Key Takeaways:

  • Dark energy, which is thought to be causing the universe’s expansion, might be decreasing over time. This is contrary to the current belief that it’s constant.

  • New observations from the Dark Energy Spectroscopic Instrument (DESI) show hints of this decrease. However, more data is needed to confirm this.

  • If confirmed, this finding could revolutionize our understanding of dark energy and the universe’s fate.

  • A decreasing dark energy might mean we’re not living in the lowest energy state of the universe, and the universe might be slowly approaching a true vacuum.

  • More powerful telescopes are being built to collect more data and solidify these findings. The next few years could be very revealing.

The mysterious “dark” energy that drives the universe’s rapid expansion may be gradually decreasing over time, according to physics’ concluded details. This discovery has the ability to challenge conventional beliefs in physics.

Adam Riess, a Johns Hopkins University physicist who shared the 1998 Nobel Prize for co-discovering dark energy, said, “If true, it would be the first real clue we have gotten about the nature of dark energy in 25 years.”

The Dark Energy Spectroscopic Instrument (DESI) team released a map of the universe with never-before-seen scales and an abundance of measurements that were obtained from it, along with the new observations. To many researchers, the highlight is a plot showing that three different combinations of observations all insinuate that the influence of dark energy may have eroded over the eons.

“It’s possible we’re seeing evolving dark energy hints,” Boston University’s Dillon Brout, a DESI team member, said.

Researchers inside and outside of the collaboration emphasize that there is insufficient evidence to declare a discovery. The observations support the erosion of dark energy with a statistical significance that is of the kind that could easily disappear with more information. However, scientists also notice that three different sets of data all suggest an intriguing conclusion that defies the conventional understanding of dark energy, which views it as the intrinsic energy of space vacuum—a quantity Albert Einstein dubbed the “cosmological constant” because of its unchanging nature.

“It’s exciting,” said Sesh Nadathur, a cosmologist at the University of Portsmouth who worked on the DESI analysis. The discovery that dark energy is not a cosmological constant will be incredible.

As the Cosmological Constant Rises

In 1998, Riess’ group and another group of astronomers under the direction of Saul Perlmutter illuminating the structure of the universe with the light of several supernovas, which are far-off, dying stars. They found that as the universe ages, its expansion is growing faster.

Cosmological expansion can be powered by any matter or energy, according to Einstein’s general theory of relativity. However, as space expands, all of the known types of matter and energy spread throughout a larger universe, becoming less dense. The expansion of the universe should slow down rather than speed up as their densities decrease.

Space itself, however, is one substance that does not dilute as space expands. As noted by Riess and Perlmutter’s teams, if the vacuum has its own energy, then it will expand more quickly as more vacuum (and hence more energy) is created. When they noticed that the universe was expanding faster than before, they realized that there was dark energy, which is a very small quantity of energy related to space’s vacuum.

It just so happens that Einstein had taken this possibility into account when he was developing general relativity. He thought that all of space might be imbued with a fixed quantity of additional energy, indicated by the symbol λ, called lambda, and referred to as the cosmological constant, in order to prevent the dilution of matter from collapsing the universe. It turns out that Einstein was not correct in his intuition, since the universe is not balanced as he had thought. But following the 1998 discovery that everything appears to be moving outward into space, his cosmological constant reappeared and became the central component of the modern standard model of cosmology, known as the “Lambda CDM model,” a complex system of interconnected elements.

It’s easy. There is just one number. You can connect a narrative to it. According to Licia Verde, a theoretical cosmologist and DESI collaboration member, this is the reason it is thought to be constant.

The first hints of a more complex story could be detected by a new generation of cosmologists using new generation telescopes.

Mapping the Heavens

On Arizona’s Kitt Peak is one of those telescopes. 5,000 robotic fibers on the telescope’s four-meter mirror, which automatically swivel toward their celestial targets, have been installed by the DESI team. Compared to the previous flagship galaxy survey, the Sloan Digital Sky Survey (SDSS), which depended on comparable fibers that had to be manually plugged in to patterned metal plates, the automation allows for lightning-fast data collection. Recently, DESI set a new record by accurately pinpointing the locations of almost 200,000 galaxies in one night.

From May 2021 to June 2022, the robotic fibers slurped up photons arriving at Earth from different eras of cosmic history. Since then, the DESI scientists have used that data to create the most comprehensive cosmic map ever created. Out of the 13.8 billion years that the universe has existed, it contains the exact locations of about 6 million galaxies as they were between 2 and 12 billion years ago. “DESI is a really great experiment producing stupendous data,” said Riess.

Image of Earth  at the center of this thin slice of DESIs 3D map of the universe. Magnification reveals the distribution...
Earth is at the center of this thin slice of DESI’s 3D map of the universe. Magnification reveals the distribution of galaxies and voids. COURTESY OF CLAIRE LAMMAN/DESI COLLABORATION; CUSTOM COLORMAP PACKAGE BY CMASTRO

The secret to DESI’s precision mapping is its ability to collect spectra of galaxies—data-rich plots recording the intensity of each hue of light. A galaxy’s spectrum indicates which era of cosmic history we are seeing it in based on how quickly it is receding from us; the faster a galaxy decreases, the older it is. That allows you to place the galaxies in relation to one another, but you need something else to calibrate the map with the correct distances from Earth—information that is necessary for a complete reconstruction of cosmic history.

That something, for the DESI collaboration, was a patchwork of frozen density ripples from the early universe. The universe was a hot, thick soup made mostly of matter and light during the first few hundred thousand years following the Big Bang. The matter was pulled inward by gravity and outward by light, causing ripples of density that spread from a few initial dense areas in the soup. The universe turned transparent when the atoms formed and the universe cooled. The ripples, known as baryonic acoustic oscillations (BAOs), remained in place as light streamed outward.

The end result was a sequence of overlapping spheres, about a billion light-years across, with slightly denser shells, the distance BAOs could travel before freezing. Researchers at DESI can find remnants of these spheres when they map millions of galaxies because those dense shells went on to form slightly more galaxies than other locations did. Since the spheres in DESI are all the same size, researchers can determine the actual distance of the galaxies from Earth and adjust the map accordingly, even though closer spheres appear larger than farther ones.

The researchers used a “blind” analysis, meaning they used measurements that had been arbitrary shuffled around to mask any physical patterns, to prevent unintentionally influencing their findings. After that, the team met in Hawaii in December of last year to figure out the findings and determine the kind of map that the Kitt Peak robotic fibers had detected.

When the map was revealed, Nadathur, who was watching live over Zoom from his home in the United Kingdom, was excited because it looked a little odd. Something a little different from the standard model was going to be required, according to Nadathur, if one had sufficient experience with BAO data. “I knew that Lambda CDM wasn’t quite the whole picture.”

The source of the oddness was found, and a flurry of Slack messages ensued as the researchers went through the new data set, analyzed it, and blended it with other significant cosmological data sets over the course of the next week.

“One of my colleagues posted a plot showing this dark energy constraint and didn’t write any words. Just the plot and an exploding head emoji,” Nadathur said.

Data for Days

By tracking various galaxy types as they emerged over seven epochs of cosmological history, DESI seeks to determine how the universe has expanded over time. Next, they assess how well the evolution predicted by Lambda CDM matches these seven snapshots. They also take into account how well other theories perform, such as those that permit variations in dark energy between snapshots.

Lambda CDM fits the snapshots almost as well as a variable dark matter model using only the first year of DESI data. The two theories don’t begin to diverge until the collaboration incorporates the DESI map with additional snapshots, such as the cosmic microwave background light and three recent supernova maps.

The results showed that, depending on which of the three supernova catalogs they used, they were 2.5, 3.5, or 3.9 “sigmas” away from the Lambda CDM prediction. Think about tossing a coin 100 times. A fair coin is predicted to yield 50 heads and 50 tails. The probability of 60 heads occurring by chance (as opposed to the coin being rigged) is 1 in 20, which is two sigma away from the mean. A five-sigma result is one that has a 1-in-2,000,000 chance of occurring at random, such as 75 heads. This is the gold standard for claiming a discovery in physics. The sigma values that DESI found are in the middle range; they may represent infrequent statistical variations or actual proof that dark energy is evolving.

Photo of the DESI instrument
The DESI instrument is equipped with thousands of robotic fibers to dramatically speed up data collection. COURTESY OF MARILYN SARGENT/THE REGENTS OF THE UNIVERSITY OF CALIFORNIA, LAWRENCE BERKELEY NATIONAL LABORATORY

Although these numbers are intriguing to researchers, they caution against extrapolating the higher values. A coin is not as complicated as the universe, and statistical significances rely on finely complex assumptions made during data analysis.

The fact that all three supernova catalogs, which cover relatively independent populations of supernovas, suggest that dark energy is changing in the same manner, is a more compelling reason to be excited: It is “thawing,” as cosmologists put it, or losing power. “When we swap out all of these complementary data sets, they all tend to converge on this slightly negative number,” Brout said. The data sets are more likely to point in different directions if the discrepancy were random.

Cosmologer Joshua Frieman of the University of Chicago, a collaborator in the DESI project who was not involved in data analysis, expressed his satisfaction with Lambda CDM’s decline. As a theorist, he put forth theories of thawing dark energy in the 1990s. More recently, he cofounded the Dark Energy Survey, which produced one of the three supernova catalogs used by DESI and looked for deviations from the standard model from 2013 to 2019. However, he also recalls having previously been burned by vanishing cosmological anomalies. “My reaction to this is to be intrigued,” but “until the errors get smaller, I’m not going to write my [Nobel] acceptance speech,” Frieman joked.

“Statistically speaking, it could disappear,” Brout said of the discrepancy with the Lambda CDM model. “We are now going all out to find out if it will.”

Following the conclusion of their third year of observations earlier this week, the DESI researchers anticipate that the number of galaxies on their next map will be almost twice as high as the one released today. Additionally, they intend to release the revised three-year map as soon as possible now that they have more experience performing the BAO analysis. A five-year map featuring 40 million galaxies follows.

A number of other instruments, in addition to DESI, will be activated in the upcoming years. These include the 8.4-meter Vera Rubin Observatory in Chile, the Nancy Grace Roman Space Telescope of NASA, and the Euclid mission of the European Space Agency.

“Our data in cosmology has made enormous leaps over the last 25 years, and it’s about to make bigger leaps,” Frieman said.

Researchers may find that dark energy appears to be as constant as it has been for a generation as they gather more and more observations. Alternatively, everything might change if the trend keeps going in the direction that the DESI results indicate.

New Physics

Dark energy cannot be a cosmological constant if it is becoming weaker. Rather, it might be the same kind of field that many cosmologists believe sparked an exponential expansion moment at the beginning of the universe. The amount of energy that this type of “scalar field” might fill space with could appear constant at first—much like the cosmological constant—but gradually begin to decrease over time.

“The idea that dark energy is varying is very natural,” said Paul Steinhardt, a cosmologist at Princeton University. Otherwise, he continued, “it would be the only form of energy we know which is absolutely constant in space and time.”

However, that variability would result in a significant paradigm change because it would mean that we would no longer be living in a vacuum, which is the universe’s lowest energy state. Rather, we would live in a state of energy that is gradually approaching a true vacuum. “We’re used to thinking that we’re living in the vacuum,” Steinhardt said, “but no one promised you that.”

How fast and how far this number, which was formerly called the cosmological constant, declines would determine the fate of the universe. The cosmic acceleration would cease if it reached zero. If it falls below zero sufficiently, space would expand initially but then slowly contract; this kind of reversal is necessary for cyclic theories of cosmology, like the ones Steinhardt developed.

The perspectives of string theorists are similar. They can create universes with various dimensions and a variety of strange particles and forces by suggesting that everything is just the vibration of strings. However, it is difficult for them to create a universe that, like ours, appears to continuously maintain a steady positive energy. Rather, in string theory, the energy must either violently collapse to zero or a negative value, or fall gently over billions of years. In essence, there is only one option for string theorists: either one or the other. We’re not sure which one it is, Harvard University’s Cumrun Vafa said.

A positive indication for the gentle-fall scenario would be observational evidence of a slow decrease in dark energy. That would be incredible. According to Vafa, it would be the most significant finding since the identification of dark energy itself.

But for now, any such speculations are rooted in the DESI analysis in only the loosest of ways. Before seriously considering a revolution, cosmologists will need to observe many millions more galaxies.

“If this holds up, it could light the way to a new, potentially deeper understanding of the universe,” Riess said. “The next few years should be very revealing.”

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