New research challenges the idea that the universe is accelerating its expansion. Astronomers at Yonsei University in South Korea corrected for age effects of stellar populations on Type Ia supernova data and found that dark energy may be losing strength—suggesting that the universe has already begun to slow down.
The expansion of the universe may no longer be accelerating as previously thought. New research suggests that cosmic growth has begun to slow down — a claim that challenges one of the most established dogmas in modern astronomy. “Remarkable” findings, published Nov. 6 in the Monthly Notices of the Royal Astronomical Society , raise serious questions about the long-held view that a mysterious force — “dark energy” — is pushing galaxies apart at an increasing rate. Instead, the researchers report that there is no clear evidence that the expansion is still accelerating today. If confirmed, the findings could change scientists’ understanding of dark energy, help resolve the “Hubble tension,” and reveal new insights into the origins and fate of the universe.
A new look at “cosmic acceleration”
Team leader, Professor Young-Wook Lee of Yonsei University, explains: “Our study shows that the universe has already entered a phase of slowing expansion in the present era, and that dark energy is evolving in time much faster than previously thought. If these results are confirmed, this is a significant paradigm shift in cosmology since the discovery of dark energy 27 years ago.”
For nearly three decades, the idea that the expansion of the universe is accelerating, driven by dark energy—a mysterious force that defies gravity—has dominated the debate. This conclusion was based on measurements of distant Type Ia supernovae, work for which the Nobel Prize in Physics was awarded in 2011.
But Yonsei's team has found evidence that could challenge this assumption. Type Ia supernovae—which have been considered reliable “standard lights” for cosmic distances—appear to be affected by the age of the stellar populations that give rise to them. Even after standardizing for brightness, supernovae from young populations tended to be systematically dimmer, while those from older populations tended to be brighter. Using a much larger dataset of 300 host galaxies, the researchers confirmed the age-dependent pattern with exceptional statistical confidence (99.999%). The finding suggests that the dimness of distant supernovae is not just due to the expansion of the universe, but also to differences in stellar evolution.
Better fit to another cosmological model
After correcting for the “age bias,” the revised supernova data no longer fit the standard ΛCDM model (which describes an accelerating universe due to a cosmological constant). Instead, they were much more consistent with the model favored by the DESI (Dark Energy Spectroscopic Instrument) project—a framework that relies on measurements of ordinary matter acoustic oscillations (BAO), ancient “sound waves” from the dawn of the universe, and observations of the cosmic microwave background (CMB).
Dark energy that weakens over time
Both the corrected supernova data and the BAO+CMB results indicate that dark energy is significantly weakening and evolving over time. More importantly, when combining the corrected supernova data with BAO and CMB, the researchers report that the standard ΛCDM model is ruled out with very high significance.
The most surprising conclusion from the combined analysis: the universe is not accelerating today, but has already entered a state of decelerating expansion. Prof. Li added: “In the DESI project, the main conclusions were obtained by combining uncorrected supernova data with BAO measurements, which led to the conclusion that while the universe will slow down in the future, it is still accelerating at present. In contrast, our analysis – which applies the age-bias correction – shows that the universe has already entered a deceleration phase today. Remarkably, this is consistent with what is predicted independently from BAO alone or BAO+CMB, although this fact has received little attention until now.”
Validation of the findings: “Evolution-free test”
To substantiate the results, the Yonsei team is now performing an “evolution-free test” using only supernovae from young and aged host galaxies across the entire redshift range. The initial results already support the main conclusion. “In the next five years, when the Vera S. Rubin Observatory will discover more than 20,000 new supernova host galaxies, precise age measurements will serve as a much more robust and conclusive test of supernova cosmology,” said Prof. Chul Chung, co-lead author of the study along with doctoral student Junheek Son.
The Vera S. Rubin Observatory, perched on a mountaintop in the Andes Mountains of Chile, is home to the world’s most powerful digital camera. It began scientific operations this year and is set to provide answers to fundamental questions about our solar system and the wider universe. After the Big Bang and rapid expansion about 13.8 billion years ago, gravity slowed the expansion. However, in 1998 it was determined that nine billion years after the beginning of the universe, its expansion began to accelerate again—a phenomenon attributed to dark energy.
The profound mystery of dark energy
Astronomers have coined the term “dark energy”—a component estimated to make up about 70 percent of the universe—but its exact nature remains one of the great scientific mysteries. Last year, data from DESI (in Tucson, Arizona) suggested that the force exerted by dark energy has changed over time—and more evidence has been accumulating since then. The hope is that with these new tools, astronomers can get closer to understanding the nature of dark energy and how it shapes the universe.
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It turns out that dark energy is too dark a matter.
Perhaps it is time to say goodbye to the concept of "dark energy," and at the same time to "dark matter" - speculations that were drawn out ad hoc to explain observational phenomena, and that in fact represent a kind of primitive, "Aristotelian" science disguised as modern science. It turns out that not only do we not have a clear concept of these speculative concepts, but that the observational phenomena that are supposed to necessitate them are quite convoluted and/or inconsistent and not always universal, and in short - do not constitute an adequate empirical basis for the enigmatic and sweeping superstructure built on them.
Well, decide now.