A recent study from Yonsei University in Seoul, South Korea, challenges the previously accepted notion that dark energy is causing the accelerated movement of distant galaxies away from us. The researchers found no evidence supporting the idea that the universe is currently accelerating. If validated, this finding could significantly alter our understanding of dark energy, address the “Hubble strain,” and provide insights into the universe’s past and future.

For over three decades, astronomers have generally accepted that the universe is expanding at an increasing rate due to a hidden force dubbed dark energy, which functions as a sort of anti-gravity.

This conclusion, derived from distance measurements of far-off galaxies using Type Ia supernovae, earned the Nobel Prize in Physics in 2011.

However, Professor Yongwook Lee of Yonsei University and his team have introduced new evidence suggesting that Type Ia supernovae, once thought to be the universe’s “standard candle,” are significantly affected by the age of their progenitor stars.

“Our findings indicate that the universe is currently in a phase of decelerating expansion, and that dark energy is evolving at a much faster rate than previously assumed,” stated Professor Lee.

“If verified, these outcomes would signify the most substantial shift in cosmology since the identification of dark energy 27 years ago.”

Even after adjusting for brightness, supernovae from younger star populations seem systematically dimmer, while those from older populations appear brighter.

Utilizing a more extensive sample of 300 host galaxies, the researchers validated these findings with remarkable significance (99.999% confidence), indicating that the dimming of distant supernovae is influenced not only by cosmological factors but also by stellar astrophysical characteristics.

After correcting for this systematic bias, the supernova data no longer aligned with the classic ΛCDM cosmology model that includes a cosmological constant.

Instead, it aligns more closely with a new model backed by the Dark Energy Spectroscopy Instrument (DESI) project, based on Baryon Acoustic Oscillations (BAO) and Cosmic Microwave Background (CMB) data.

Both the adjusted supernova data and the results from BAO+CMB demonstrate that dark energy diminishes and evolves significantly over time.

Importantly, when the corrected supernova data were integrated with BAO and CMB findings, the traditional ΛCDM model was decisively ruled out.

Most notably, this comprehensive analysis reveals that the universe is not accelerating as much as once believed, but has already transitioned into a state of slowing expansion.

“The DESI project has yielded significant results by merging unadjusted supernova data with baryon acoustic vibration measurements, concluding that while the universe will decelerate in the future, it is still accelerating at present,” remarked Professor Lee.

“Conversely, our analysis, which incorporates an age-bias correction, indicates that the universe is already entering a slowing phase today.”

“Surprisingly, this aligns with predictions made independently from BAO analyses, which has yet to receive much attention.”

To further validate their findings, the researchers are now conducting an evolution-free test using only supernovae from young, contemporaneous host galaxies across the entire redshift range.

Initial results already support their primary conclusion.

“With the Vera C. Rubin Observatory set to discover more than 20,000 new supernova host galaxies within the next five years, accurate age measurements will provide a more robust and conclusive examination of supernova cosmology,” stated Yonsei University professor Chul Chung.

The team’s paper published today in Royal Astronomical Society Monthly Notices.

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Song Joon Hyuk et al. 2025. Strong founder age bias in supernova cosmology – II. Alignment of DESI BAO with signs of a non-accelerating universe. MNRAS 544 (1): 975-987; doi: 10.1093/mnras/staf1685