New Study Challenges Cosmic Acceleration: Could Dark Energy Be Evolving?

A new peer-reviewed study argues that the universe’s expansion may already have entered a decelerating phase, calling into question the long-standing view that cosmic expansion is currently accelerating under a constant dark energy force.

What the study claims

Published on November 6 in Monthly Notices of the Royal Astronomical Society, the paper—led by Young‑Wook Lee of Yonsei University—analyzes a sample of about 300 galaxies that host Type Ia supernovas. The team reports a strong age–brightness correlation for these explosions: supernovas from younger progenitor systems are slightly dimmer, while those from older progenitors are brighter. The authors quote a statistical confidence of 99.99% for this relation and argue that correcting for it changes cosmological inferences so that the present-day universe may no longer be accelerating. They further suggest dark energy might not be a constant but an evolving phenomenon.

Context and related findings

The original discovery of cosmic acceleration in 1998 relied on Type Ia supernovas as standard candles; some distant supernovas appeared dimmer than expected, interpreted as evidence of accelerating expansion. More recently, large-scale surveys such as the Dark Energy Spectroscopic Instrument (DESI) produced a three-dimensional map of the large-scale structure of the universe and produced hints that dark energy's influence could change over time. The new Yonsei-led study builds on and amplifies these lines of inquiry by emphasizing a progenitor-age bias in supernova luminosities.

Responses from the scientific community

The study has provoked strong skepticism. Adam Riess (Johns Hopkins University; 2011 Nobel laureate) said the analysis “rests on a flawed premise,” arguing that present-day Type Ia supernovas are typically associated with young star-forming regions and that similar ideas were tested and refuted previously. Dan Scolnic (Duke University) cautioned that the leap from host-galaxy age to supernova progenitor age is not straightforward, and that larger data sets already account for environmental effects that affect brightness. Dillon Brout (Boston University) noted that the proposed evolutionary modeling lacks empirical support and that modern analyses already capture most environment-linked brightness variations. Dragan Huterer (University of Michigan) emphasized that extraordinary claims demand extraordinary evidence and that the result does not yet meet the threshold to overturn the prevailing cosmological model.

Young‑Wook Lee: “Dark energy is there, but the present universe has already entered a decelerating phase, today. So the fate of the universe could change.”

Implications and caution

If the age–brightness relation and its cosmological correction are confirmed, cosmologists would need to revise measurements of the expansion history and rethink models of dark energy—possibly replacing a simple cosmological constant with a time-evolving field. In an extreme scenario, continued deceleration could eventually lead to recontraction, a so‑called "big crunch," though the paper’s authors acknowledge that more evidence is needed before such conclusions gain traction.

What comes next

Lee and colleagues point to imminent large surveys as tests of their claim. The Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST), with early data releases expected after it began operations, will produce orders of magnitude more supernova discoveries and enable more precise host‑galaxy age measurements. Lee estimates Rubin data could provide a decisive test within three to five years. Meanwhile, independent reanalyses and larger samples will be essential to evaluate whether the age–brightness correlation persists and whether it substantially alters cosmological conclusions.

Bottom line

The new study raises an important and testable challenge to the use of Type Ia supernovas as cosmological probes and to the assumption that dark energy is constant. The claim is provocative and potentially far-reaching, but it remains controversial: respected experts have raised methodological concerns, and further data and independent confirmation are required before the standard model of cosmic acceleration is overturned.

Sources: Young‑Wook Lee (Yonsei University), Junhyuk Son (Yonsei University), comments from Adam Riess, Dan Scolnic, Dillon Brout, and Dragan Huterer; journal: Monthly Notices of the Royal Astronomical Society; related DESI and Vera C. Rubin Observatory projects.