The standard cosmological model is simple. The expanding Universe has the total mass density corresponding to a nearly flat geometry and its mass budget today is dominated by two components: a cosmological constant — labeled as “dark energy” and weakly-interacting particles — labeled as “dark matter”.
The simplest incarnation of dark energy is a constant energy density of the vacuum, commonly called “the cosmological constant”. This possibility was conceived by Albert Einstein as a source of repulsive gravity that would balance out the attractive gravity of cosmic matter, before it became clear that the Universe is expanding. In the case of a cosmological constant, dark energy is a constant of Nature, just like other constants of the Standard Model of Particle Physics, such as the electron mass and charge.
The most natural value of the cosmological constant is the Planck energy density, which is 122 orders of magnitude larger than the observed value. In collaboration with Mark Hertzberg, I wrote a paper recently that relates this small value to the electron mass and charge in the context of a set of underlying principles for quantum gravity. However, string theory — the most popular contender for a fundamental theory of quantum-gravity, forecasts a huge number of possible vacuum states. It explains the observed value of the cosmological constant in terms of anthropic reasoning. Much larger values would have limited the growth of structure and not allowed galaxies like the Milky-Way to form, inside of which stars like the Sun are made, next to which planets like the Earth emerge. In this anthropic interpretation, our Universe appears to be “fine-tuned” for us to exist in it because other regions of spacetime in the multiverse do not lead to the emergence of conscious observers like us.
Last week, the DESI collaboration announced a set of papers with a new analysis of dark energy using three years of collected data, which spans nearly 15 million galaxies and quasars. The DESI observations were combined with data from studies of the cosmic microwave background, supernovae, and weak gravitational lensing. The standard model of cosmology struggles to explain all the observations when taken together, but a model in which the dark energy density declines with cosmic time fits all current data well.
If confirmed by future data, this inference spoils the simplicity of the standard cosmological model. It suggests that dark energy evolved over time and is not a cosmological constant. The evidence is still tentative with a statistical discrepancy between the data and the standard cosmological model of at most 4.2 standard deviations, representing one in 50,000 chances that the results are a fluke.
If the discrepancy with the standard model will be firmed up within a year or two, the implications will be difficult to understand theoretically. In such a case, not only the value of the dark energy needs to be fine-tuned to within 122 orders of magnitude below the Planck scale but also its evolution timescale should be similar to the current age of the universe. There is no anthropic argument or a combination of fundamental constants that would explain this evolution time coincidence. The intriguing possibility is that perhaps we do not understand gravity on cosmic scales.
But there are cosmic benefits to the breakdown of the standard model. With a cosmological constant, the accelerated expansion of the Universe will continue forever, leaving our own galaxy in empty space once the Universe will age by a factor of ten. As I pointed out in a 2001 paper, shortly after a non-vanishing cosmological constant was discovered, all distant galaxies will exit from our cosmic event horizon and their images will freeze and fade away, just like fireflies falling into the event horizon of a black hole. The cosmic microwave background will fade exponentially with time to an undetectable level and our descendants will be surrounded by a dark and lonely space. No spacecraft limited by the speed of light will be able to escape this unavoidable prison cell set by the cosmic horizon of a spacetime which is dominated by a cosmological constant. In contrast from the prison of a black hole, where the prisoner’s body gets torn apart over a finite time by tides near the central singularity, in a Universe dominated by a cosmological constant the prisoner freezes to death in the darkness of empty space.
But if dark energy will vanish over time, then the long-term future of extragalactic astronomy will be much brighter. With improved instruments, future observers will have access to more galaxies than we see today and the Universe will not be a lonely place. For those of us who are not suffering from cosmic claustrophobia and wish to enjoy the company of alien civilizations in other galaxies, this is a far better future.
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.
