Dark energy may be weakening, shaking the foundations of modern cosmology

Latest Dark Energy Study Suggests the Universe Is Even Stranger Than We Thought

DESI's revolutionary data challenge our understanding of dark energy, igniting debate over the universe’s fate

A new cosmic twist has emerged in 2025 that may reshape our understanding of the universe’s structure and destiny. According to the latest findings from the Dark Energy Spectroscopic Instrument (DESI), dark energy—the mysterious force that drives the universe’s accelerating expansion—may not be a fixed constant, as previously thought. Instead, it could be slowly weakening over time.

This bold claim has sparked intense debate across the astrophysics community. If verified, it could spell the end of the ΛCDM (Lambda Cold Dark Matter) model—the longstanding foundation of modern cosmology—and usher in new, exotic physics.

A cosmic shock from Arizona

DESI, based at Kitt Peak National Observatory in Arizona, has been measuring the positions and redshifts of millions of galaxies to study how the universe expanded over the past 11 billion years. Using techniques like baryon acoustic oscillation mapping and redshift tracking, DESI builds a 3D map of the cosmos to understand the influence of dark energy over time.

In 2024, initial findings hinted that the universe’s expansion rate might not align with the constant dark energy model of ΛCDM. By March 2025, those hints had solidified into statistical significance, suggesting that dark energy may be changing with time—a hypothesis previously considered fringe.

“We tend to stick with the simplest theory that works—until it doesn’t,” said physicist Joshua Frieman of the University of Chicago.

Not all scientists are convinced

The reaction from the scientific community has been mixed. Daniel Green, a physicist at the University of California, San Diego, cautioned against prematurely interpreting DESI’s findings.

“They only looked under one lamppost,” Green said, comparing DESI’s analysis to searching for lost keys only where there’s light.

Green and others argue that decaying dark matter or other unknown phenomena could also explain the data, without requiring a radical revision of dark energy's nature. He advocates for broader analysis before drawing conclusions.

Breaking the standard model

The ΛCDM model has served as the backbone of cosmology for decades, assuming cold dark matter explains galaxy formation and that dark energy—represented by Einstein’s cosmological constant (lambda)—is uniform over time. DESI’s findings threaten that balance, challenging the assumption of a time-invariant lambda.

To account for a dynamic dark energy, cosmologists use a parameter called w(z), representing the ratio of dark energy’s pressure to its energy density. If w(z) equals –1, it matches ΛCDM. But if it rises above –1, dark energy dilutes; if it drops below –1, it could trigger a "big rip" that tears apart galaxies, stars, and even atoms—violating key physical principles like the null energy condition.

Controversies and caveats

Some graphs in DESI’s preprint show w(z) dipping below –1, raising concerns about a violation of physical laws. But others argue these graphs rely on simplistic linear models. Paul Steinhardt of Princeton University and others point out that a curved fit to DESI’s data shows w(z) staying just above the critical threshold—consistent with a fading but not catastrophic dark energy.

“They draw the wrong conclusions,” Steinhardt said of critics focusing on linear fits.

A fifth force—or failing neutrinos?

One revived explanation suggests a fifth fundamental force, potentially arising from a new ultra-light dark matter particle, could be responsible for the changing behavior of dark energy. DESI’s precision even allows researchers to estimate the theoretical particle’s mass—about 10⁻³³ electron volts, an almost incomprehensibly tiny figure.

Meanwhile, other theories invoke neutrinos, particles already known to be extraordinarily light. However, accommodating a dynamic dark energy in current models would require these neutrinos to have negative mass—a scenario many physicists reject as physically implausible.

“You really need to explore every alternative explanation,” Green said. “Evolving dark energy is the absolute last one I would be willing to believe.”

Dark matter decay: a simpler solution?

One promising alternative comes from physicist Gabriel Lynch and advisor Lloyd Knox. Their new model suggests the same observational effects DESI sees could result from a decaying form of dark matter, without invoking negative neutrino mass or violating core principles. If dark matter decays over billions of years into lighter “dark radiation,” it could mimic a weakening dark energy signature.

This scenario might also allow scientists to estimate the actual masses of dark matter particles and neutrinos—a long-sought milestone in particle physics and cosmology.

What's next for cosmology?

The DESI debate is far from settled. Experts emphasize that more data and broader analyses are needed before any theory can claim victory. Nevertheless, the consensus is clear: ΛCDM is under strain, and new physics may be required to explain DESI’s results.

“We’re beyond LCDM both ways,” said DESI co-spokesperson Nathalie Palanque-Delabrouille. “We just want to know the truth.”

New instruments, such as the Vera C. Rubin Observatory and the Euclid Space Telescope, promise to deliver even richer datasets in the coming years. As the tools of cosmology sharpen and theories evolve, humanity may be on the brink of a radical new understanding of the universe’s deepest forces.

Stay tuned to The Horizons Times as the future of dark energy—and cosmology itself—unfolds before our eyes.