Japan's Fugaku supercomputer has propelled cosmology into a new era by enabling some of the most extensive and precise simulations of the universe’s formation to date.
By harnessing its extraordinary computational resources, researchers have explored how dynamic dark energy may impact the cosmos in ways previously uninvestigable.
The international team led by Tomoaki Ishiyama from Chiba University joined forces with leading astrophysicists from Spain and the US. In August 2025, they published breakthrough findings in Physical Review D, changing prevailing assumptions about the universe’s evolution by directly comparing three massive simulation runs against new observational data.
What Makes Fugaku’s Simulations Unique?
Fugaku is equipped to perform trillion-cell calculations, dwarfing past cosmological efforts both in speed and scope. The research team leveraged this power to conduct three distinct, high-resolution simulations, pushing the boundaries of standard cosmic models.
These runs included one adhering to the traditional cosmological constant framework and two with time-varying dark energy, offering a rare side-by-side test of evolving cosmic forces.
The primary innovation lay in altering key cosmic parameters, particularly the matter density and characteristics of dark energy.
Such variations would have required prohibitive computing time elsewhere, but Fugaku completed them efficiently, supporting robust results on galaxy formation and clustering patterns.
Did you know?
Fugaku is the fastest supercomputer in Japan, capable of over 442 petaflops of peak performance, making it ideal for large-scale cosmological simulations.
How Did Dynamic Dark Energy Challenge Existing Models?
Contrary to longstanding assumptions, the simulations found that dynamic dark energy alone exerts a surprisingly modest influence on large-scale cosmic structure.
Early results showed only minor deviations from the predictions of the standard model, which treats dark energy as a fixed “cosmological constant.”
The real shift emerged when dynamic dark energy was examined alongside adjusted parameters, particularly those sourced from recent Dark Energy Spectroscopic Instrument (DESI) surveys.
This interplay revealed how the evolution of dark energy, coupled with changes in matter density, could dramatically shape the growth and distribution of galaxies across cosmic history.
Why Is Matter Density Crucial for Cosmic Structure?
When the DESI-derived 10% increase in matter density was incorporated into the simulations, gravitational effects intensified. This caused galaxy clusters to emerge sooner and in greater numbers, a finding not anticipated by conventional frameworks.
The model projected as many as 70% more massive clusters in early epochs, an outcome directly linked to stronger gravitational interactions from heightened matter density. In this scenario, dynamic dark energy amplified the density-driven changes.
The results revealed a picture of cosmic architecture far more sensitive to its underlying parameters than previously acknowledged, with observable implications for the universe's makeup at large scales.
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Do Observations support these Findings?
DESI’s March 2025 observations strongly suggested that dark energy may vary over time, a possibility gaining support through data with near-4.2 sigma significance.
This strengthens the evidence for non-constant dark energy, a prospect that has long been debated in astrophysics.
In addition, the Fugaku simulations showed a 3.71% shift in baryonic acoustic oscillation patterns, ancient sound wave relics now used to measure cosmic distances.
These simulated results closely matched DESI’s empirical data, building confidence in the models and testing the limits of current cosmic standard candles.
What Could This Mean for Scientific Theories?
The evolving picture suggested by Fugaku’s breakthroughs places new pressure on the notion of a static cosmological constant. If dark energy and matter density can shift, the theoretical foundations of cosmology may need substantial revision.
Dynamic models are becoming ever more plausible, driven by both simulation and observation.
Researchers anticipate further tests as upcoming supercomputers become operational and observational campaigns expand.
The synergy between computational power and collaboration across borders is accelerating our understanding of cosmic history, potentially reshaping textbooks about the universe's fundamental forces.
Looking ahead, the assimilation of Fugaku’s simulation results with ongoing DESI and future collaborative efforts promises more revelations.
As both simulations and telescopes probe deeper, the story of dark energy is set to unfold in unprecedented detail, challenging scientists to adapt their theories to a universe more complex and dynamic than ever thought possible.
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