The more we learn about planets inside and outside our Solar System, the more difficult the question "Well, where the hell are aliens then?" seems to become.
As well as finding a slew of planets in the habitable zones around their stars and locating new classes of exoplanets that may be good candidates for life, we have discovered life on Earth enduring extreme environments we hadn't thought possible. So where is everybody?
Among early attempts to answer this question, known as the Fermi Paradox, one astrophysicist tried to investigate how long it would take for one (or many) civilizations to colonize the galaxy. In 1974, Michael Hart's dramatically-titled paper "An Explanation for Absence of Extraterrestrials on Earth" argues that if a civilization tried, it would be able to colonize the galaxy in a relatively short time span.
"Assume that we eventually send 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 of eventually sending out their own expeditions, and their colonies in turn can colonize, and so forth," Hart wrote.
"If there were no pause between trips, the frontier of space exploration would then lie roughly on the surface of a sphere whose radius was increasing at a speed of 0.10c. At that rate, most of our Galaxy would be traversed within 650,000 years. If we assume that the time between voyages is of the same order as the length of a single voyage, then the time needed to span the Galaxy 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 commenced space exploration less than 2 million years ago."
If this is anywhere close to correct, and given that our galaxy is around 13.6 billion years old, it's puzzling that we have not found of extraterrestrial civilizations. In fact, the paper argues, the absence of aliens on Earth could be seen as evidence that there is no alien life out there to find at all. If there is, Hart said, it would require a sociological explanation for why they did not colonize the galaxy when they have had ample chance to do so. Unless a convincing one can be found, and until we have more information on planetary formation and other factors, Hart said that this is "strong evidence that we are the first civilization in the galaxy".
So, as we look to the stars should we expect to see a colonized galaxy? According to a team in a new preprint paper that attempted to explore how quickly a civilization can occupy the planets within their reach, expansion is not quite this uniform.
They created a simplified model of the universe composed of cells representing habitable planets. A 3D spherical section of this model was then isolated, with a "civilization" at its center. This civilization moves out to the other cells at a constant speed and uses the resources from the new planet to keep spreading further around the galaxy.
Complicating matters, 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 has it that the universe was matter-dominated. In both matter and dark matter-dominated universes, there are planets that are outside of the reach of any civilization, as they are receding (from their point of view) faster than the speed of light.
We can only see light that has made it to our vantage point, meaning that there is a limit to how much of the universe we can see, known as the observable universe, as the light has not yet made it to us.
As the universe expands (as it does in a dark matter-dominated universe) the distance between us and all other stars increases, and our observable universe shrinks. In a dark-matter-dominated universe, there is a limit to how far civilizations can expand, and that limit is shrinking as the universe continues to expand.
"However, for a matter-dominated universe, in accordance with Friedmann Equation, the comoving Hubble Sphere is shrinking instead of expanding," Allan L. Alinea, who co-authored the paper, told Universe Today. "Put simply and informally, those planets far away from a reference planet in this Universe (that are initially 'moving' faster than the speed of light) are 'slowing down,' making them reachable, at least in principle."
Though civilizations in a dark matter-dominated universe will be able to reach fewer and fewer planets as time goes on, they were still able to colonize the universe available to them in a reasonable time frame. In all types of universe, the expansion began slowly, before speeding up as they spread, giving themselves more and more options for nearby planets to inhabit. Finally, as they colonize their bubble, expansion slows as they run out of options.
While it's an annoyance that we won't be able to colonize beyond our (massive) observable universe, the study suggests that maybe aliens haven't colonized the galaxy yet, and are in the initial slow phase of expansion. We shouldn't be worried, as Hart was, by a lack of contact thus far. Once things start cooking up, perhaps billions of years after we're all dead, contact with alien life forms becomes much more likely.
The study is published on pre-print server arXiv.