You come for the king, you best not miss.
In cosmology, the King is the standard model, also known as the ΛCDM. The CDM stands for Cold Dark Matter, which comprises the bulk of matter in the Universe. The Λ, or Lambda, is the symbol used for the cosmological constant. The Hubble parameter that drives the ever-increasing expansion of spacetime. The cosmological constant was first proposed by Einstein, and it has been a central tenet of the standard model for nearly a century. And with good reason. Nobel Prizes have been awarded for research proving its validity. The cosmological constant, though weird, is very real.
But a new paper in *Monthly Notices of the Royal Astronomical Society* says it is not.[^1] That Nobel-winning study that proved cosmic expansion is accelerating? Wrong. The concept of dark energy and cosmic expansion as an inherent structure of space and time? Wrong. The cosmic distance ladder we use to measure galactic distances? Also wrong.
Wait, what?
To be fair, this result doesn't come entirely out of the blue. As far back as 2015, there was tentative evidence that our distance measures of supernovae might be biased. And difficulties with the standard model, such as the Hubble tension, have led astronomers to at least consider alternatives. But this new study isn't tentative, so it's worth exploring in detail.
*The unique brightness curve of a Type Ia supernova. Credit: Wikipedia*
The study focuses on Type Ia supernovae. Triggered by binary white dwarfs, these particular supernovae can be identified by the presence of silicon in their spectral lines. Their brightness curves, which describe how they brighten and fade over time, are dominated by the decay of Nikel-56 to Cobalt-56 to Iron-56. Since the rate of radioactive decay is always the same for a particular element, the light curves of Type-Ia supernovae can be used as a standard candle. No matter where we observe them, we can compare the observed brightness to the actual brightness to find their distance.
*A correlation between galactic age and supernova brightness. Credit: Son, et al*
We've long known that the standard candle method isn't exact. Even with its roots in radioactive decay, there are statistical variances in the ratio of peak brightness to light curve half-width. But this new study finds that there is a strong correlation between the maximum brightness of a Type-Ia supernova and the age of its host galaxy. The younger the galaxy, the fainter the supernova is likely to be. The strange part about this is that there's no clear reason why this would be. Observationally, however, the result is straightforward.
We can determine the age of a galaxy by observing the overall spectrum of galactic light. Bright blue stars die young, and small red dwarfs remain. New stars that form have a higher amount of heavier elements. So the presence or absence of certain spectral lines in a galaxy gives us a good handle on its age. This works for both distant and nearby galaxies. When you plot the peak brightness of Type-Ia supernovae against the age of their host galaxies, the correlation is clear. Studies by other teams indicate that the correlation is about 5σ, which is quite strong.
*This new result disagrees with ΛCDM but agrees with BAO and Planck. Credit: Son, et al*
With this correlation in mind, the team then went back to the studies of cosmic acceleration. Rather than treating variations in SN brightness as a statistical fluctuation, they introduced the correlation into the mix. The early Universe had many more young galaxies than the middle-aged Universe, and that bias increases the more distant the galaxy is. When they put it all together, the result was striking. The ΛCDM model is contradicted with a certainty of over 9σ! Based on the results, cosmic expansion isn't accelerating. The Universe continues to expand, but the rate of expansion is slowing down. It's been decelerating for about a billion years.
If these results are confirmed, then it means that cosmic expansion can't be entirely due to the structure of space and time. In general relativity, the Hubble parameter is an absolute universal constant. It can't vary in time and space, and it can't cause expansion to decelerate. Einstein was a little bit wrong after all.
But there is a bright side. This result could also solve the biggest mystery in modern cosmology. I'll talk about that next time.
Reference: 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.
