The universe is still accelerating, and we're on the right track.
Since 1998, the accelerating expansion of the universe has stood as one of cosmology's most consequential pillars, pointing to a mysterious force — dark energy — that comprises the majority of all that exists. A new study led by Nobel laureate Adam Riess and colleagues has reaffirmed that acceleration, rebutting a 2025 claim that dark energy was weakening and the universe was slowing down. Using the reliable brightness of Type Ia supernovae as cosmic measuring sticks, the researchers found no evidence for the 'age effect' the opposing team had proposed. Humanity's oldest question — what is the universe made of, and where is it going — remains open, but the map we have drawn so far appears to hold.
- A 2025 study shook the foundations of modern cosmology by suggesting dark energy was fading and the universe's expansion was decelerating — contradicting nearly three decades of accepted science.
- The challenge triggered an urgent reexamination: if the acceleration had stopped, the standard model of the cosmos would need to be fundamentally rewritten.
- Nobel laureate Adam Riess and a team of astrophysicists fought back, combing through the largest supernova datasets of the past decade and finding no trace of the 'age effect' the rival study had claimed was being ignored.
- Their findings, published in the Monthly Notices of the Royal Astronomical Society, reassert that cosmic acceleration is ongoing and that the opposing analysis carries methodological inconsistencies.
- The deeper mystery endures — dark energy makes up 68% of the universe yet remains physically unknown — and two powerful new observatories are now poised to push the boundary of what we can learn.
In 1998, astronomers discovered that the universe was not merely expanding but accelerating — a finding that introduced the concept of dark energy, an invisible force accounting for roughly 68 percent of everything that exists. For nearly three decades, this became a cornerstone of cosmology. Then, in 2025, a research team proposed that the acceleration had stopped, suggesting dark energy was weakening. It was a direct challenge to one of physics' most fundamental ideas.
Now, a new analysis has pushed back. Astrophysicist Adam Riess of Johns Hopkins University — a Nobel laureate who helped make the original discovery — and his colleagues reexamined the evidence using observations of Type Ia supernovae, stellar explosions that all share roughly the same intrinsic brightness. Because their apparent brightness from Earth reveals their distance, and because light takes time to travel, these explosions allow scientists to map the universe's expansion going back billions of years.
The 2025 study had argued that supernova distances needed recalibration to account for the ages of the stars that exploded — an overlooked 'age effect,' its authors claimed. Riess and his team found no evidence for this when examining the largest supernova samples used by the cosmology community over the past decade, and they identified what they consider methodological flaws in the opposing work.
Researcher Brodie Popovic of the University of Southampton acknowledged that dark energy itself remains deeply mysterious — known only through its gravitational push on the universe, not through any direct understanding of what it is. Two new instruments may soon change that: the Vera Rubin Observatory in Chile is already operational, and the Nancy Grace Roman Space Telescope is set to launch in August, both capable of gathering far richer data on cosmic expansion. For now, the reaffirmation of acceleration stands — a hard-won confirmation that the universe's most unsettling secret is still very much intact.
In 1998, astronomers made a discovery that upended our understanding of the cosmos: the universe wasn't just expanding, it was speeding up. That finding pointed to the existence of dark energy, an invisible force making up roughly 68 percent of everything that exists. For nearly three decades, this accelerating expansion became foundational to modern cosmology. Then, last year, a team of researchers published a study suggesting the acceleration had stopped—that dark energy was weakening and the universe's expansion was slowing. It was a challenge to one of the most fundamental ideas in physics.
Now, a new analysis has pushed back against that conclusion. A team of astrophysicists, including Adam Riess of Johns Hopkins University—a Nobel laureate who helped discover the acceleration in the first place—has reexamined the data and confirmed that cosmic expansion is still accelerating. The study, published this month in the Monthly Notices of the Royal Astronomical Society, uses observations of Type Ia supernovae, a specific kind of stellar explosion, to measure how fast the universe is expanding and whether that rate is changing over time.
Type Ia supernovae occur when a white dwarf—the dense, collapsed remnant of a dead star—pulls material from a companion star until it can no longer support itself and explodes. What makes these explosions useful for cosmology is that they all have roughly the same intrinsic brightness. By measuring how bright they appear from Earth, astronomers can calculate their distance. Since light takes time to travel across the universe, observing distant supernovae is like looking backward in time, allowing researchers to map how the expansion rate has changed since the Big Bang roughly 13.8 billion years ago.
The 2025 study that challenged the acceleration hypothesis argued that supernova distances needed to be recalibrated by accounting for the ages of the stars that eventually exploded. The authors of that work suggested this "age effect" was being overlooked. But Riess and his colleagues found no evidence for this effect when they examined the largest supernova samples used by the cosmology community over the past decade. The new study's authors maintain confidence in their methods and argue that the opposing analysis contains methodological flaws or internal inconsistencies.
Brodie Popovic of the University of Southampton, one of the leaders of the new research, acknowledged the uncertainty that still surrounds dark energy itself. "The universe is still accelerating," Popovic said. "There's still a lot we don't know and are excited to learn, but we think we're on the right track." The physical nature of dark energy remains one of the deepest mysteries in science. It's known only through its gravitational effects—the way it pushes the universe apart—but what it actually is remains unknown.
Two new observatories may soon shed light on the question. The Vera Rubin Observatory in Chile is already operational, and the Nancy Grace Roman Space Telescope is scheduled to launch in August. Both instruments will gather far more data about distant supernovae and the universe's expansion history than current telescopes can provide. Popovic expressed hope that these new observations will help narrow down what dark energy truly is. For now, the confirmation that cosmic acceleration persists stands as a reaffirmation of one of modern astronomy's most consequential findings—even as the deeper mystery of what drives that acceleration remains unsolved.
Notable Quotes
The universe is still accelerating. There's still a lot we don't know and are excited to learn, but we think we're on the right track.— Brodie Popovic, astrophysicist, University of Southampton
Type Ia supernovae are the premier tool for measuring the expansion history of the universe, and provided the first evidence in 1998 that cosmic expansion is accelerating due to dark energy.— Adam Riess, astrophysicist, Johns Hopkins University and 2011 Nobel laureate
The Hearth Conversation Another angle on the story
Why does it matter whether the universe's expansion is accelerating or slowing down? What changes if dark energy is weakening?
It changes everything about how we understand the fate of the cosmos. If acceleration is real, the universe will expand forever, growing colder and emptier. If it's slowing, gravity might eventually pull everything back together. And fundamentally, it tells us whether our best model of reality is correct.
But you said dark energy makes up 68 percent of the universe. How can we not know what it is?
That's the humbling part. We know it exists because we can measure its effects—the way it pushes galaxies apart. But we can't see it, can't touch it, can't replicate it in a lab. It's like knowing wind exists because you see trees bend, but not understanding what wind actually is at a molecular level.
So this new study is saying the 2025 researchers got it wrong?
Not exactly wrong—they asked a legitimate question about whether the old measurements accounted for something they thought was being missed. But when the new team looked at the same data more carefully, they didn't find evidence for what the 2025 team was looking for. Science works this way. Someone challenges the consensus, and then you either confirm the challenge or you don't.
These Type Ia supernovae—why are they so special for measuring the universe?
Because they're standardized explosions. Imagine if every lightbulb in the world had exactly the same wattage. Then the only reason one looks dimmer than another is distance. That's what these supernovae give us—a cosmic measuring stick. We see one that's faint, we know it's far away. We see one that's bright, it's closer. From that, we can map the universe's expansion.
What happens when those new telescopes launch?
They'll see farther and clearer than we can now. More supernovae, better measurements, less room for interpretation. It's possible they'll settle this debate once and for all—or they might reveal something nobody expected.