The brilliant 87-year-old astrophysicist Jerry Ostriker, called me on zoom today to ask: “What is dark energy?” He had been troubled by this question even during an election week and came up with an out-of-the-box idea. I explained why his idea does not work. “I agree,” he noted in disappointment and added: “So what do you think dark energy is?”
This is a challenging question. Dark energy is the only example we have of repulsive gravity. The vacuum contributed equally to matter in the cosmic mass budget 3.7 billion years ago, nearly a billion years after the Sun was born. At that time, dark energy accelerated cosmic expansion. Albert Einstein’s gravity allows for accelerated cosmic expansion, as long as the vacuum is endowed with some energy per unit volume. Matter and radiation get diluted by the cosmic expansion while the vacuum remains fixed. As a result, the vacuum eventually dominates the cosmic mass budget and generates repulsive gravity with twice its mass density, because of its negative pressure.
In 1998, two observational teams discovered that the cosmic expansion accelerates in the present-day Universe. One of the Nobel laureates for this discovery, Adam Riess, told me that he was inspired to pursue his work after taking my class on “Cosmology” as a graduate student at Harvard in 1994, where I explained how an accelerated or decelerated expansion can be inferred from cosmic distance measurements. I regarded this a trivial insight, but it led Adam to reveal a fundamental property of the vacuum.
Prior to this discovery, physicists reasoned that the natural energy scale for the vacuum energy density is the Planck scale. At the same time, they also realized that the Planck energy density is 123 orders of magnitude larger than the value that would have allowed galaxies like the Milky-Way to exist. Hence, it was argued that the vacuum density is probably zero because of some unknown physical principle. Surprisingly, the observed value is non-zero and happens to correspond to an energy scale that is 31 orders of magnitude below the Planck energy (with the energy density proportional to this scale to the fourth power). Another way to phrase Jerry’s question is: “Where does this energy scale come from?”
One could treat the vacuum energy density like all other fundamental constants in the standard model of particle physics, such as the electron mass or charge, Newton’s constant, Planck’s constant, and the speed of light. We need a complete theory of quantum-gravity to figure out where these constants came from.
In a novel paper that I wrote with Mark Hertzberg last year, we proposed possible principles of quantum-gravity that relate the observed value of the dark energy density to other fundamental constants of the standard model of particle physics. Our reasoning was based on a thought experiment in which we scatter hypothetical black holes of minimal electric charge. We demanded that a scattering process involving the black hole and an electron would take place within the expansion time of an accelerating Universe before the black hole would evaporate away by Hawking radiation. This requirement allows the state of a discretely charged black hole to be well defined in quantum mechanics. By imposing that the black hole’s charge be detectable, we derived a relation between the Hubble expansion time — which is related to the dark energy density, and the electron’s mass and charge, Newton’s constant, Planck’s constant and the speed of light. This relation yields a prediction which agrees with the observed value of the dark energy density and explains the gap between its low energy scale and the Planck scale.
If indeed the value of dark energy is dictated by fundamental constants, as Mark and I proposed, then it should remain constant over cosmic time. Upcoming data from the Dark Energy Spectroscopic Instrument (DESI) will be able to constrain the time evolution of dark energy by recording optical spectra of tens of millions of galaxies and quasars and mapping their three-dimensional distribution over the latest 11 billion years of cosmic history.
Another way to falsify our quantum-gravity proposal for dark energy is to discover a dark matter particle which is charged to a fraction of the electron charge. If such a particle exists, it would have dictated a smaller dark energy. However, in a new paper with Misha Medvedev, I showed that milli-charged dark matter would have shown plasma-like structures that are not seen in gravitational-lensing maps of colliding galaxy clusters. We concluded that the charge-to-mass ratio of dark matter particles must be smaller by at least 19 orders of magnitude than the electron charge-to-mass ratio. Such a tight upper limit is fully consistent with the quantum-gravity argument for dark energy that Mark and I proposed.
Manipulation of dark energy could lead to an exotic substance which induces repulsive gravity. Such a substance could then be used in fancy construction projects by quantum-gravity engineers of an advanced scientific civilization. If these engineers are able to excavate dark energy and make objects out of it, these objects could have a negative gravitational mass. As realized by the physicist Herman Bondi in 1957, placing a negative mass next to a positive mass of equal value would allow for propulsion without fuel, because the negative mass would push the positive mass away while the positive mass would attract the negative mass with it. The exotic dark energy substance might also allow us to construct wormholes for faster-than-light travel between widely separated spatial regions as well as to possibly build time machines.
If we ever encounter extraterrestrial visitors who figured out how to use dark energy in traveling quickly through space, or we find terrestrial visitors from our own future, we might gain new insights into the nature of dark energy. However, if all visitors were forced to move slower than light before arriving at our doorstep, we would have no evidence that repulsive gravity can be used for propulsion. Time will tell if the sky’s the limit on dark energy.
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.
