Explorers of the ocean realized that the world does not end at the horizon. They discovered new continents that were not visible from the starting point of their journey.
In the same way, we do not expect our universe to end at the cosmic horizon observable to our telescopes. There must be galaxies like the Milky Way beyond our current cosmic horizon, which we cannot see because the light emitted by them takes more time to reach us than the 13.8 billion years that have elapsed since the Big Bang. Is there any way for us to figure out what lies beyond our cosmic light horizon?
Evidence on super-horizon scales of the terrestrial ocean had been provided through debris carried by ocean currents, long before explorers ventured into the unknown. Is there a cosmic analog? For example, if there is a “cliff” outside our cosmic horizon, then the gravitational tide it induces could have left a detectable imprint inside the region we observe.
In 1978, Leonid Grishchuk and Yakov Zel’dovich wrote a paper which showed that the large-scale anisotropies of the microwave background radiation, left over from the Big Bang, impose tight constraints on the homogeneity and isotropy of the Universe on length scales much larger than the cosmic horizon. State-of-the-art measurements of these microwave background anisotropies rule out a “cliff” associated with a large variation in the mean cosmic density to be on scales that are at least 4,000 times farther away from us than the cosmic horizon. This means that there are at least 64 billion (4,000 cubed) more galaxies than the trillions of galaxies observable in the deepest images of the Webb telescope.
One might naively assume that if we wait for a while, we might see beyond the current horizon because light emitted by super-horizon galaxies will have time to reach us. Unfortunately, this is not the case because the cosmic expansion is accelerating at the present epoch. By now, the cosmic matter and radiation were diluted to a level that dipped below the energy density of the vacuum. According to Albert Einstein’s theory of General Relativity, a constant vacuum density — labeled as the “cosmological constant”, generates a repulsive gravitational force that pushes galaxies apart at an ever-increasing speed.
As a result, the sources of light beyond our horizon are receding away from us faster than light. In other words, the gap between us and them is opening up at a rate that cannot be bridged by light. Since nothing can travel faster than light, this means that no ships can sail beyond our cosmic horizon. How can this be understood?
Consider ladybugs walking on the surface of an expanding balloon. If the balloon expands at a rate faster than the walking speed of the ladybugs, they will never be able to visit a new region on the balloon’s surface. Hence, their horizon will never expand. The ladybugs in this metaphor are analogous to photons, the particles of light, and the expanding balloon is analogous to the expanding space. In Einstein’s theory of gravity, the fabric of space can expand faster than light, even though particles cannot move locally faster than light.
In 2001, I wrote a paper titled “The Long-Term Future of Extragalactic Astronomy”, in which I showed that we will never see light from galaxies that are not visible to us now. Moreover, if we had an Instagram account shared with friends in distant galaxies, then we will stop getting updates from them beyond a certain time in their future. This would be the time when their recession speed would reach the speed of light, implying that after this time — they move away from us faster than light and exit our horizon.
The situation is similar to considering friends who specialize in string-theory and decide to test the theory by bravely venturing inside the horizon of a black hole. We would lose contact with these friends beyond the time tag that marks their entry into the black hole horizon — from where no signal can escape. As they approach the horizon, their image will freeze and fade away, just like cowboys riding into the horizon at the end of a Western movie. Crossing of the cosmic horizon in an accelerating universe looks just the same.
If the accelerated cosmic expansion will continue as the Universe ages by a factor of ten, then all galaxies beyond our own will exit from our horizon, as I showed in a 2002 paper with my former postdoc Ken Nagamine. By that time, the Milky Way and its sister galaxy, Andromeda, would have merged as described in a 2008 paper I wrote with my former postdoc T.J. Cox. Although the Sun would have died, most stars — like the nearest one, Proxima Centauri — would still be burning their nuclear fuel. Future generations of astronomers could continue to monitor the cosmic expansion by residing on habitable planets around dwarf stars like Proxima Centauri.
At that time, the cosmic microwave background will fade to an undetectable level and the Hubble expansion will be unobservable for lack of visible galaxies outside our own. However, in a 2011 paper I showed that dwarf stars ejected from the Milky-Way could serve as lampposts that are carried away by the cosmic expansion, allowing future observers to trace the cosmic dynamics. Cosmology, therefore, has a future even in the darkness that awaits us. There will be some lights at the end of the tunnel.
And as long as there is light, there is hope that the accelerated expansion will cease as a result of the decay of the vacuum to a zero-energy state. If that happens before the lights turn off and the party is over, we could be united again with our extragalactic Instagram followers.
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.