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The Dark Energy Spectroscopic Instrument (DESI) collaboration just dropped their first official data release, covering the first 13 months of the instrument’s operation.
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While many conclusions were drawn from this data, one stood out—dark energy may not be a cosmological constant after all. In fact, its effects on the universe may be weakening over time.
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If this were to prove correct upon further data collection and analysis, it would punch a massive hole in the standard model of cosmology, known as the LCDM model.
Hey, big news—everything we thought we knew about the universe might be wrong.
Well… not everything. But scientists may have just blown a serious hole in the standard model of cosmology—known scientifically as ΛCDM—which is the theory underpinning the entirety of astrophysics. Like, a hole big enough to potentially change the name. So… also not not everything.
Here’s the deal. For the last 13 months, astronomers around the world have been collaborating on the use of the Dark Energy Spectroscopic Instrument (DESI) and the analysis of the data it has collected. You may have already guessed from the name, but DESI is primarily meant to study dark energy—as part of the team puts it, “DESI aims to place unprecedented constraints on the equation of state of dark energy, the gravitationally driven growth of large-scale structure, and the sum of the neutrino masses, as well as to explore the observational signatures of primordial inflation.”
Lofty goals, indeed.
This week, DESI dropped its Data Release 1, which encompasses all of the observations it’s made over its first 13 months of operation. It was accompanied by a slew of papers detailing the first round of analysis done on this absolute treasure trove of data. While numerous exciting conclusions were drawn, one result stood head and shoulders above everything else: dark energy might not be a cosmological constant.
That’s a small statement with a big impact. See, when dark energy was discovered in 1998, our picture of the universe fundamentally changed. Instead of living in an infinite bubble that had been inflated by the Big Bang and was slowing down in its expansion over time, we suddenly knew that our universe was, in fact, accelerating outward. All that energy had to come from somewhere, and the idea of dark energy was made known to humanity.
The new findings from the DESI teams don’t negate that picture, but they do modify it significantly. Rather than accelerating into infinity forever, the new data seems to show that the acceleration attributed to the influence of dark energy is slowing down over time. Not only is it not a constant influence on the cosmos—it seems to be a weakening one.
The team made use of two massive sets of data to draw this conclusion. The first, obviously, was Data Release 1. From this massive release, the team was able to make measurements of the large-scale fluctuations of visible matter (known as baryon acoustic oscillations, or BAO) and the behavior of the intergalactic medium (through proxy observations of what is known as the Lyman-α forest) across nearly the entire history of the universe. Because DESI can see so far into the universe, it can also see billions of years into the past.
The second major data set was the yet-unreleased DESI Data Release 2, from which they were able to pull a second round of BAO measurements. They also made use of external data sets describing the behaviors of the cosmic microwave background and type Ia supernovae. Combining all of that data, analyzing what it tells us (or, more accurately, what it tells the people who have studied enough physics to understand it), and comparing it to existing theories and calculations, scientists came to the conclusion that the equations that best explain what we are seeing are ones where dark energy is expressed as a variable entity, rather than a constant.
To understand how big a deal this is, refer for a second back to the scientific name for the standard model of cosmology. Breaking down ΛCDM, CDM stands for Cold Dark Matter, and Λ is the cosmological constant that represents the effects of dark energy. If this finding is confirmed, the Λ in ΛCDM would be no more, as that component would be inherently incorrect.
“It’s fair to say that this result, taken at face value, appears to be the biggest hint we have about the nature of dark energy in the ~25 years since we discovered it,” Adam Riess, one of the astrophysicists who first discovered dark energy, told .
This isn’t the first time the idea of variable dark matter has been proposed—not by a long shot, and not even by researchers on the DESI team, who started hinting at the possibility in their early data release a few months ago. But it’s easily the best indication we’ve ever gotten. To try and contextualize how certain the team is about their results: scientists express certainty of discovery in units of statistical significance known as sigma (σ) in order to describe how likely it was that the thing they detected was a fluke. Experts claim that something is a true ‘discovery’ when its statistical significance reaches 5σ, which means there’s only a 1-in-3.5 million chance that the finding was the result of random fluctuation. The statistical significance of this evolving-dark-energy detection peaked at 4.2σ, which means about a 1-in-50,000 chance of fluke. So, not a “eureka” yet, but definitely exciting. As cosmologist Wendy Freedman told The Washington Post: “We are still at the ‘interesting’ or ‘stay tuned’ level. Very intriguing but not yet definitive.”
Especially (as the NYT points out) considering that another similarly large and impressive recent study, undertaken by a consortium using the Atacama Cosmology Telescope, seemed to confirm that the ΛCDM model is accurate—at least, in the very early universe. In part by further confirming the Hubble tension (full explanation on that idea here), the team concluded that our current model still seems good to go.
“It just blew me away that we didn’t see even, like, a hint of one of these new physics extensions,” Colin Hill, a cosmologist who worked on the team, told Science News. “It indicates that we might need to go back really to some of the foundational assumptions of our understanding of cosmology.”
Critically, the Atacama study being correct does not preclude the DESI study from also being correct. It just adds more puzzles to scientists’ plates.
So, what does this all mean? Well, outside of increasing our understanding of the vast and varied universe in which we live, it means we might be able to paint a better picture of how the universe will end. As it stands right now, the ΛCDM model implies that everything in the universe will fly apart faster and faster as time goes on, until everything is so far apart from each other that it’s all functionally destroyed.
But if the Λ part of ΛCDM is more of an equation than a constant—and, especially, an equation indicating that the effects of dark energy are weakening over time—our universe may not be doomed to rip itself apart. Rather, it may collapse back into an infinitely dense point in a sort of reverse-Big Bang, as described by a hypothesis known as the Big Crunch. Or, if we’re very lucky, it could completely stabilize. An infinite frozen universe, forever.
All of that is still left to scientists to figure out. But luckily, that’s what they want to do, anyway.
“This is actually a little bit of a shot in the arm for the field,” Will Percival, a cosmologist and spokesperson for DESI, told the NYT. “Now we’ve got something to go after.”
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