The dark energy that affects the universe is slowly decaying

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Physicists have deduced subtle hints that the mysterious “dark” energy that drives the universe to expand faster and faster may be slightly weakening with time.
“If true, it would be the first real clue we have gotten about the nature of dark energy in 25 years,” said Adam Riess, an astrophysicist at Johns Hopkins University who won the Nobel Prize for co-discovering dark energy in 1998.
Their discovery of the accelerating expansion of the universe revealed the presence of a tiny amount of energy associated with the vacuum of space—dark energy.
“One of my colleagues posted a plot showing this dark energy constraint and didn’t write any words.
They also consider how well other theories do—such as theories that allow dark energy to vary between snapshots.
“The idea that dark energy is varying is very natural,” said Paul Steinhardt, a cosmologist at Princeton University.
Observational evidence for a gradual decline of dark energy would be a boon for the gentle-fall scenario.
It would be the most important discovery since the discovery of dark energy itself,” Vafa said.

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This story’s initial publication was in Quanta Magazine.

The enigmatic “dark” energy that propels the universe’s rapid expansion may be gradually waning over time, according to physics’ inferred subtleties. It’s a discovery that might upend the fundamentals of 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 a wealth of measurements that were gleaned from it, along with the new observations. Plotting three different combinations of observations together suggests that dark energy’s influence may have diminished over eons, which is what many researchers consider to be the most compelling finding.

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

Scholars from both within and outside the collaboration emphasize that there is insufficient evidence to declare a discovery. The data point to a slight statistical significance in favor of the erosion of dark energy, which could easily disappear with more information. The standard understanding of dark energy as the intrinsic energy of the vacuum of space—a quantity that Albert Einstein dubbed the “cosmological constant” due to its unchanging nature—is contradicted by three different sets of observations, according to researchers.

One University of Portsmouth cosmologist who worked on the DESI analysis, Sesh Nadathur, described it as “exciting.”. It will be a tremendous discovery if dark energy is not a cosmological constant. “.

As the Cosmological Constant Rises.

Using the light of several far-off, dying stars known as supernovas, Riess’ group and another team of astronomers under the direction of Saul Perlmutter illuminated the structure of the universe in 1998. They found that as the universe ages, its expansion is growing faster.

By the general theory of relativity, cosmic expansion can be accelerated by any kind of matter or energy. However, as space grows, all of the known forms of matter and energy disperse throughout a larger universe, becoming less dense. The universe should expand more slowly, not faster, as their densities decrease.

Nonetheless, space itself is one substance that does not dilute as it expands. The expansion will accelerate if the vacuum has its own energy, as the teams led by Riess and Perlmutter found, as more vacuum (and hence more energy) is created. Dark energy, or the minuscule amount of energy connected to the vacuum of space, was discovered by them when they noticed that the universe was expanding faster than before.

Luckily, Einstein had taken into account the possibility of this when he was developing general relativity. He proposed that a constant amount of additional energy, denoted by the symbol λ and known as the cosmological constant, might permeate all of space in order to prevent the universe from collapsing due to the dilution of matter. The universe isn’t balanced the way Einstein thought it should be, proving his intuition to be incorrect. However, his cosmological constant reappeared and assumed a central role in the current standard model of cosmology, known as the “Lambda CDM model,” which is composed of a complex web of components, following the 1998 discovery that space appears to be pushing everything outward. “.

It’s easy. There is only one number. There is a tale associated with it. It is thought to be constant because of this, according to Licia Verde, a theoretical cosmologist and DESI collaboration member.

Now, a new generation of cosmologists with access to newer telescopes might be beginning to hear the first hints of a more detailed tale.

Charting the Sky.

Perched atop Arizona’s Kitt Peak is one of those telescopes. 5,000 robotic fibers that automatically swivel toward their celestial targets are installed on the telescope’s four-meter mirror by the DESI team. Automation makes data collection lightning fast compared to the previous flagship galaxy survey, the Sloan Digital Sky Survey (SDSS), which used analogous fibers that needed to be manually plugged into patterned metal plates. DESI recently achieved a record-breaking night when it was able to pinpoint the locations of almost 200,000 galaxies.

The robotic fibers slurped up photons that came to Earth from various cosmic eras between May 2021 and June 2022. Since then, the DESI scientists have created the most comprehensive cosmic map to date using that data. 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 about 2 and 12 billion years ago. “DESI is an incredible experiment generating incredible data,” Riess remarked.

DESI’s capacity to gather galaxy spectra—data-rich plots that describe the intensity of each hue of light—is the key to its accurate mapping. A spectrum indicates the era of cosmic history that a galaxy is visible in based on how quickly it is receding from us; the faster a galaxy recedes, the older it is. This enables you to place the galaxies in relation to one another, but you still need something else to calibrate the map with the accurate 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, viscous soup made mostly of matter and light during the first few hundred thousand years following the Big Bang. There were a few initial dense spots in the soup, but as light pushed the matter outward and gravity pulled it in, density ripples began to spread outward. The universe turned transparent when atoms formed and it cooled. After the ripples, also known as baryonic acoustic oscillations (BAOs), stopped moving, light began to stream outward.

The outcome 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 a little more galaxies than other places. As DESI scientists know that all spheres are the same size, they can determine the actual distance of the galaxies from Earth and adjust the map to reflect that difference. Closer spheres appear larger than farther ones.

The researchers used measurements that had been randomly shuffled around to mask any physical patterns in order to perform a “blind” analysis and prevent unintentionally influencing their findings. In December of last year, the team convened in Hawaii to decipher the findings and determine the kind of map that the Kitt Peak robotic fibers had detected.

When the map was shown, Nadathur, who was watching live over Zoom from his home in the United Kingdom, got excited because it looked a little odd. “One could see that something a little different from the standard model was going to be needed if one had enough experience with BAO data,” Nadathur said. “I was aware that Lambda CDM was not the complete 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 sizable cosmological data sets over the course of the next week.

“Without adding any words, one of my coworkers shared a plot that illustrated this dark energy limitation. Nadathur stated, “Just the plot and an exploding head emoji.”.

Information for the 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 alternative theories perform, such as those that permit variations in dark energy between snapshots.

Just using the data from the first year of DESI, Lambda CDM fits the snapshots almost as well as a variable dark matter model. The two theories don’t begin to diverge until the collaboration adds more snapshots to the DESI map, like the cosmic microwave background light and three more recent maps of supernovae.

They discovered that, depending on which of the three supernova catalogs they used, the results deviated by 2.5, 3.5, or 3.9 “sigmas” from the prediction of Lambda CDM. Picture yourself tossing a coin 100 times. A fair coin is predicted to yield 50 heads and 50 tails. The probability that 60 heads will occur by chance (as opposed to the coin being rigged) is 1 in 20, which is two sigma away from the mean. The gold standard for claiming a physics discovery is 75 heads, which has a 1-in-2,000,000 chance of occurring at random. This is known as a five-sigma result. 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.

Researchers are intrigued by these numbers, but they caution against drawing undue conclusions from the higher values. The statistical significances rely on narrow assumptions made during the data analysis process, and the universe is far more intricate than a coin.

Enthusiasm is further bolstered by the observation that dark energy appears to be changing in the same manner across all three supernova catalogs, which cover relatively distinct populations of supernovas: its strength is decreasing, or “thawing,” as cosmologists refer to it. According to Brout, “all of these complementary data sets tend to converge on this slightly negative number when we swap them out.”. The likelihood of the data sets pointing in different directions would increase if the discrepancy were random.

Not involved in the data analysis, Joshua Frieman is a cosmologist at the University of Chicago and a member of the DESI collaboration. He expressed his satisfaction with Lambda CDM falling. As a theorist, he put forth theories of thawing dark energy in the 1990s. More recently, he cofounded the Dark Energy Survey, an initiative that 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. “I find this fascinating,” but Frieman quipped, “I’m not going to write my [Nobel] acceptance speech until the errors get smaller.”.

About the disparity with the Lambda CDM model, Brout stated, “Statistically speaking, it could disappear.”. Now, everything we have is focused on determining whether it will. “.

The DESI researchers anticipate that their next map will have nearly twice as many galaxies as the one released today, having completed their third year of observations earlier this week. Furthermore, 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 is presented next.

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

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

Scientists might discover that dark energy seems to be as stable as it has been for a generation as they gather more data. Or it might completely alter things if the trend keeps going in the direction that the DESI results indicate.

Fresh Physics.

A cosmological constant cannot explain dark energy’s weakening. Rather, it might be the same kind of field that many cosmologists believe ignited an exponential expansion at the beginning of the universe. Space may be filled with a certain amount of energy that, like the cosmological constant, appears to be constant at first but gradually decreases with time under this type of “scalar field.”.

“It makes perfect sense that dark energy is changing,” Princeton University cosmologist Paul Steinhardt stated. If not, he went on, “it would be the only known form of energy that is completely 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 state of complete vacuum. Steinhardt observed, “We’re accustomed to believing that we’re alone in the universe, but no one ever 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. Cosmic acceleration would cease if it reached zero. The expansion of space would reverse to a slow contraction if it falls below zero, as needed for cyclic theories of cosmology like those proposed by Steinhardt.

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. As opposed to this, string theory requires that energy 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. Cumrun Vafa of Harvard University stated, “We are unsure which one.

The gentle-fall scenario would benefit from observational evidence of a slow decrease in dark energy. That’d be incredible. Vafa stated that it would be the most significant finding since the identification of dark energy itself.

Any such conjectures, however, are for the time being only loosely grounded in the DESI analysis. Cosmologists will need to study a great number of galaxies before they can begin to seriously consider a revolution.

According to Riess, “if this holds up, it could light the way to a new, possibly deeper understanding of the universe.”. “The upcoming years ought to be extremely instructive. “.

Reprinted from the original work by permission of Quanta Magazine, an editorially independent publication of the Simons Foundation whose goal is to improve public science literacy through reporting on scientific advances and trends in mathematics, the physical sciences, and the biological sciences.

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