New research from the Royal Astronomical Society casts doubts on the long-standing theory that a mysterious force known as ‘dark energy’ is driving distant galaxies away at an increasingly faster rate.
Instead, they show no evidence of an accelerating Universe, instead suggesting it might have started to slow.
If the results are confirmed, it could open an entirely new chapter in scientists’ quest to uncover the true nature of dark energy, resolve the ‘Hubble tension,’ and gain a deeper understanding of the Universe’s past and future.
Lead researcher Professor Young-Wook Lee, of Yonsei University in South Korea, explained: “Our study shows that the Universe has already entered a phase of decelerated expansion at the present epoch and that dark energy evolves with time much more rapidly than previously thought.
“If these results are confirmed, it would mark a major paradigm shift in cosmology since the discovery of dark energy 27 years ago.”
Astronomers thought the Universe was expanding over time
For the past three decades, astronomers have generally believed that the Universe is expanding at an ever-increasing rate, driven by an unseen phenomenon called dark energy that acts as a kind of anti-gravity.
However, a team of astronomers at Yonsei University have now put forward new evidence that type Ia supernovae, long regarded as the Universe’s “standard candles,” are in fact strongly affected by the age of their progenitor stars.
Even after luminosity standardisation, supernovae from younger stellar populations appear systematically fainter, while those from older populations appear brighter.
Dark energy weakens and evolves over time
Based on a much larger host-galaxy sample of 300 galaxies, the new study confirmed this effect at extremely high significance (99.999% confidence), suggesting that the dimming of distant supernovae arises not only from cosmological effects but also from stellar astrophysics effects.
When this systematic bias was corrected, the supernova data no longer matched the standard ΛCDM cosmological model with a cosmological constant.
Instead, it aligned far better with a new model favoured by the Dark Energy Spectroscopic Instrument (DESI) project.
The corrected supernova data and the BAO+CMB-only results both indicate that dark energy weakens and evolves significantly with time.
More importantly, when the corrected supernova data were combined with BAO and CMB results, the standard ΛCDM model was ruled out with overwhelming significance.
Paving the way for more accurate cosmological testing
To further confirm their results, the team are now carrying out an “evolution-free test,” which uses only supernovae from young, coeval host galaxies across the full redshift range.
“Within the next five years, with the Vera C. Rubin Observatory discovering more than 20,000 new supernova host galaxies, precise age measurements will allow for a far more robust and definitive test of supernova cosmology,” said research professor Chul Chung, a co-lead on the study along with PhD candidate Junhyuk Son.
Last year, data from DESI in Tucson, Arizona, suggested that the force exerted by dark energy had changed over time, a finding that subsequent observations have corroborated.
The hope is that with these new tools, astronomers will now be better equipped to find clues about what exactly dark energy is and how it influences the Universe.
