In the cosmic playbook, few mysteries grip scientists quite like dark energy. It’s the unseen hand pushing galaxies apart, accelerating the universe’s expansion, and defying everything we thought we knew about gravity and the cosmos. Dark energy isn’t just an unsolved problem; it’s a profound puzzle, a whisper from the universe that our understanding is incomplete. The deeper scientists dig, the more unsettling and thrilling the mystery becomes.
The journey to untangle this mystery has led to the creation of extraordinary tools like the Dark Energy Spectroscopic Instrument (DESI), cutting-edge simulations powered by the world’s most powerful supercomputers, and theories that challenge the foundations of modern physics. What we’re learning suggests that the cosmos may be far stranger than we ever imagined.
The Great Discovery: A Universe in Motion
Let’s rewind a century. Back in the early 20th century, most scientists assumed the universe was static—unchanging, eternal, and peaceful. That illusion was shattered when astronomers noticed something peculiar: distant galaxies were shifting toward the red end of the light spectrum, a phenomenon known as redshift. This suggested they were moving away from us.
It was Edwin Hubble who cracked the code. By comparing distances and velocities, he discovered that the farther a galaxy was, the faster it receded. Hubble’s revelation forced humanity to abandon the idea of a static universe. We were adrift in a cosmos that was growing larger every moment.
But as groundbreaking as this was, it wasn’t the whole story. Astronomers assumed gravity—always the great cosmic tether—was slowing down this expansion. They thought the universe was coasting to a gentle halt, or maybe even poised to reverse.
Then came 1998, and everything changed.
The Acceleration Surprise
Two independent teams of astronomers, studying Type Ia supernovae, found something no one had expected. Instead of slowing, the universe’s expansion was speeding up. Galaxies were racing away faster and faster. It was as if some unknown energy was overcoming gravity, pulling the cosmos apart at an accelerating pace.
This mystery force was dubbed dark energy, and it wasn’t just a minor player—it made up a staggering 68% of the universe. For perspective, dark matter accounts for about 27%, and the stuff we can actually see—stars, planets, galaxies—barely makes up 5%. We are living in a universe dominated by things we cannot see or fully understand.
The Cosmological Constant and the Hubble Constant: Conflicting Clues
Einstein himself once added a fudge factor to his equations, called the cosmological constant, to keep the universe static. He later discarded it when Hubble’s discovery proved the universe was expanding. Ironically, the cosmological constant is back—this time to explain dark energy. It’s thought to represent the energy density of empty space, driving the acceleration we observe.
But there’s a catch. Measuring the universe’s expansion rate—the Hubble constant—has given us different answers depending on how and where we look. Measurements of nearby supernovae yield one number, while observations of the early universe (via the cosmic microwave background) give another. The discrepancies suggest something is missing from our picture—maybe even the whole framework.
Enter DESI: The Ultimate Cosmic Cartographer
The thirst to decode dark energy has driven scientists to build powerful instruments. One of the most ambitious is the Dark Energy Spectroscopic Instrument (DESI), which began its main survey in 2021. Perched on a telescope in Arizona, DESI is tasked with mapping the 3D distribution of galaxies across 11 billion years of cosmic history.
DESI works by analyzing baryon acoustic oscillations (BAO)—subtle ripples in the distribution of matter, remnants of pressure waves from the early universe. By creating a precise map of where galaxies are and how they’re moving, DESI can track how fast the universe expanded over billions of years. It’s a powerful probe of dark energy’s influence.
The early results from DESI were both reassuring and tantalizing. Much of what it observed fit neatly with our Standard Model of Cosmology, known as Lambda-CDM, where Lambda represents the cosmological constant. But there were cracks. Subtle discrepancies hinted that dark energy might not be constant after all.
Is Dark Energy Changing Over Time?
If dark energy’s influence is shifting, it would upend one of the most basic assumptions in cosmology—that the cosmological constant is fixed. Instead, we might be dealing with something dynamic: a mysterious field or fluid, often referred to as quintessence, that evolves over time and space.
And if this dynamic dark energy exists, it would mean the universe is permeated by an exotic, ever-changing energy with negative pressure—an effect that’s never been replicated or even observed in a lab on Earth.
Physicists like Andrew Hearin from Argonne National Laboratory and Katrin Heitmann, a cosmologist and deputy director at Argonne, aren’t disappointed by the potential upheaval. They’re exhilarated. This isn’t the universe closing its doors—it’s throwing them wide open.
Simulating the Universe: The Power of Aurora
To test these ideas, Hearin and his colleagues have turned to one of the most powerful tools in science: supercomputer simulations. Specifically, they’ve harnessed Aurora, an exascale supercomputer at Argonne capable of performing a quintillion (that’s a billion billion!) calculations per second.
Using Aurora, they created a pair of virtual universes—what they call the Discovery Simulations. Both simulations started with identical initial conditions, but one had dark energy as a constant, and the other allowed dark energy to change over time.
“These simulations are a pair of boxes with identical initial conditions,” the team explained. “The first box uses a Lambda-CDM cosmology, while the second box contains a dark energy equation of state, w, which evolves in time.”
The goal? To see how these virtual universes grow and change, and to compare them to DESI’s real-world data.
What the Simulations Revealed
The simulations have been nothing short of eye-opening.
- Dark Matter Halo Mass Function: Halos of dark matter are the scaffolding on which galaxies are built. The simulations showed that halos formed differently in the two universes. In the evolving dark energy model (called w₀wₐCDM), halos were more abundant and grew differently, especially at earlier cosmic times.
- Accretion Rates: The rate at which dark matter halos gathered mass differed too. In the evolving dark energy model, halos accumulated mass slightly faster at lower redshifts (closer to the present day).
- Star Formation Rates (SFRs): Star formation wasn’t immune to the effects. Galaxies in the dynamic dark energy universe formed stars at different rates compared to their constant dark energy counterparts—particularly among massive galaxies at earlier times.
These are small differences, but they matter. They point to distinct cosmic histories that DESI’s observations can help verify.
Why Supercomputers Matter
Running these simulations used to take weeks. With Aurora’s processing power, they’re done in two days. That rapid turnaround allows researchers to quickly compare theories to real data and adjust accordingly.
“This pair of simulations really illustrates our ability to take a result that’s hot off the presses from a collaboration like DESI, immediately run a simulation based on those results, and then see what it looks like,” said Gillian Beltz-Mohrmann, a postdoctoral fellow at Argonne.
Where Do We Go From Here?
The Discovery Simulations don’t provide a final answer—yet. They’re a step in a careful scientific process, where simulations and observations inform each other in a feedback loop. As DESI gathers more data and as supercomputers become more powerful, the models will get sharper and more precise.
If evolving dark energy is confirmed, it could signal a new era in physics, perhaps even requiring a new theory of gravity. It could also hint at previously unknown forces or particles—or something stranger still.
“The cosmological constant is the simplest solution,” said Hearin. “But if the data shows us it’s not right, we need to embrace the more exciting alternatives.”
The Ultimate Cosmic Mystery
For now, dark energy remains the greatest enigma in cosmology. We’ve named it, measured it, and mapped its effects, but we don’t know what it is. It’s the blank space on our cosmic maps, the deep ocean yet to be explored.
DESI, Aurora, and the scientists behind them are charting a path into that unknown. What they find could reshape our understanding of reality, gravity, time, and space.
In the end, dark energy is more than just an astronomical puzzle. It’s a challenge to human curiosity and ingenuity—the kind of mystery that pushes us to build bigger telescopes, faster computers, and deeper theories. And as long as there’s a mystery out there, there will be those who try to solve it.
Because, as Einstein once said, “The important thing is not to stop questioning.” And dark energy may be the biggest question of them all.
Reference: Gillian D. Beltz-Mohrmann et al, Illuminating the Physics of Dark Energy with the Discovery Simulations, arXiv (2025). DOI: 10.48550/arxiv.2503.05947