For more than two decades, astronomers have wrestled with one of the biggest mysteries in science: what is driving the universe’s expansion to speed up instead of slow down?
This mysterious force is called dark energy, and though it makes up about 70% of the universe, no one knows what it actually is.
The simplest and most widely accepted explanation has been that dark energy is constant—an unchanging property of empty space itself.
This idea traces back to Einstein’s “cosmological constant” from more than a century ago.
But now, new studies are giving scientists reason to think that dark energy may not be so constant after all.
Last year, data from the Dark Energy Survey and the Dark Energy Spectroscopic Instrument created a stir among cosmologists by suggesting that the strength of dark energy may be evolving.
If true, it would be the first evidence that dark energy is not Einstein’s cosmological constant but something more dynamic.
Josh Frieman, professor emeritus at the University of Chicago, and Anowar Shajib, a NASA Hubble Fellowship Einstein Fellow, have taken a closer look at this possibility.
In a new paper published in Physical Review D, they analyzed the combined results from multiple surveys and compared them with models of evolving dark energy.
Their findings suggest that these dynamic models fit the data better than the standard, constant model.
The idea is simple but profound.
If dark energy is not fixed, then something else in the universe—possibly a new, undiscovered particle—could be driving cosmic acceleration. According to their models, this particle would be unimaginably light, many orders of magnitude smaller than even an electron.
So why does this matter? Frieman explains it this way: “We know exactly how much dark energy there is, but we don’t know what it is. And whatever it is will determine the universe’s future.”
Until recently, the data fit comfortably with the constant dark energy model. But newer observations combining supernova explosions, galaxy clustering patterns, and the cosmic microwave background suggest otherwise.
The evidence points to a small but noticeable decline in dark energy’s density over the last several billion years—about a 10% drop. That may sound tiny, but it is significant enough to challenge the standard picture.
To make sense of this, Frieman and Shajib turned to particle physics. Their models use a version of the axion, a hypothetical particle first proposed in the 1970s. Normally, axions are considered candidates for dark matter.
But in this scenario, an “ultra-light” axion could instead act as dark energy.
For billions of years, its density would remain constant, but eventually it would start to evolve—like a ball sitting on a slope that finally begins to roll. As it does, the density of dark energy would slowly decrease.
What would this mean for the future of the universe? If dark energy weakens over time, cosmic acceleration would slow down too.
That rules out some of the more dramatic scenarios scientists have speculated about, like the “Big Rip” where everything—even atoms—is torn apart by runaway expansion, or the “Big Crunch” where expansion reverses and the universe collapses back in on itself.
Instead, their models predict a slower but steady expansion that will eventually leave the universe cold, dark, and empty—a “Big Freeze.”
For Frieman, this result feels like déjà vu. “When we began the Dark Energy Survey in 2003, our main goal was to figure out whether dark energy was constant or changing. For twenty years, the data kept telling us it was constant.
We almost gave up on the question. Now we finally see a hint that it may be evolving. If that’s true, it changes everything we thought we knew.”
Shajib agrees. “We brought together all the major datasets available today—from supernova surveys to cosmic microwave background maps—to get the tightest constraints yet. What excites me is that these measurements represent the collective knowledge of our entire field, and together they may be pointing to something new.”
Future projects, such as the Vera Rubin Observatory’s Legacy Survey of Space and Time, will provide even sharper data.
Within the next decade, scientists expect to know definitively whether dark energy really is evolving—or whether the universe is still ruled by Einstein’s century-old cosmological constant.
Either way, the answer will reshape our understanding of the cosmos and perhaps reveal new physics beyond anything we have discovered before.
