Dark Energy May Not Be Constant
The great Russian physicist and Nobel laureate Lev Landau once remarked that “cosmologists are often in error, but never in doubt”. In studying the history of the universe itself, there is always a chance that we have got it all wrong, but we never let this stand in the way of our inquiries.
A few days ago, a new press release announced groundbreaking findings from the Dark Energy Spectroscopy Instrument (DESI), which is installed on the Mayall Telescope in Arizona. This vast survey, containing the positions of 15 million galaxies, constitutes the largest three-dimensional mapping of the universe to date. For context, the light from the most remote galaxies recorded in the DESI catalogue was emitted 11 billion years ago, when the universe was about a fifth of its current age.
DESI researchers studied a feature in the distribution of galaxies that astronomers call “baryon acoustic oscillations”. By comparing it to observations of the very early universe and supernovae, they have been able to suggest that dark energy – the mysterious force propelling our universe’s expansion – is not constant throughout the history of the universe.
An optimistic take on the situation is that sooner or later the nature of dark matter and dark energy will be discovered. The first glimpses of DESI’s results offer at least a small sliver of hope of achieving this.
However, that might not happen. We might search and make no headway in understanding the situation. If that happens, we would need to rethink not just our research, but the study of cosmology itself. We would need to find an entirely new cosmological model, one that works as well as our current one but that also explains this discrepancy. Needless to say, it would be a tall order.
To many who are interested in science this is an exciting, potentially revolutionary prospect. However, this kind of reinvention of cosmology, and indeed all of science, is not new, as argued in the 2023 book The Reinvention of Science.
THE SEARCH FOR TWO NUMBERS
Back in 1970, Allan Sandage wrote a much-quoted paper pointing to two numbers that bring us closer to answers about the nature of cosmic expansion. His goal was to measure them and discover how they change with cosmic time. Those numbers are the Hubble constant, H₀, and the deceleration parameter, q₀.
The first of these two numbers tells us how fast the universe is expanding. The second is the signature of gravity: as an attractive force, gravity should be pulling against cosmic expansion. Some data has shown a deviation from the Hubble-Lemaître Law, of which Sandage’s second number, q₀, is a measure.
No significant deviation from Hubble’s straight line could be found until breakthroughs were made in 1997 by Saul Perlmutter‘s Supernova Cosmology Project and the High-Z SN Search Team led by Adam Riess and Brian Schmidt. The goal of these projects was to search for and follow supernovae exploding in very distant galaxies.
These projects found a clear deviation from the simple straight line of the Hubble-Lemaître Law, but with one important difference: the universe’s expansion is accelerating, not decelerating. Perlmutter, Riess, and Schmidt attributed this deviation to Einstein’s cosmological constant, which is represented by the Greek letter Lambda, Λ, and is related to the deceleration parameter.
Their work earned them the 2011 Nobel Prize in Physics.
DARK ENERGY: 70% OF THE UNIVERSE
Astonishingly, this Lambda-matter, also known as dark energy, is the dominant component of the universe. It has been speeding up the universe’s expansion to the point where the force of gravity is overridden, and it accounts for almost 70% of the total density of the universe.
We know little or nothing about the cosmological constant, Λ. In fact, we do not even know that it is a constant. Einstein first said there was a constant energy field when he created his first cosmological model derived from General Relativity in 1917, but his solution was neither expanding nor contracting. It was static and unchanging, and so the field had to be constant.
Constructing more sophisticated models that contained this constant field was an easier task: they were derived by the Belgian physicist Georges Lemaître, a friend of Einstein’s. The standard cosmology models today based on Lemaître’s work, and are referred to as Λ Cold Dark Matter (ΛCDM) models.
The DESI measurements on their own are completely consistent with this model. However, by combining them with observations of Cosmic Microwave Background and supernovae, the best fitting model is one involving a dark energy that evolved over cosmic time, and that will (potentially) no longer be dominant in the future. In short, this would mean the cosmological constant does not explain dark energy.
THE BIG CRUNCH
In 1988, the 2019 physics Nobel laureate P. J. E. Peebles wrote a paper with Bharat Ratra on the possibility that there is a cosmological constant that varies with time. Back when they published this paper, there was no serious body of opinion about Λ.
This is an attractive suggestsion. In this case the current phase of accelerated expansion would be transient and would end at some point in the future. Other phases in cosmic history have had a beginning and an end: inflation, the radiation-dominated era, the matter-dominated era, and so on.
The present dominance of dark energy may therefore decline over cosmic time, meaning it would not be a cosmological constant. The new paradigm would imply that the current expansion of the universe could eventually reverse into a “Big Crunch.”
Other cosmologists are more cautious, not least Carl Sagan, who wisely said that “extraordinary claims require extraordinary evidence”. It is crucial to have multiple, independent lines of evidence pointing to the same conclusion. We are not there yet.
Answers may come from one of today’s ongoing projects – not just DESI but also Euclid and J-PAS – which aim to explore the nature of dark energy through large-scale galaxy mapping.
While the workings of the cosmos itself are up for debate, one thing is for sure – a fascinating time for cosmology is on the horizon.
(Authors: Bernard J.T. Jones, Emeritus Professor, University of Groningen; Licia Verde, Profesor ICREA de Cosmologia en el ICCUB de la Universidad de Barcelona, Universitat de Barcelona; Vicent J. Martínez, Catedrático de Astronomía y Astrofísica de la Universitat de València, y miembro del Observatorio Astronómico de la misma institución, Universitat de València, and Virginia L Trimble, Physics and Astronomy, University of California, Irvine)
(Disclosure Statements: Licia Verde receives funding from the AEI (Spanish State Research Agency) project number PID2022-141125NB-I00, and has previously received funding from the European Research Council. Licia Verde is a member of the DESI collaboration team | Vicent J. Martínez receives funding from the European Union NextGenerationEU and the Generalitat Valenciana in the 2022 call “Programa de Planes Complementarios de I+D+i”, Project (VAL-JPAS), reference ASFAE/2022/025, the research Project PID2023-149420NB-I00 funded by MICIU/AEI/10.13039/501100011033 and ERDF/EU, and the project of excellence PROMETEO CIPROM/2023/21 of the Conselleria de Educación, Universidades y Empleo (Generalitat Valenciana). He is a member of the Spanish Astronomy Society, the Spanish Royal Physics Society and the Royal Spanish Mathematical Society | Bernard J.T. Jones and Virginia L Trimble do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment)
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