I am sorry to be the bearer of bad cosmic news. But our supplies are limited in the long-term future of the Universe. If we stay within our host Milky-Way galaxy, the supplies would at most double after the merger with our sister galaxy, Andromeda, in a few billion years. But beyond that time, the standard cosmological model forecasts that our reservoir will not grow much even if we wait forever. In a paper that I wrote with my former postdoc, Ken Nagamine, we simulated the motion of all nearby systems and demonstrated that all other galaxies will be carried away from us by the cosmic expansion.
If the Universe had been dominated by matter, the matter’s attractive gravity would have slowed down the cosmic expansion. This, in turn, would have implied that by boarding a fast-enough spacecraft we would have been able to reach any galaxy within our cosmic horizon, because the recession speed of every galaxy would have slowed down over time. As more galaxies would have entered our horizon, we could have accessed more resources at our disposal. The unlimited cosmic resources in a matter dominated universe could have fueled our economy forever.
However, the universe we live in is not as generous as we would have wished for. Rather than decelerating, the cosmic expansion is accelerating as a result of the repulsive gravity of the vacuum, the so-called “cosmological constant”. As I showed in a recent paper, the accelerated expansion means that we can only visit a finite number of galaxies in our cosmic neighborhood irrespective of how fast our spacecraft is moving. Given the limited supplies within these systems, where should we go?
In an email exchange about this gloomy cosmic future that I had with Freeman Dyson in January 2011, I suggested that we aim to reach a nearby rich cluster of galaxies, such as the Coma cluster, which contains a thousand times the mass of the Milky-Way galaxy or equivalently over a quadrillion times the mass of the Sun. Clusters of this scale represent the biggest reservoirs of mass that would stay gravitationally bound because of their high matter density, despite the accelerated expansion of the Universe. Their total mass reservoir would allow us to survive for a thousand times longer than the content of the Milky-Way. In addition, they offer a thousand times more opportunities to meet advanced technological civilizations from whom we can learn and with whom we can socialize.
About 83% of the mass in these clusters is composed of dark matter and most of the remaining ordinary matter is in the form of hot gas at a temperature of tens of millions of degrees, hotter than the center of the Sun. The cluster gas does not burn as nuclear fuel because of its low density. The cores of these clusters also host the biggest black holes in the Universe, containing tens of billions of solar masses. The fraction carried by the central black hole mass out of the total cluster mass, about a part in a hundred thousand, is similar to that of the Milky-Way which hosts a black hole of 4 million solar masses in a dark matter halo of a trillion solar masses.
But there is a challenge. To reach the Coma cluster at a distance of 330 million light years before it exits from our cosmic horizon, we will need to board a spacecraft that moves at a speed of 4% of the speed of light, or 12,000 kilometers per second. Such a craft would traverse a distance equal to the diameter of the Earth in one second. This speed is 740 times faster than our most advanced interstellar spacecraft, New Horizon. This is roughly the factor by which New Horizons is faster than the first affordable car from 1908, Ford’s Model T. Our engineers have a lot of work to do before we reach this technological milestone. The Starshot Initiative, for which I chair the Science Advisory Board, is the first attempt to develop the technology to propel a spacecraft to a significant fraction of the speed of light.
If we ever observe through our telescopes signs of migration by other civilizations towards the Coma cluster, we will know that this is the wise strategic decision for our civilization as well.
Once the Universe will age by another factor of ten, a hundred billion years from now, all other galaxies will exit from our event horizon. Most stars — having a tenth of the mass of the Sun, will still be burning their nuclear fuel at that time. If we manage to reach the Coma cluster by then, we would have maximized our fuel supplies for the long period of loneliness that awaits us in our cosmic future when our host system would be surrounded by the darkness of vacuum in a space empty of matter.
This forecast of cosmic loneliness relies on the stability of the cosmological constant. If our vacuum will decay before we lose most of the matter out of our cosmic horizon, then we would all heave a sigh of relief that we were saved from the cosmic doomsday of death by starvation.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He chairs the advisory board for the Breakthrough Starshot project, and is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. His new book, titled “Interstellar”, was published in August 2023.
