Imagine sitting in the center of a firework that has just exploded. After the first flash of light and heat, sparks fly off in all directions, with some streaming together into fiery filaments and others fading quickly into cold, ashy oblivion. After a moment more, the smoke is all that remains—the echo, if you will, of the firework’s big bang.
Now imagine the firework is the universe, which scientists think began with a similar explosion. Where the firework’s expansion is propelled by a chemical reaction, the expansion of the cosmos comes from the energy of empty space itself. From where we sit, it seems that the universe is expanding in all directions, faster and faster at every moment.
This spring scientists announced that something is wrong with the fireworks. For the first time since the discovery of dark energy—the mysterious force that is accelerating our cosmic fireworks show—cosmologists think we may be on the cusp of something new. Two prominent dark energy surveys seeking to measure the nature of this force found evidence that dark energy seems to have weakened over time.
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“If it is true, it is a big deal,” says Licia Verde, a theoretical cosmologist at the Institute of Cosmos Sciences of the University of Barcelona in Spain and a member of the collaboration reporting the oddity. “But as usual, extraordinary claims require extraordinary proofs.”
Dark energy was assumed to be a constant force in the universe, as unchanging and reliable as the forward march of time. If the new results are right, it is changeable after all. “It’s mega important,” says Paul J. Steinhardt, a cosmologist at Princeton University, who did not work on the data, adding that this is only true if the results hold up to scrutiny. “But it’s still early days.”
The news is based on a combination of two dark energy studies, called the Dark Energy Survey (DES) and the Dark Energy Spectroscopic Instrument (DESI), with a third set of preexisting data. DES measures distant supernovae, and the DESI experiment measures galaxies and sound waves from the early universe. The third component measures the cosmic microwave background (CMB)—the smoke ring of the cosmic firework.
DES yielded new findings back in February, and DESI came out with novel results in April. The DESI data produced a detailed three-dimensional map of the universe. It showed that galaxies appear to be spread apart less than they should be if dark energy’s role was unchanging through cosmic time.
The DESI telescope is perched on Kitt Peak in Arizona and measures the positions of millions of galaxies as they existed between 12 billion and two billion years ago. Astronomers compared these observed galactic locales against where galaxies are expected to be based on dark energy predictions and saw the lack of expected spreading.
A bigger surprise came when cosmologists combined the DESI galaxies, DES’s supernovae and the cosmic microwave background. The map of reality began to drift apart from theory.
Theorists have been buzzing since: if the results are true, a bedrock assumption of cosmology is incorrect. Scientists might have to throw out the widely held idea that dark energy is a “cosmological constant”—a static element of the universe.
“If the cosmological constant is wrong, all bets are off about what’s right,” says Adam G. Riess, a cosmologist at Johns Hopkins University, who shared the 2011 Nobel Prize in Physics for the discovery of dark energy and did not work on the new results.
To understand what’s wrong, we have to go back to Albert Einstein. When he was formulating his general theory of relativity, he assumed that the universe was evenly spread out and stationary. This idea was bold in 1917, when we didn’t even know there were other galaxies and the evidence suggested that stars were not spread out evenly. But in Einstein’s equations, gravity and uniformity don’t get along. Gravity causes instability. If gravity was dominant in a curved universe, then everything in the cosmos should clump together into one big blob—but it doesn’t. Einstein assumed there must be some cosmic force counteracting gravity, which he called a “cosmological constant” and described using the Greek letter Lambda. In 1929, however, Edwin Hubble showed that the universe was not static but expanding, so Einstein abandoned this constant counteracting force, calling it his “biggest blunder.”
In 1998 Riess, as well as cosmologists Saul Perlmutter of the University of California, Berkeley and Brian P. Schmidt of the Australian National University showed that Einstein was right the first time. The researchers saw that supernovae that exploded when the universe was young were fainter than would be expected, which implied the universe was expanding outward ever faster, presumably because of an omnipresent and unchanging force. The cosmological constant was resurrected. Scientists believe that the force behind the constant must come from energy present in empty space, which they call vacuum energy, or dark energy. In this view, as the universe expands, each new bit of the growing vacuum comes with its own vacuum energy, so the total amount of dark energy grows, causing the cosmic expansion to continue to accelerate. (Dark energy is still a constant because, although its total rises as the universe grows, the amount of energy in each piece of space—the energy density—is constant.) What’s more, the constant value of dark energy was set at the big bang and then never lessened. Bang, zap: the universe has the same inherent energy everywhere, all at once.
All observations since the late 1990s have seemed to confirm this scenario. Lambda is now the heart of the standard model of cosmology, which combines dark energy with the gravitation of copious, invisible “cold dark matter” (CDM), known as Lambda-CDM. This standard model holds that about 68 percent of the universe is made up of dark energy, another 27 percent is dark matter, and the remaining 5 percent is everything we can see and measure: galaxies, stars, whales, us. Surveys like DESI were designed to measure dark energy precisely enough to understand its quirks.
Still, not everyone has been satisfied with the Lambda-CDM model. “It seems like a very peculiar set of affairs,” Steinhardt says. “The only thing that’s nice about it is that it is described by a single number. But that doesn’t mean you should believe it. And if it turns out that dark energy is time-varying, that opens up a lot of possibilities.”
DESI has made the largest 3D map of our universe to date. Earth is at the center of this thin slice of the full map. In the magnified section, it is easy to see the underlying structure of matter in our universe.
Claire Lamman/DESI collaboration; custom colormap package by cmastro
Theorists have been increasingly busy since the DESI results and the combined 3D map both dropped in April. So far, no single theory can supplant Lambda with some other nonconstant cosmological force. Even before the new results, some cosmologists favored alternatives to a constant dark energy, in part because the idea is so weird—other known forces are not constant but vary with time, pressure and other factors—and in part because Lambda is nonsensical when inserted into other physics theories. “In quantum theory, if you calculate the energy of empty space, you don’t get a sensible answer. You get infinity,” says Joshua A. Frieman, a cosmologist at the University of Chicago, who co-founded DES to study this problem and is not involved in the new DESI results. “That’s one reason people have looked for alternatives: we don’t understand why it would have this value.”
Theorists have several ideas for new kinds of dark energy, most of which involve a fluidlike energy field, reminiscent of the Higgs field that endows normal particles with mass. The proposed dark energy field is often called “quintessence,” after a classic fifth element first imagined in antiquity. There are a few ways such an energy field, also called a scalar field, could work. It would produce the same results we can see—galaxies flung apart from one another and an apparently expanding empty canvas on which to see them—but the difference is that the cosmological force is temporary, not everlasting and unchanging.
Some theorists favor quintessence because they already study a potential earlier version of a scalar field called cosmic inflation. This field would have affected the universe immediately after the big bang, driving it to expand exponentially before eventually calming down and continuing to accelerate at a slower rate. A scalar field underlying modern dark energy is like “inflation lite,” according to Riess. Where there is energy in the physical space of the universe, the universe will accelerate. The dark energy field would be exceptionally weak, about 30 orders of magnitude lighter than the Higgs field. And it would be temporary like inflation was.
One popular version, first proposed by Frieman and his collaborators in 1995, is called “thawing” or “slowly rolling” dark energy. Its effects on the universe would be similar to those of a cosmological constant—to a point, says Jessie Muir, a theoretical cosmologist at the Perimeter Institute. “It acts like empty space has some intrinsic energy density,” she says, “but because it changes in the later universe, you can get some deviations at later times.”
Think of a ball rolling down one side of a hill toward a shallow, U-shaped valley. If there is no friction, the ball will roll up the opposite side, then roll back down and oscillate back and forth. The ball represents the field’s potential, which describes how easy it is to move that field in relation to the density or expansion of the universe, Muir says. This is also one way to understand the Higgs field, which physicists think underwent some changes early in the universe before it reached its current state. A similar but much heavier field could have driven cosmic inflation. If dark energy works in the same way, there’s precedent for it, Frieman says.
“In the only other case of acceleration we know about (inflation), we know it wasn’t dark energy, the constant. It was something else,” he says. “I’ve always felt we need to keep an open mind about what is driving the current accelerated expansion of the universe.” In Frieman’s versions of this theory, you put the ball on one side of the hill at the beginning of the universe. The ball is stuck at first because the universe is too dense for it to roll quickly. As the universe expands and matter dilutes, the ball can begin to roll. This is called a “thawing” dark energy model because the ball unfreezes and begins to move. “That is acting like dark energy but dark energy that has a different impact on the expansion of the universe than if it was exactly constant,” Frieman says. “And it can look exactly like what the recent results from DESI and the Dark Energy Survey and CMB data seem to be suggesting.”
Theorists are also testing ideas such as the “big bounce,” a cyclic universe in which the big bang happens again and again, as well as variations on general relativity in which gravity behaves differently in the very early universe or on different scales, among other possibilities.
A flurry of scientific papers are being uploaded to the preprint server arXiv.org, where cosmologists are sharing ideas and paths forward. Everything is on the table, from arguments about the masses of particles called neutrinos to discussions of the best statistical methods for comparing data. “I wouldn’t say there is a specific model so far that seems to be taking the lead,” says Nathalie Palanque-Delabrouille, a cosmologist at Lawrence Berkeley National Laboratory and a spokesperson for the DESI project. “There are too many hypotheses out there, and many models can fit the data. That’s why it’s important to keep going.”
A coming generation of new observatories will shed light on dark energy—or whatever other force is driving the expansion of the universe. The Euclid space telescope, operated by the European Space Agency, launched last year and will work until 2030 to create a map of almost one third of the sky, charting dark matter and dark energy. NASA’s future Nancy Grace Roman Space Telescope will measure more than a billion galaxies to study dark energy over time. And the DESI survey will continue through 2026.
For Lambda to fall, cosmologists would want a five-sigma level of confidence, which means about a one-in-a-million chance that the findings are a result of error or random chance. So far, the DESI, DES and cosmic microwave background results from the Planck satellite show a probability of three sigma, which is about a 0.3 percent probability of something happening by chance. While this sounds like strong evidence, three-sigma results can fail under scrutiny, so a five-sigma finding is necessary for a real discovery. DESI is continuing its work, but the team already has another year of galactic data to examine, and Verde says her colleagues are clamoring for it. “I am working until my hands are on fire,” she says.
Muir, who also studies general relativity and tests of gravity at different scales, says the universe as it is will provide the best clues. If dark energy is a fluidlike energy field like quintessence, then models would predict a certain type of relationship between how the universe has expanded over time and how cosmic structures have come together. Cosmologists can look for correlations between expansion and growth, such as the formation of galaxy clusters, to understand both quintessence and gravity beyond general relativity, she says.
Even Verde, who is working on the DESI analysis, remains skeptical that Lambda-CDM will be overturned. “I am really conservative, but on whether I am willing personally to throw constant dark energy out the window based on this—not yet,” she says. “Right now we need to keep looking at it and understand it better.”
Many cosmologists are paying close attention but not eulogizing Lambda quite yet, Riess says. Steinhardt suggests systematic errors could play a role in the new findings, especially when three different types of data are combined to arrive at one sweeping conclusion. “Everyone is doing the best they can, but you should take it with a large grain of salt,” he says.
If Lambda lives, in some ways, that will be a very boring outcome—and a philosophically challenging one. The future of the universe will be cold, empty, distant and quiet. Expansion will accelerate forever until atoms themselves are stretched so thin that their centers will not hold, and they fall apart.
But maybe the future is brighter than that, Frieman says. “These hints from DESI and DES are telling us to keep going,” he adds.
