Unlocking Cosmic Mysteries: A Nobel Laureate’s Insight on the Universe’s Deceleration Debate

This is Part 4 of a series exploring a study that claims the Universe is slowing down. You can find earlier parts here.

Recently, I received an email from Adam Reiss. He’s a well-known physicist and a Nobel Prize winner in 2011 for his work on cosmic expansion. He shared some concerns about the decelerating Universe paper, and I’d like to pass on his insights.

The original paper discusses research involving around 300 supernovae. The authors discovered that the brightness of Type Ia supernovae relates to the age of their host galaxies. In simpler terms, younger galaxies show dimmer supernovae. They argue this means our measurements of galactic distances are incorrect and that the Universe is decelerating, challenging the widely accepted ΛCDM model. But Reiss pointed out some significant flaws in their claims.

First, the authors focus too much on galaxy ages. They say that the light curves of supernovae don’t consider the ages of their host galaxies. While that is partly true, these curves do factor in galactic mass, which is easier to measure. Determining a galaxy’s age is complex and often depends on the model used, making it less reliable. In contrast, galactic mass can be accurately assessed, as explained in articles like this one on galactic scales.

Contemporary catalogs, such as Pantheon+, account for mass since it correlates closely with age. After around 2010, catalogs began including this mass adjustment, effectively using it as a proxy for age. The authors of the new paper relied on older data without this crucial adjustment, raising some red flags when disputing established theories.

The next issue Reiss identified is the assumption that a galaxy’s age reflects the age of its progenitor stars. The authors use galaxy age as a proxy, assuming progenitor stars formed when the galaxy did. However, nearby supernovae often appear in star-forming regions, suggesting they are relatively young. Research indicates that Type Ia supernovae typically occur less than a billion years after their progenitors form. This weakens the authors’ argument significantly.

As a final note, it’s essential to wait for further peer-reviewed studies that will scrutinize these claims. The scientific community is always evolving, and new insights emerge regularly.

Thanks to Professor Reiss for sharing his valuable feedback.

References:

Son, Junhyuk, et al. “Strong progenitor age bias in supernova cosmology–II. Alignment with DESI BAO and signs of a non-accelerating universe.” *Monthly Notices of the Royal Astronomical Society* 544.1 (2025): 975-987.

Brout, Dillon, and Daniel Scolnic. “It’s dust: solving the mysteries of the intrinsic scatter and host-galaxy dependence of standardized type Ia supernova brightnesses.” *The Astrophysical Journal* 909.1 (2021): 26.

Rose, B. M., et al. “Host Galaxy Mass Combined with Local Stellar Age Improve Type Ia Supernovae Distances.” *The Astrophysical Journal* 909.1 (2021): 28.

Mannucci, F., et al. “The supernova rate in local galaxy clusters.” *Monthly Notices of the Royal Astronomical Society* 383.3 (2008): 1121-1130.

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