A new study from Yonsei University suggests the universe's expansion has begun slowing down, potentially due to biases in supernova measurements. Researchers argue this indicates dark energy is weakening over time. However, other scientists express skepticism about the findings.
For nearly three decades, astronomers have relied on type Ia supernovae as 'standard candles' to measure cosmic distances, concluding that the universe's expansion, which began after the Big Bang 13.8 billion years ago, started accelerating around nine billion years later due to dark energy comprising about 70 percent of the universe. This discovery earned the 2011 Nobel Prize in Physics. Now, a study led by Professor Young-Wook Lee of Yonsei University in South Korea challenges that view.
Published on November 6 in Monthly Notices of the Royal Astronomical Society, the research analyzed data from 300 host galaxies and found that supernova brightness varies with the age of their progenitor stars, with 99.999 percent confidence. Younger stars produce fainter supernovae, while older ones appear brighter. Correcting for this 'age bias,' the team says the evidence for ongoing acceleration disappears. Instead, the universe has entered a phase of decelerated expansion at the present epoch, aligning with models from the Dark Energy Spectroscopic Instrument (DESI) project using baryonic acoustic oscillations (BAO) and cosmic microwave background (CMB) data.
Professor Lee stated, '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.' The findings suggest deceleration began about 1.5 billion years ago, potentially leading to a 'big crunch' where expansion reverses.
Skeptics, including Nobel laureate Adam Riess, argue the work repeats a refuted 2020 claim by the same group and that measuring stellar ages at vast distances is unreliable. Mark Sullivan of the University of Southampton notes that age effects are already accounted for in dark energy measurements and doubts a decelerating universe. Future observations from the Vera C. Rubin Observatory in Chile, starting this year, could test these ideas by cataloging tens of thousands of supernovae for precise age analysis.
Recent DESI results also hint that dark energy may vary over time, adding intrigue to the debate without confirming deceleration today.