Some of my colleagues are disappointed that the cosmic expansion is accelerating. Their frustration stems from my 2002 paper, which showed that once the Universe will age by a factor of ten, all distant galaxies will exit from our event horizon and our galaxy will be left alone in darkness. To lift up their spirits, I published a decade later a more positive cosmic forecast, showing that escaping stars from our own galaxy would still allow astronomers to monitor the cosmic expansion just as distant galaxies do today.
Personally, I feel fortunate to live in an expanding Universe because as I age I prefer to have more free space for myself and a contracting Universe would have cluttered my environment with distractions. But I realize that social inhabitants would have preferred to reside in a contracting Universe, where they get closer to their extragalactic friends over time. Let us review this alternative more closely to decide upon its pros and cons.
The underlying equations of cosmology are time symmetric and the arrow of time is chosen by the initial conditions. A priori, we had a 50% chance of being born in a contracting universe instead of an expanding one. This would have traded the familiar cosmological redshift for a blueshift. In order to probe distant galaxies, we would have needed to build space telescopes that are sensitive to ultraviolet light as appropriate for blueshift of starlight, instead the recently launched JWST — which is tuned to the infrared to accommodate the redshift from cosmic expansion.
It is straightforward to figure out what is needed for reversing expansion to contraction. A ball thrown up at less than the escape speed from Earth would initially recede away from us, then slow down by gravity, and finally return back down. Similarly, if the initial expansion rate of the Universe was low enough, the recession of galaxies away from us would have been eventually reversed by the attractive nature of gravity.
In particular, if the Hubble constant was half of its measured value while the cosmic matter density was kept the same, then the Universe would have already been contracting at this cosmic time, because the gravity induced by matter would have stopped the cosmic expansion and reversed it to a contraction even before dark energy became important or the Sun was born.
My colleagues could not have been happier under these circumstances. As time would have progressed into our cosmic future — the night sky would have become brighter, making astronomy easier. Instead of building ever bigger telescopes to resolve distant galaxies, astronomers could have waited until these galaxies got closer to us.
The reversal of the familiar Hubble expansion would have implied that the farther a galaxy is, the faster it approaches us. This is bad news because it would have set a finite lifespan for our future, namely the ratio between the distance and the approach speed of a galaxy at that distance. The density of matter would have diverged at one instant in our future, the time of the Big Crunch. Nothing in our reality would have survived beyond that time.
As the Universe would have contracted, the cosmic radiation background would have gotten brighter, ultimately exceeding the surface brightness of the Sun. We are used to the notion that stars radiate energy into space, but this only happens because they are hotter than their environment. Once the contracting Universe would have been hotter, stars would have absorbed energy instead of radiating it away, ultimately getting crushed by the ever-growing cosmic pressure and gravity. Eventually all atoms would have been broken into their constituent electrons and quarks and mixed together into a uniform soup that gets hotter and denser over time. Some of the matter would have been funneled into pre-existing black holes, but those would have represented a negligible fraction of the cosmic mass budget.
Near the Big Crunch, quantum gravity theorists would have had an opportunity to test their ideas and celebrate their success if they could have engineered a capsule that protected them and Nobel committee members from the cosmic doomsday. Of course, there is also the hypothetical possibility that the universe would have bounced before reaching the Planck density, as postulated in cyclic cosmological models which have a following expansion phase.
For socially inclined inhabitants of the cosmos, the contracting phase offers the advantage of bridging extragalactic distances without the need to board a spaceship. But for me the realization that a Big Crunch is awaiting in our future would have been as claustrophobic as falling towards the singularity of a black hole. Yes, you get closer to cosmic neighbors, but no companionship can compensate for the fact that the end is near. Another way to view a Big Crunch is as a mass grave, a relic of quantum gravity at the Planck density. The farewell parties to life-as-we-know-it would ultimately be forgotten in the final soup of hot and dense gas of elementary particles. No monument would be left to celebrate our transient past existence. No tear will be shed given the extreme density and temperature of the end.
Is it merely a matter of cosmic luck that we live in an ever-expanding Universe? The answer is linked to the unknown process that triggered the Big Bang.
Given this ignorance, let us bravely embrace the cosmic loneliness awaiting in our future and be grateful for the fortune of cosmic expansion. All in all, the alternative sounds worse. The grass does not appear to be greener in contracting parts of the multiverse.
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”, is scheduled for publication in August 2023.
