The universe may be more intricate than we assumed
For decades, cosmology has rested on a quiet assumption: that dark energy, the invisible force accelerating the universe's expansion, is eternal and unchanging. Now, simulations run on Japan's Fugaku supercomputer — informed by fresh observational data from the Dark Energy Spectroscopic Instrument — suggest that dark energy may evolve across cosmic time, and that when paired with a denser universe than previously modeled, the consequences for how galaxies form are profound. The work does not overturn the standard model, but it introduces a principled crack in its foundation, reminding us that the cosmos may be stranger, and more dynamic, than our most elegant equations have allowed.
- Decades of cosmological consensus are under pressure as new observational data hints that dark energy — long assumed to be a fixed constant — may shift and evolve across billions of years.
- Researchers at Chiba University deployed the Fugaku supercomputer to run simulations eight times larger in volume than prior efforts, stress-testing the standard model against a dynamic dark energy alternative.
- The most dramatic finding was not dark energy's variation alone, but its combination with a 10 percent higher matter density suggested by DESI data — together predicting up to 70 percent more massive galaxy clusters in the early universe.
- Simulated signatures of ancient cosmic sound waves shifted by 3.71 percent toward smaller scales, closely matching what DESI actually observed — a rare moment where computational prediction and telescope data converge.
- The results do not yet confirm that dark energy varies, but they establish a rigorous framework for testing that possibility, signaling that multiple cosmic parameters must be refined simultaneously to understand the universe's true architecture.
For more than a century, astronomers have watched the universe expand at an accelerating pace, attributing the push to dark energy — an invisible, constant force embedded in spacetime itself. This assumption anchored the Lambda Cold Dark Matter model, the standard framework guiding cosmological research for decades. But recent data from the Dark Energy Spectroscopic Instrument has begun to whisper a different story: dark energy may not be fixed at all, but dynamic, shifting across cosmic time.
To probe that possibility, Tomoaki Ishiyama and colleagues at Chiba University turned to the Fugaku supercomputer, running three vast simulations of cosmic evolution — each eight times larger in volume than comparable prior studies. One modeled the standard unchanging universe; the others incorporated dynamic dark energy, one tuned to match the latest DESI observations. The research, published in Physical Review D in August 2025, was a collaboration spanning Japan, Spain, and New Mexico.
The results were surprising in their nuance. Dynamic dark energy alone produced only modest changes in how cosmic structure evolved. The real transformation emerged when the team also incorporated DESI's suggestion that the universe contains roughly 10 percent more matter than the standard model assumes. That combination — dynamic dark energy plus higher matter density — predicted up to 70 percent more massive galaxy clusters in the early universe.
The simulations were tested against real observations with notable success. Ancient acoustic signatures frozen into the universe's structure shifted by 3.71 percent toward smaller scales in the DESI-derived model, closely matching telescope data. Galaxy clustering patterns, too, aligned with observations, particularly at smaller scales where the denser matter made its presence felt most strongly.
What the work ultimately reveals is a universe more intricately woven than the standard model suggests — one where dark energy's possible evolution cannot be understood in isolation, but only in concert with matter density, gravitational structure, and the full interplay of cosmic forces across time. The question of whether dark energy truly varies remains open, but Fugaku has given researchers a rigorous framework for pursuing the answer.
For more than a century, astronomers have watched the universe expand—not slowly, as gravity might suggest, but faster and faster. The culprit, they concluded, was dark energy, an invisible force woven into the fabric of spacetime itself, pushing galaxies apart. The explanation was elegant and simple: dark energy was constant, unchanging, a fixed property of the cosmos. This assumption became the foundation of modern cosmology, the Lambda Cold Dark Matter model, or ΛCDM, that has guided research for decades.
But what if that assumption was wrong? What if dark energy was not static but shifting, evolving across billions of years? Recent observations from the Dark Energy Spectroscopic Instrument, a massive astronomical survey mapping distant galaxies, have begun to hint at exactly that possibility. The data suggests dark energy might be dynamic—time-varying rather than eternal and fixed. If true, it would represent a fundamental crack in the edifice of modern cosmology, and it would open a new question: how would a changing dark energy reshape the universe's history?
To explore this possibility, a team of researchers led by Tomoaki Ishiyama at Chiba University in Japan turned to one of the world's most powerful machines. Using the Fugaku supercomputer, they ran three massive simulations of cosmic evolution, each eight times larger in volume than previous studies of this kind. One modeled the standard, unchanging dark energy universe. The other two incorporated dynamic dark energy—one with fixed parameters, another tuned to match the latest observational data from DESI. The work, published in Physical Review D in August 2025, was conducted in collaboration with Francisco Prada in Spain and Anatoly Klypin in New Mexico.
The simulations revealed something unexpected. Dynamic dark energy alone, when isolated from other cosmic ingredients, had surprisingly modest effects on how the universe evolved. The real transformation came when the researchers adjusted another parameter: the density of matter itself. DESI observations suggested the universe contained about 10 percent more matter than the standard model assumed. When the team incorporated this higher matter density into their dynamic dark energy model, the effects became dramatic. The simulations predicted up to 70 percent more massive galaxy clusters in the early universe—the cosmic scaffolding where galaxies and galaxy clusters form and congregate.
The researchers tested their model against real observations in multiple ways. They examined baryonic acoustic oscillations, ancient sound waves frozen into the structure of the universe that serve as a cosmic ruler for measuring distances. In their simulations, the DESI-derived model shifted this signature by 3.71 percent toward smaller scales—a prediction that closely matched what DESI actually observed. They also studied how galaxies cluster together across space, finding that the higher matter density produced stronger clustering signals, especially at smaller scales, again aligning with the observational data.
What emerged from these massive computational experiments was a portrait of a universe more intricate than the standard model suggested. The dynamic dark energy component itself was not the dominant force reshaping cosmic structure. Instead, it was the interplay between time-varying dark energy and a denser universe that produced measurable, significant changes in how galaxies formed and distributed themselves across space. As Ishiyama explained, variations in matter density had a greater influence on structure formation than the dynamic dark energy alone.
The implications are still unfolding. The simulations validate the possibility that dark energy is not constant, lending credibility to hints from observational data. But they also suggest that understanding the universe's evolution requires precision in multiple parameters simultaneously—not just dark energy, but the total amount of matter, the way gravity shapes structure, the interplay of forces across cosmic time. The work does not settle the question of whether dark energy truly varies. Rather, it provides a framework for testing that possibility against reality, and it shows that if dark energy does change, its effects would be woven together with other cosmic properties in ways that demand careful, detailed study.
Notable Quotes
Our large simulations demonstrate that variations in cosmological parameters, particularly the matter density in the universe, have a greater influence on structure formation than the DDE component alone— Tomoaki Ishiyama, Chiba University
The Hearth Conversation Another angle on the story
Why does it matter whether dark energy is constant or changing? Doesn't the universe expand either way?
It matters because the history of structure—how galaxies and clusters form—depends on it. If dark energy changes over time, the universe's expansion rate changes, which changes how gravity competes with that expansion. That reshapes everything that grows from gravity's pull.
So the simulations showed dynamic dark energy is the big driver of change?
Actually, no. That was the surprise. Dynamic dark energy alone had modest effects. The real transformation came from matter density—the universe appears to have about 10 percent more matter than we thought. That's what dramatically changed structure formation.
Then why focus on dark energy at all if matter density is the bigger player?
Because DESI observations hint that dark energy might be varying, and we need to understand what that would mean if it's true. The simulations show how to test it. You can't separate one ingredient from the others—they work together.
Did the simulations match what astronomers actually see?
Yes. The predictions for galaxy clustering and the cosmic ruler signature aligned with DESI data. That validation is crucial. It means the model has real predictive power, not just theoretical elegance.
What happens next?
More observations, more refinement. The standard model may need updating. But we're still in the early stages of understanding whether dark energy truly varies and how deeply that variation shapes the cosmos.