Scientists with Yonsei University in South Korea report that the universe’s expansion may not be accelerating as was previously believed, but is instead slowing down.
The findings, based on new research that corrects a previous supernova age-bias, also suggest that dark energy is not static but evolves over time, which the researchers say helps account for seemingly contradictory data regarding past observations of our universe.
The research team behind the groundbreaking discovery says a future confirmation of their results could usher in an entirely new chapter in the search for the true nature of dark energy, helping move toward resolving the controversial Hubble Tension, and aid scientists in better understanding “the past and future of the universe.”
“This points to a time-varying dark energy and a universe that is no longer accelerating, representing a potential paradigm shift in cosmology if confirmed,” explained Professor Chul Chung, co-lead author on the new study, in an email to The Debrief.
Early cosmological models suggested that the initial rapid expansion of the universe, which occurred approximately 13.8 billion years ago just after the Big Bang, began slowing down due to gravity. However, a 1998 study, which ultimately earned a 2011 Nobel Prize, showed that the universe’s expansion began to accelerate around 9 billion years after the Big Bang due to the influence of what astronomers later termed dark energy.
Nearly three decades since that discovery, the nature of dark energy, which models suggest makes up about 70 percent of the universe, remains a cosmological mystery. Another unresolved issue, known as the Hubble Tension due to contradictory readings with relation to the rate of universal expansion, has only added to the confusion.
According to the latest study’s lead researcher, Professor Young-Wook Lee of Yonsei University, the original estimates made in 1998 were based on what are known as Type Ia supernovae. Because astronomers previously believed they understood the lifecycles of these supernovae, they were often used as the universe’s “standard candles” for age calculations.
In their new study, Lee, Chung, and colleagues directly measured the light from the host galaxies of approximately 300 Type Ia supernovae. Chung told The Debrief that this information “simply wasn’t available” to the Nobel Prize-winning team 27 years ago.
After performing a luminosity standardization on the results, the research team found that the closer supernovae from younger stellar populations appeared “systematically fainter,” while those from older, more distant populations appeared consistently brighter.
“We found a 5.5σ correlation between Type Ia supernova brightness and progenitor age, creating a redshift-dependent bias in previous analyses,” Chung told The Debrief.
In effect, the analysis suggested that the brightness of the supernovae decreased with age. Follow-up analysis confirmed the data finding with a confidence level of 99.999%. Lee’s team suggests this unexpected dimming of distant supernovae is caused by “cosmological effects” and stellar astrophysics effects.
In the study’s conclusion, the team stated that their corrected supernova data no longer matched the standard ACDM cosmological model, which includes a cosmological constant. In fact, after correcting for their newly discovered bias, Chung told The Debrief that their findings make cosmological distances “producing a >9σ tension with the ΛCDM model.”
Next, Chung said that his team cross-checked their age-corrected supernova distances against independent ‘standard ruler’ results from the DESI BAO project and CMB data “to confirm consistency across multiple cosmological probes.” Derived from baryonic acoustic oscillations (BAO), often referred to as the sound of the Big Bang, and cosmic microwave background (CMB) data, the new model for the universe’s expansion is favored by the Dark Energy Spectroscopic Instrument (DESI) project.
After running the comparison, Chung says that correcting for the dimming bias made “supernova distances align with BAO and CMB results.”
Professor Lee explained that in the DESI project, the key results were obtained by combining “uncorrected supernova data with baryonic acoustic oscillations measurements,” which seemed to indicate that although the universe will decelerate in the future, “it is still accelerating at present.”
“By contrast, our analysis — which applies the age-bias correction — shows that the universe has already entered a decelerating phase today,” Lee said.
Although the potentially historic findings suggest that dark energy isn’t static, but instead evolves and weakens significantly over cosmological time periods, the researchers cautioned that they are not suggesting dark energy is entirely illusory.
“We’re not saying dark energy doesn’t exist,” Chung affirmed in a statement provided to The Debrief.
In contrast, the researcher said their findings simply show that the standard assumption that dark energy is a constant “no longer fits” after correcting the previous data with supernovae brightness based on the age of their progenitor galaxy. Instead, Chung said the new evidence points to a “time varying form” of dark energy and shows that the universe is no longer accelerating, but is instead decelerating.
“Dark energy may still exist,” Chung told The Debrief, “but it likely evolves over time rather than remaining constant.”
When asked if the findings could impact studies of theoretical dark matter, Chung said it could, albeit “indirectly.”
“Cosmological inferences about matter density and structure growth depend on the assumed expansion history,” the researcher told The Debrief. “If supernova distances change after correcting for age bias, some derived parameters may need to be re-evaluated.”
Still, he clarified, his team’s study focused primarily on dark energy and cosmic expansion, “not dark matter particles.”
When discussing possible methods for confirming their findings, the team pointed to the Vera C. Rubin Observatory in Chile, which is scheduled to go into operation within the next year. Once operational, the team expects that the observatory, which is home to the world’s most powerful digital camera, will discover more than 20,000 new supernovae within the next five years.
Chung said measurements of these newly discovered supernovae will allow for a “far more robust and definitive test of supernova cosmology.”
In a follow-up effort to confirm their results, the Yonsei University team has already begun a series of tests they call “evolution-free.” Chung said these tests involve restricting observations to Type Ia supernovae in young, coeval host galaxies “across the full redshift range.” By keeping the stellar population age effectively fixed, Chung said their new study eliminates “the very effect” that can give a false reading of accelerating cosmic expansion.
“If these ‘age-controlled’ supernovae reproduce our main result, it would provide a powerful and age-bias-free confirmation of our findings,” the researcher told The Debrief, though adding that “our first results already support a time-varying form of dark energy.”
The paper, “Strong Progenitor Age-bias in Supernova Cosmology. II. Alignment with DESI BAO and Signs of a Non-Accelerating Universe,” was published in Monthly Notices of the Royal Astronomical Society.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.