Will the Universe Die?
In a television interview today, accessible at this link, I was asked about the possible death of the Universe in the long-term future.
Fifty years ago, Stephen Hawking calculated that black holes evaporate by the escape of particles which cannot be trapped within their event horizon. Such particles behave like plump prisoners that cannot be confined to stay within the prison walls. Hawking showed that the escape of radiation with a wavelength larger than the event horizon leads to the evaporation of black holes.
My interview was triggered by a new paper arguing that Hawking radiation should be emitted by any spacetime curvature, even in the absence of an event horizon. In a paper that I wrote with Mark Hertzberg, we showed that the newly proposed formalism is inconsistent with established results for Hawking radiation. Nevertheless, the authors ignored our criticism and applied their formalism to any gravitating body. They suggested that stars and dark matter halos should evaporate after a very long time, triggering the death of all matter in the Universe.
This proposal raised an avalanche of doubts. First, it is unclear how this evaporation process is able to violate the conservation of quantum numbers, such as the baryon number of a star that evaporates into radiation. But even ignoring this conceptual hurdle, we cannot be sure that the laws of physics apply over the calculated lifespans of 10^{68} years for neutron stars and 10^{135} years for superclusters of galaxies.
The interviewer asked me to clarify whether the death of the Universe should be of any concern.
I was relaxed in my response for a variety of reasons. First, the fundamental building blocks of atomic nuclei, such as protons, are expected to decay on a much shorter timescale in the simplest extensions of the standard model of particle physics. But even before that, the dark energy that dominates the cosmic mass budget today might evolve and lead to a Big Crunch as part of a cyclic pattern that alternates periodically between Big Bangs and Big Crunches. Finally, we must keep in mind that we do not know if the laws of physics-as-we-know-it hold on timescales as long as 10^{68} to 10^{135} years. The real future might be completely off the rails of the physical reality we experience in the past 13.8 billion years after the Big Bang. For all these reasons, I argued that we should not lose any sleep about this forecasted death of our Universe.
But there are bigger existential issues to worry about in the immediate future. Within a billion years, the Sun will brighten up and boil off all liquid water on the surface of Earth through a greenhouse effect, transforming our planet to an inhospitable desert to life-as-we-know-it. By that time, we will need to construct a space platform powered by its internal nuclear reactor to replace the Earth-Sun habitat for our descendants.
Within 7.6 billion years, the Sun will die, shedding its outer envelope and turning the remaining sixty percent of its mass into a white dwarf, a cooling metallic remnant of Earth’s size. Occupying the Moon or Mars will not solve our existential risk, because the brightening and expansion of the Sun prior to its death would have devastating effects on the Moon or Mars as well. By that time, it would make most sense for humanity to become an interstellar species. Our travels may take our descendants to the habitable zones around the most common red dwarf stars. These abundant stars have down to 7% of the mass of the Sun, and possess natural nuclear fusion reactors which would last for up to ten trillion years, a thousand times longer than the Sun.
Most Sun-like stars already went through this life-threatening evolution because they were born billions of years before the Sun. This implies that many of the technological civilizations that predated us by billions of years had to solve the same existential problems that we are about to face due to stellar evolution. Studying the innovative technologies that they developed as solutions to their existential threats would allow us to cope with our own challenges. The fact that the Sun was born in the last third of cosmic history is a great boon. It allows us to learn from those who predated us.
Based on the latest statistical data on exoplanets, there are billions of Earth-Sun analogs in the Milky Way galaxy. Applying the principle of “survival of the fittest” to the thousands, or perhaps millions, of technological civilizations that predated us, implies that those who were able to escape their star are most likely to survive the longest. They will also be the ones whose technological products were most likely to reach us over the past billions of years. Rather than focus on the discovery of microbes which are inferior to us, we better focus on those lifeforms which are superior to us in the interstellar food chain. The 2020 Decadal Survey of U.S. astronomers recommended as its highest priority to invest over ten billion dollars in the search for the chemical fingerprints of microbes in exoplanet atmospheres. My recommendation is to hedge our bets by investing billions of dollars in the search for technological interstellar objects. Otherwise, the more advanced civilizations would find our scientific ambitions pathetic. They might already be laughing at our mainstream scientific notion that the existence of something like us on an exoplanet like Earth is an extraordinary claim.
This diversification in search strategies will not only improve our chances of finding something. Most importantly, if we find the products of more advanced lifeforms, we might be able to learn new science and technologies from them. And if we ever establish contact with alien scientists, we can ask them: “what is your best forecast for the long-term future of the Universe?”
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 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. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
