Hold on to your hats, space enthusiasts! A recent study has stirred up the cosmic pot, claiming the universe isn't expanding at an accelerating rate, but rather, is slowing down. This directly challenges the current understanding of our universe, and it's causing quite a stir. This is Part 4 of a series, and you can find the earlier parts to catch up on the details. But what does a Nobel laureate have to say about this? Let's dive in.
I recently received an email from Adam Reiss, the 2011 Nobel Prize winner in Physics, who, along with Saul Perlmutter and Brian Schmidt, discovered the accelerating expansion of the cosmos. He was kind enough to share his thoughts on the matter, and with his permission, I'm passing them along to you.
So, what's the buzz about this new study? The authors analyzed observations of roughly 300 supernovae, specifically Type-Ia supernovae, and found a correlation between their peak brightness and the age of their host galaxies. Essentially, the younger the galaxy, the dimmer the supernova. Their conclusion? Our measurements of galactic distances are off, leading them to suggest the universe is decelerating. This also means the standard ΛCDM model might be incorrect.
However, Reiss points out some key issues.
First, there's the matter of galactic ages. The original paper argues that SN-Ia light curves don't account for the age of their host galaxies. While partly true, they do account for galactic mass. Determining a galaxy's age is tricky and model-dependent, which can lead to tweaked results. Galactic mass, on the other hand, is much easier to measure.
Studies have shown the mass of a supernova's host galaxy is a crucial factor. This is why modern catalogs, like Pantheon+, adjust for mass. They don't worry about galactic age because a galaxy's age and mass are closely linked. Once you account for mass, adjusting for age doesn't add much value.
Here's where it gets controversial... Since around 2010, Type-Ia supernova catalogs have included mass adjustments, which also act as an age proxy. The authors of this new paper chose to focus on age directly and used older databases that didn't include the mass adjustment. This raises a red flag. If you're trying to challenge existing theories, using outdated data might not be the best approach.
And this is the part most people miss... This leads to the second issue: the connection between galaxy age and progenitor age. The authors focused on the measured age of host galaxies because it's something they can measure. They didn't focus on the age of a supernova's progenitor star because it's difficult to measure. The team used galaxy age as a proxy for progenitor age, assuming the progenitor formed when the galaxy did. Thus, distant supernovae progenitors are young, while nearby ones are old. But here's the kicker: local supernovae are typically found in young, star-forming regions. Research suggests Type-Ia supernovae occur less than a billion years after their progenitor star forms. So, the very foundation of their argument is shaky, to say the least.
Of course, don't take my word for it. Peer-reviewed papers will soon delve into these issues and more.
What do you think? Do you agree with Reiss's points, or do you see merit in the original study's findings? Share your thoughts in the comments below!
With thanks to Professor Reiss for his kind 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. (2025)" *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. (2021)" *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." (2021) *The Astrophysical Journal 909.1 (2021): 28.*
* Mannucci, F., et al. "The supernova rate in local galaxy clusters. (2008)" *Monthly Notices of the Royal Astronomical Society 383.3 (2008): 1121-1130.*
