Sphere, in accordance with the Friedmann equation, shrinks rather than expanding,” Allan L. Alinea, co-author of the paper, told Universe Today. “Simply and colloquially, the planets that are far from a reference planet in this universe (which initially ‘move’ faster than the speed of light) ‘slow down’, making them, at least in principle, reachable.”
Although civilizations in a dark matter-dominated universe will be able to reach fewer and fewer planets over time, they were still able to colonize the universe available to them in a reasonable time frame. In all types of universes, expansion started slowly, before accelerating as they spread out, providing more and more opportunities for nearby planets to inhabit. Finally, as they colonize their bubble, expansion slows because they run out of options.
While it’s an annoyance that we won’t be able to colonize beyond our (huge) observable universe, the study suggests that aliens may not have colonized the galaxy yet and are in the first slow phase of expansion. Like Hart, we don’t have to worry about a lack of contact so far. Once things start to boil, perhaps billions of years after we’re all dead, contact with alien life forms becomes much more likely.
The research has been published on pre-print server arXiv. An earlier version of this article appeared in October 2023.