The Evolving Debate Around Dark Energy

In the beginning, the Big Bang happened, sending everything in the universe expanding outward and apart, from a dense hot point. Since then, all that matter and energy has continued to move outward, carried along with the cosmos’ expansion.

That expansion is fueled by dark energy, a mysterious force that is fundamental to scientists’ understanding of the past and future of the universe. Since dark energy’s discovery a quarter-century ago, scientists have assumed its influence to be constant, its force exerted the same way 5 billion years ago, today, and forever; a sort of steady foot on the gas pedal.

But new results, from an instrument called the Dark Energy Spectroscopic Instrument, or DESI, located at the Kitt Peak National Observatory in Arizona, suggest that might not be true: Dark energy may, in fact, evolve—its influence changing over time. Data now suggest dark energy has weakened in more recent epochs, essentially lessening its pressure on the accelerator. The results potentially “really change your understanding of what dark energy actually is,” said Ashley Ross, a cosmologist at The Ohio State University who is working on a project to measure how galaxies are distributed, as part of the DESI collaboration.

If DESI’s recent finding holds up, it means scientists’ current conception of the universe’s past, present, and future is mistaken. And news stories about the findings were quick to point that out: “We might have gotten dark energy totally wrong,” proclaimed Live Science. “This could change everything,” wrote Futurism.

Headlines like those may be true, but the verdict isn’t yet in. Some scientists take the possible error as exciting, since it could provide a path to better understanding the most fundamental physics, for which details have so far been elusive; others doubt the finding will stand time’s test.

To understand the universe, scientists use telescopes to observe as much of it as they can, gathering and characterizing patterns—how galaxies tend to form, for instance, or how stars tend to die. They use those observations to create, and bolster or refute, theories: the underlying, usually mathematical models that explain why they see what they see through their telescopes.

But any human-made model is likely to be incomplete, oversimplified. And data that potentially conflicts with existing ideas, as DESI’s data might, raises questions about the costs and benefits of being wrong, like spending hundreds of millions of dollars of federal research money on instruments and human capital, as dark-energy studies have, that ultimately upend a particular idea about the universe.

Getting closer to cosmic truth, though, experts point out, often requires fumbling through uncertainties and smashing into dead ends. That mental maze, which is typical in this area of research, experts say, is something that sensationalized news coverage often fails to acknowledge.

And making that incremental progress and taking advantage of its fruits, like potential practical applications of fundamental science, asks scientists to be willing to change their minds about even their closest-held foundational theories in favor of creative new lines of inquiry. But that line can be subjective, said Melissa Jacquart, a philosopher of science at the University of Cincinnati: “When do we have enough evidence to make us shift our perspective or think that we need to be approaching it differently?”

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Dark energy seems distant from daily life on Earth, but its presence has made the universe what it is today, maybe even enabling that life to arise. And though it’s not apparent on this planet, dark energy causes the universe to grow larger, and faster, with each passing picosecond.

For a long time, scientists thought the expansion rate was slowing with time, like a coasting car. But in 1998 astronomers discovered that the opposite was true: Cosmic expansion was actually speeding up. The universe seemed to be pressing the gas pedal pretty hard.

Something had to be providing that fuel, counteracting the gravity that naturally draws things together. Scientists didn’t know what that something was, so they called it dark energy. Decades later, they still can’t explain it: Dark energy is “dark” because it remains a mystery.

Dark energy is ubiquitous, though. It’s estimated to make up about 70 percent of the universe, and together with dark matter—another scientific shoulder-shrug—the two account for a staggering 95 percent of the universe. “We don’t know what most of the universe actually is,” said Ross.

Still, despite that lack of knowledge, scientists assumed dark energy forced a constant acceleration since it fit with the data they had gathered thus far on the universe’s history and evolution.

Cosmologists are not naive: They knew that assumption could be incorrect. And, in fact, analyzing it—along with other hypotheses—was one of DESI’s goals.

The instrument, which started its main work in 2021, kicked toward that goalpost by peering at various galaxies across the universe. By analyzing the light emitted from those galaxies, DESI scientists were able to measure their distance from Earth and how fast they were moving outward and made a three-dimensional map of the cosmos to understand how its expansion has changed.

DESI’s data, when combined with other observations, suggested that the universe’s expansion rate has actually shifted over time—and if dark energy dictates that rate, the energy itself must be changing.

If that’s the case, it could alter scientists’ prediction of the universe’s fate: With constant dark energy, the cosmos is doomed to expand faster and faster forever, pushing everything so far apart that other galaxies will recede beyond the view of even the most powerful telescopes; our cosmic neighborhood will appear to be alone. If dark energy can change over time, though, that dark ending may be avoided.

Even though this evolving possibility was DESI scientists’ idea, it’s nevertheless a big departure from scientists’ current cosmological model of the universe, appropriately called the “standard model.” That model postulates, in mathematical equations, that after the Big Bang, the universe experienced a period of rapid inflation. Since then, it has continued to expand, in a way dictated by the balance of its contents: regular atomic material and dark matter, both influenced by gravity, and dark energy—the latter assumed to exert a constant acceleratory force in opposition to gravity.

But cosmologists have actually been searching for holes in the standard model—holes that might lead to a more complete understanding of spacetime, because the standard model has limitations. Dark energy and dark matter, for instance, have never been directly detected—only their effects. Scientists have also seen discrepancies in the measurement of the universe’s expansion rate based on different methods of measurement. And the light left over from the Big Bang shows wonky anomalies that don’t necessarily line up with the standard model’s predictions.

To James Overduin, a theoretical physicist at Towson University who co-wrote a book about dark energy called The Weight of the Vacuum, the concept of dark energy itself is an opaque placeholder—something hand-wave-y that explains a physical behavior that astronomers observe. That kind of a cover is something scientists have created for centuries, when they wanted to hang onto their current conception of the universe in the face of evidentiary challenges.

In some sense, making models of the universe always involves those kinds of simplifications—something scientists don’t always like to admit. In physics, we often think of the universe as a set of facts waiting to be discovered, said Jacquart. “But we can’t really just know those facts of the matter,” she said. “And so, in terms of how to explain everything, there are all of these spaces along the way where the scientists have to make either assumptions or idealizations.”

Data and analysis that poke holes in those smooth models can push science in new directions, acting as their own kind of dark energy. The question, always, is how big those holes must be before a theory—like the standard model, or dark energy’s constancy—rips.

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Whether the DESI results rise to that level remains a debate among scientists. Zachary Slepian, an astrophysicist at the University of Florida and a member of the DESI team, doesn’t think the new data represent enough evidence to abandon current cosmological conceptions. What appears to be creepily evolving dark energy could, in fact, be some kind of experimental error, or an instrumental quirk. At the lower end of calculations, scientists estimate that the odds the DESI results are due to random chance are about one in 385—close to a statistical significance known as three-sigma. Five sigma is the field’s standard for a real discovery—something that has a one in 3.5 million chance of being a random fluke.

Colin Hill, a Columbia University cosmologist who works with the Atacama Cosmology Telescope in Chile, also isn’t convinced. “There’s sort of a borderline hint that maybe there’s something going on,” said Hill. But the statistical significance could vanish with more data, and extensions of the standard model—rather than a whole new model—could also explain the galactic findings.

Besides, he said, if dark energy really is evolving, it could imply a scenario where, as the universe expands, more dark energy is created. “That’d be truly, truly wild,” he said.

That data from DESI and other experiments doesn’t necessarily indicate dark energy is evolving, he added; the DESI measurement could be attributed to other phenomena. “It’s a little bit of a messy situation.”

Ross, though, sees the discovery as more solid: The statistical significance has increased as more DESI data has come in, for instance. “That’s what makes me excited and feel that it could all be pretty real,” he said, adding that data from other instruments also increased the analysis’ rigor.

Ross, along with other physicists, would actually be excited if the current model of dark energy were proven wrong, because it could help his crew think differently about the best direction for cosmology. Overduin agrees: Cosmologists haven’t made much headway in using theories to explain the universe’s nuances and contradictions, said Overduin. And the buzz about the DESI results, despite their preliminary nature, illuminate scientists’ hope that discrepancies like this could be wormholes to new ideas, and so progress. “There’s a bit of desperation there,” said Overduin.

Learning that dark energy may be fundamentally different—and so, too, may the universe—than scientists thought could be an important step toward the truth. Because, presumably, if scientists are on track for the truth, progress will come more easily. “If you look at the history of science it’s entirely filled with us throwing out theories,” said Jacquart—or, more accurately, using mounting evidence to keep what seems right, toss what seems wrong, and getting closer to “the actual reality of the world,” she said.

Jacquart likens this stepwise process to a choose-your-own-adventure book. If one choice is a dead end, “let’s go back a few steps and figure out where in our journey we could have gone on a different path.”

But in science, taking those steps back can be difficult. “Especially when you have theories that astronomers hold so dear,” Jacquart said. Dearly held theories are often ones whose tenets line up with a preferred modus operandi for the physical world, revealing a human bias in the search for scientific truth—not something that’s unique to cosmology, since human bias can pervade any scientific field.

Dark energy’s constant form may fall into the natural-bias category. “So many spaces of physics focus on consistency,” Jacquart said. “The physics always works the same way. And, in some ways, that shows sort of a preference towards simplicity.”

The DESI results are hinting that the universe isn’t so simple, or consistent. It “adds complexity that I don’t think we always want to lean towards,” she said.

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Leaning where evidence points, though, is important—even though how dark energy behaves can seem lightyears away from everyday life on Earth. For one, pursuing dead-end theories spends research time, and tax dollars, that some argue would be better spent on ideas that open new doors to understanding.

More than 900 scientists are part of the DESI collaboration; getting that large dark-energy cohort mobilized around the most fruitful ideas, as DESI’s results may, could prevent them from simply spinning their wheels. And being wrong, or holding onto ideas longer than data suggests is prudent, could lead to building expensive instruments that may not add much new knowledge to the world, if they are not engineered to pursue what the universe actually has on offer.

In particle physics, for instance, the Large Hadron Collider cost nearly $5 billion to build, not to mention operational costs. While it discovered a particle that validated scientists’ existing understanding, it didn’t find any of the new physics some were hoping for. Now proposals are on the table for a new machine that would cost tens of billions of dollars but doesn’t have a clear road sign in the right direction from the previous experiment.

To Ross, that’s part of why the new results are important for dark energy research: They might change how future experiments are designed. That would save science from wasting money and scientists’ time on an outdated idea.

On a more long-term and abstract note, if scientists get closer to characterizing dark energy and its place in cosmic evolution, as the largest ingredient in the universe, that could someday benefit humans. Einstein, after all, probably didn’t envision GPS satellites when he came up with general relativity, but those satellites nonetheless rely on his discovery.

If DESI does ultimately show astronomers, to their consensus satisfaction, that their existing models of the universe and its dark places are wrong, Jacquart doesn’t think time spent on current ideas was a waste. Slepian, from his perch at the University of Florida, sees the DESI collaboration, which includes hundreds of scientists, as a physics incubator—kind of like the Manhattan Project, he said. The project built the atomic bomb and altered the world forever, but it also united some of the 20th century’s greatest scientific minds: “That seeded American particle theory and particle physics dominance for the next 50 years.”

Perhaps DESI could do the same for cosmology.

Maybe someday those scientists and their instruments will tell us what dark energy actually is, said Ross: “The whole point to me is to not have to call it dark energy.”

But, even if that’s the case, Slepian doesn’t think physicists will ever fully understand the fundamental truths of the universe. “I see science as something where you can never really be right,” he said. “You can just be asymptotically less wrong.”

This article was originally published on Undark. Read the original article.