Message for You, Sir!

ESO/L. Calçada

So I got an email from Adam Reiss. You know, the guy who was awarded the 2011 Nobel Prize in Physics along with Saul Perlmutter and Brian Schmidt for discovering the rate of cosmic expansion is accelerating. He pointed out a few issues with the decelerating Universe paper, and with his permission I’d like to share them with you.

So let’s start with the central claims of the original paper.¹ Based on observations of about 300 supernovae, the authors found a correlation between the peak brightness of Type-Ia supernovae and the age of its host galaxy. Basically, the younger the galaxy, the dimmer the supernova. As a result, the authors argue, our measure of galactic distances is wrong. Based on their results, the Universe is decelerating, which would also mean the standard ΛCDM is wrong. Although the paper is peer reviewed, Reiss finds a couple of major flaws.

The first is on the issue of galactic ages. The authors emphasize that SN-Ia light curves don’t take the age of their host galaxies into account. That’s somewhat true, but they do take galactic mass into account. Determining the age of a galaxy is difficult to do. It’s also model dependent, so the results can be a bit tweaked. Galactic mass, on the other hand, is much simpler to measure.

Studies have shown that the mass of a supernova’s host galaxy should be considered.² This is why modern catalogs such as Pantheon+ adjust for mass. The reason they don’t worry about galactic age is because the age of a galaxy and its mass correlate pretty strongly.³ Once you adjust for mass, adjusting for age buys you nothing.

Since around 2010, Type-Ia supernova catalogs all include the mass adjustment, which also serves as an age proxy. Since the authors wanted to focus on age directly, they used older databases without the mass adjustment. That’s a bit of a red flag. If you want to disprove the current theory, don’t use old data. But this leads to the second issue, which is the connection between galaxy age and progenitor age.

The authors focus on the measured age of the host galaxies, since that’s something that can be measured. They don’t focus on the age of a supernova’s progenitor star because we don’t have a good way to measure that. In the paper, the team uses galaxy age as a proxy for progenitor age, assuming that the progenitor formed when the galaxy formed. Thus, distant supernovae progenitors are young, while the progenitors of nearby supernovae are old. But local supernovae are typically found in young star-forming regions. In fact, studies suggest that Type-Ia supernovae occur less than a billion years after the formation of their progenitor star.⁴ So that very basis of their argument is shaky at best.

Of course, don’t take my word for it. Just wait for the peer-reviewed papers that will look at all these issues and more. I don’t think we’ll have to wait long.

With thanks to Professor Reiss for his kind feedback.