Is Dark Energy Evolving?

Avi Loeb
5 min readApr 7, 2024
The LSST (Legacy Survey of Space and Time) Camera with 3.2 billion pixels of the Rubin Observatory in Chile. (Image credit: Jacqueline Ramseyer Orrell/SLAC National Accelerator Laboratory)

Life is shaped by choices. The fundamental debate is whether to live life for learning or for pleasure. This weekend, my existential tension boiled down to a simple question: why am I writing my next book and my next scientific paper rather than having fun in the sun?

After some contemplation, I came up with the realization that learning is pleasure. But there is another benefit to writing. Most people will live in the future and I wish to communicate my thoughts to those who will be born long after I am gone. I weigh my priorities in life based on the number of people who might benefit from my actions.

There are currently 8.1 billion people on Earth, about 7% of the total number of humans who have ever lived since the Big Bang, 117 billion. Based on the star count from the Gaia sky survey, the number of stars in the Milky-Way galaxy is comparable to this total value within a factor of a few. This implies that for the foreseeable future, Milky-Way stars could be named after each person who ever lived on Earth.

But there are two important distinctions between stars and people. First, in difference from people, most of the Milky-Way stars were already born. The reservoir of cold interstellar gas that can make new stars is significantly depleted. The same holds true throughout the cosmos. The average rate of star formation per unit mass in the Universe peaked 9 billion years ago and declined by a factor of ten by the present time, 13.8 billion years after the Big Bang. The Sun formed in the last third of cosmic history, 4.6 billion years ago, halfway back to the time when most stars were born. This establishes the rationale for the Galileo Project’s search for interstellar technological objects: advanced technological civilizations could have existed billions of years before us. It takes chemical rockets less than a billion years to cross the Milky-Way disk from one side to the other.

Second, in contrast to the solemn commemoration of the death of people in funerals, massive stars die in explosions that display splendid fireworks in the form of supernovae. The most powerful super-luminous supernovae are detectable all the way to the edge of the observable universe.

Starting in 2025, the 3.2-billion-pixel camera on the Vera C. Rubin Observatory in Chile will document the explosive deaths of massive stars across the entire southern sky every four days. Its cosmic movie will constitute the biggest cinematic production ever made, with the identity of the director only known to religious viewers. Learning about the death of loved ones is the hardest news each of us absorbs in a lifetime. But detecting the death of stars is the most thrilling experience for observers, since it allows them to gauge unknown properties of the Universe. Instead of writing an obituary, they honor the memory of dead stars with scientific papers.

Recently, the Dark Energy Spectroscopic Instrument (DESI) collaboration posted a new preprint, in which they analyzed their 1st year data on the large-scale clustering of 6.4 million galaxies and quasars across a third of the sky all the way back to 1.5 billion years after the Big Bang. The preliminary DESI data confirmed the standard cosmological model, known as Lambda-CDM, in which the dark energy density is constant over time, constituting the so-called cosmological constant, Lambda. In this framework, DESI measured a Hubble constant consistent with the value inferred from the anisotropies of the cosmic microwave background, 68 kilometers per second per megaparsec, arguing against the Hubble tension suggested by supernova data sets.

On its own, the 1st year DESI data did not reveal evolution in dark energy, but when the DESI team combined their data with studies of supernovae, the combined data fit better an evolving dark energy with a suggestive statistical significance of about 3-sigma. Supernova explosions are not fully understood from first principles, and using them for cosmology is akin to gauging the cosmic scale with a ruler that could have evolved over cosmic time. The question therefore remains whether the independent supernova data suffers from unknown systematic uncertainties, in which case we better wait for more DESI data. By now, DESI has three years of data in the can, and within two more years — the DESI team is expecting to complete their full five years of data collection on 37 million galaxies and 3 million quasars. This will improve significantly the statistical confidence in the DESI clustering analysis, as their sample of galaxies and quasars will increase by a factor of 6.2 relative to the latest report. And additional data from the Rubin Observatory will help as well. If we encounter extraterrestrials before 2026, they could give us the answer for less effort, although it may feel like copying during an exam from a smarter student in our class.

An evolving dark energy would be surprising as it would argue for two unexpected coincidences. One is that dark energy started dominating the cosmic mass budget recently and the second is that it evolved over a timescale comparable to the current age of the Universe. The two are independent facts, as one can imagine the vacuum density being a fundamental constant in the present-day Universe, just like the mass or charge of the electron. In fact, in a recent paper I published in Physical Review with Mark Hertzberg, we proposed a relationship between Lambda and the electron mass and charge.

When I started my career in cosmology forty years ago, string theorists argued that the cosmological constant must be zero because a finite value would appear unnatural, more than 122 orders of magnitude smaller than the Planck scale. Later, when dark energy was discovered in 1998 with roughly this value, a prominent string theorist asked the observers to check their data because their finding “did not make sense theoretically.” Six years ago, a team of string theorists led by my Harvard colleague Cumrun Vafa argued for an evolving dark energy, similar to the preliminary DESI results.

This demonstrates that while nature can be more imaginative than we are, once we know what nature is — theorists would find a “natural” way to explain it and claim that they knew it all along. For me, confessing that we learned something new and unexpected brings more pleasure, as in the example of the newly discovered interstellar objects. Learning is pleasure, as long as it is accompanied by humility.

ABOUT THE AUTHOR

Image credit: Chris Michel (October 2023)

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. His new book, titled “Interstellar”, was published in August 2023.

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Avi Loeb

Avi Loeb is the Baird Professor of Science and Institute director at Harvard University and the bestselling author of “Extraterrestrial” and "Interstellar".