The best place to find advanced extraterrestrial civilizations may be at the heart of matter-rich galaxy clusters, according to a top Harvard astronomer.
Harvard University's Abraham Loeb, head of the Astronomy Department, says aliens could have traveled to galaxy clusters in preparation for the impending isolation that could happen to intelligent civilizations as the universe rapidly expands far beyond its current size.
In a paper published in the pre-print server ArXiv, Loeb describes how advanced alien civilizations could have congregated in clusters of thousands of galaxies akin to the Milky Way, similar to how Earth's ancient peoples have flocked to the rivers and lakes.
Preparing For The Expansion Of The Universe
In the 1990s, scientists discovered that the universe was expanding at an exponential rate, contrary to the popular theory at the time that gravity will eventually cause the universe to collapse in on itself.
This has led to the theory of dark energy, a mysterious force believed to comprise three-fourths of the entire universe. Experts theorize that dark energy may be responsible for the expansion of the universe against the inward pull of gravity.
By the time the universe turns 138 billion years old, dark energy will have become the most dominant force in the cosmos and all objects in space will have moved away far from each other that they are virtually inaccessible.
This applies to the Local Group as well, the group of galaxies containing the Milky Way, Andromeda, and all their satellites.
Assuming that humanity will still be around when this happens, Loeb recommends it is best to prepare for such drastic isolation by migrating to galaxy clusters.
Migration To Galaxy Clusters
The story of civilizations, according to Loeb, is like that of "The Ants and the Grasshopper." Variations of Aesop's famous fable abound, but the central lesson is that it is best to be prepared for rainy days, or in this case, isolation from the rest of the universe.
Galaxy clusters are groups of thousands of galaxies as big as the Milky Way. They are believed to have formed 10 billion years ago, when perturbations in the density of the primordial universe caused large regions to implode and assemble all that matter to form multiple galaxies. These clusters are held together by the mutual gravitational attraction between the galaxies. Loeb believes this gravity is strong enough to resist the expansion of the universe.
"Clusters of galaxies [...] host the largest reservoirs of matter bound by gravity against the accelerated cosmic expansion," Loeb says.
The nearest candidate is the Virgo Cluster, which can be found toward the Virgo constellation 50 million light-years away. The Virgo Cluster contains a thousand times more matter than the Milky Way. Another viable destination would be the Coma Cluster, which has thousands of galaxies and can be found around 300 million light-years from Earth.
Advanced Civilizations May Have Gotten There First
Anticipating the effects of the expansion of the universe, alien species that have developed technologies far more advanced than humans may have already begun migrating to these rich galaxy clusters.
However, it would be impossible for scientists to detect them unless they emit powerful light signals to propel their spaceships or communicate with one another.
For this to be possible, Loeb says civilizations must have devised a spacecraft that can travel at a speed faster than 1 percent the speed of light. The fastest man-made spacecraft is hundreds of times slower than that, although Russian billionaire Yuri Milner's Breakthrough Starshot Initiative is raising funds to address that.
If aliens have indeed settled in galaxy clusters, Loeb suggests they could be using the energy of the cluster's stars to collect fuel for the isolation that could take place a hundred billion years from now.
Most of the stars in these clusters have masses far less than the sun's, although they burn fuel slower and shine far longer. This means civilizations nestled in galaxy clusters would have access to energy sources for trillions of years. Loeb says it may be best for humans follow to suit.
"It would be beneficial for us to reside in the company of as many alien civilizations as possible with whom we could share technology," he says, "for the same reason that animals feel empowered by congregating in large herds."
Loeb first began studying the distant future of the universe in 2001. His research has led him to correspond with theoretical physicist Freeman Dyson, who suggested that humans undertake a "cosmic engineering project" to move stars in a large region and concentrate them around the Earth in such a way that it could withstand the force of expansion.
The idea is similar to what Dan Hooper of the Fermi National Accelerator Laboratory suggested. In a recent study, Hooper proposed that advanced alien civilizations could use energy harvested from stars to push these stars toward the center of their civilizations.
Hooper said that, if humanity's current understanding of dark energy is correct, extraterrestrials would gather fuel from stars with solar masses between 0.2 and 1.
However, Loeb points out the limitations of Hooper's theory.
"First, we do not know of any technology that enables moving stars around," he says, "and moreover Sun-like stars only shine for about ten billion years and cannot serve as nuclear furnaces that would keep us warm into the very distant future."
For this reason, he believes humans do not need to harvest stellar fuel; we only need to migrate outside of the Local Group to a rich galaxy cluster.