The Universe's Expansion May Be Slowing Down, Not Speeding Up ?

Pinball1970 said:
I try not post YouTube videos, here is a quote on PF from Sabine Hossenfelder (the Hoss)

"Basically, they say that it's wrong to conclude that the expansion of the universe is accelerating. It's just that younger galaxies have on average dimmer supernovae. They don't say why this might be the case. They simply say this is what the data really say. And this isn't one of those maybe results with low statistical significance. This is a whopping 5.5 sigma correlation which they call a serious systematic bias."

One interesting comment was that this factor is not new to the cosmological community, just that it is not as significant as this paper suggests.
We can expect more on this since so much is at stake.
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Yes, interesting that doubt about the validity of these standard candles seems to have been floating around for a while. Do you know anything about this baryon acoustic oscillation, or whatever it is, that they mention as corroborating, at least directionally, their findings?
 
geordief said:
if the acceleration is lessening,what happens when (if?) it becomes less than zero?

Does expansion gradually stop and if it does, does it turn into contraction or might it find an equilibrium which could look like the Steady State even if not the Steady State that Hoyle assumed?(well I am not really familiar with what the Steady State was actually meant to be)
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It's worth remember what the position was in cosmology before we found data that seemed to show that the expansion of the universe is accelerating.

Prior to that, we had relativistic models with cosmological constant set to zero. With that setting, the ultimate fate of the universe is determined entirely by the density of matter in the universe. More than a critical density means the expansion will slow, stop then reverse, and we get a Big Crunch. Less than the critical density means the expansion continues forever and we get an eventual heat death of the universe. With exactly the critical density, we also get an expansion that continues forever, but at an ever-decreasing rate.

Interestingly, it appears that the average density of the universe is right at the critical value, or at least so close that we can't tell whether it is slightly more or slightly less. That seems unlikely to be a coincidence, but I don't think it has been explained either.

The density has to include dark matter, of course. Even if we abolish dark energy, dark matter will still be a problem that needs a solution.
 
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James R said:
It's worth remember what the position was in cosmology before we found data that seemed to show that the expansion of the universe is accelerating.

Prior to that, we relativistic models with cosmological constant set to zero. With that setting, the ultimate fate of the universe is determined entirely by the density of matter in the universe. More than a critical density means the expansion will slow, stop then reverse, and we get a Big Crunch. Less than the critical density means the expansion continues forever and we get an eventual heat death of the universe. With exactly the critical density, we also get an expansion that continues forever, but at an ever-decreasing rate.

Interestingly, it appears that the average density of the universe is right at the critical value, or at least so close that we can't tell whether it is slightly more or slightly less. That seems unlikely to be a coincidence, but I don't think it has been explained either.

The density has to include dark matter, of course. Even if we abolish dark energy, dark matter will still be a problem that needs a solution.
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How can we know the average density of the universe if we only have the observable unierse to look at?

(I understand that it may be that the ratio of unobservable to observable could be anything -even some exponentially huge number)

Maybe the CMWB has information that applies to the unobservable universe and allows us to calculate its average density?
 
exchemist said:
Yes, interesting that doubt about the validity of these standard candles seems to have been floating around for a while. Do you know anything about this baryon acoustic oscillation, or whatever it is, that they mention as corroborating, at least directionally, their findings?
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Yes I have read up not or at least tried to. Simply put they are the sound waves of the plasma of the very early universe. These became "fixed" when the plasma cooled enough to hold onto the electrons.
 
exchemist said:
Yes, interesting that doubt about the validity of these standard candles seems to have been floating around for a while. Do you know anything about this baryon acoustic oscillation, or whatever it is, that they mention as corroborating, at least directionally, their findings?
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This is closely linked to the CMBR because that was the point when the oscillations were frozen. The separation of these sound waves are being measured in surveys like DESI.
These frozen sound waves are seen in the distribution of galaxies. They have worked out that the sound waves could have moved 150 Mpc before decoupling when the CMBR was produced.

Edit: the amount of separation "equates to" 150Mpc.
 
Thanks to C C , we now have, if not exactly a rebuttal then some reasons to doubt this finding. https://www.universetoday.com/artic...ng-heres-what-a-nobel-laureate-has-to-say-abo

I’ve copied this reference from her evergreen thread on “Compromised Science”.

Two objections. One is that while the supernova standard candle light curves don’t correct for galactic age they correct for galactic mass, which apparently correlates with age. The authors of the paper however went back to earlier versions without this correction, so their criticism of the light curves currently used is rather unfair. The other is that the authors assume young galaxies have supernovae produced by young progenitors. This, according to the critic, himself a Nobel Prize winner, is a false assumption.

So the empire strikes back already. It will be interesting to see how this plays out.
 
I remember us all rassling in November with galactic (host) mass, metallicity, 1n SN progenitor observation, dust densities, etc. Yeah this sounds like more "game on" in astrofiz. This bit...

Thirdly, we demonstrate that the claimed 5 Gyr progenitor age difference between nearby and distant supernovae is overstated by factors of three to five largely due to a conflation of host galaxy age with supernova progenitor age. We conclude that type Ia supernova cosmology remains robust for current measurements of dark energy.

...tells me there's a brawl ahead. In my naivete I did not think it was conflation.
 
I put it on PF Also, see what those guys say. Early doors suggest last year's paper may have not factored some things in. I will keep you posted.
 
Given the rate of current expansion, how accurate do our measurements and calculations need to be before we can truly tell? How long do we need to leave measurement intervals? Are we accurate enough that the gap of a year will be sufficient? Or do we need to wait 1,000 years to be sure?
 
Peeples said:
Given the rate of current expansion, how accurate do our measurements and calculations need to be before we can truly tell? How long do we need to leave measurement intervals? Are we accurate enough that the gap of a year will be sufficient? Or do we need to wait 1,000 years to be sure?
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It is not a matter of waiting, different teams are making different claims, some using the same data set.
Something is wrong.
Assumptions about the standard candles, SN, Cepheid variables.
The galaxies they are in.
The stars the associated with
Dust.
was the acceleration uniform in the past, current thinking says things started to speed up about 9gya.

Then there is the CMBR
Lambda CDM
 
Peeples said:
Given the rate of current expansion, how accurate do our measurements and calculations need to be before we can truly tell? How long do we need to leave measurement intervals? Are we accurate enough that the gap of a year will be sufficient? Or do we need to wait 1,000 years to be sure?
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Just to add, I'm not qualified, i am just an interested person following the discussion on the publications as they are released.
 
(google)
The universe's expansion is speeding up. While gravity originally slowed the cosmic expansion following the Big Bang, scientists discovered in 1998 that the expansion rate began to accelerate roughly 5 billion years ago. This ongoing acceleration is widely attributed to a mysterious force called dark energy.

Current Scientific Consensus
While a brief 2025 study challenged this model—suggesting supernova data might indicate a slowing expansion rate due to aging stars—major reviews published by institutions like the Royal Astronomical Society have overwhelmingly reaffirmed that the universe is accelerating.

The paper:
Published:

10 June 2026


Still accelerating: type Ia supernova cosmology is robust to host galaxy age evolution:

ABSTRACT​

Type Ia supernovae are a cornerstone of modern cosmology, providing first evidence for cosmic acceleration and new tests of dark energy. Son et al. (S25) claim a strong redshift evolution in standardized supernova luminosities driven by supernova progenitor age, with dramatic cosmological implications: rapidly evolving dark energy, decelerating expansion, and a tension with CDM. We show that the underpinning evidence required for this conclusion – the supernova progenitor-age dependence, the redshift-dependent age difference, and their combined impact – is either negligible or relies on effects already corrected for in modern supernova analyses. First, the S25 analysis omits the standard host-galaxy stellar mass correction that captures known environmental dependencies that also correlate with stellar age. Applying this correction to the S25 sample, we find no dependence of standardized supernova brightness on host age. Independent data also show no significant difference at low-redshift in standardized brightness between star-forming galaxies and several 5Gyr older quiescent galaxies of the same stellar mass. Secondly, the S25 scenario predicts strong redshift evolution of the host-mass effect. Data from the Dark Energy Survey supernova survey measure evolution of ⁠, consistent with zero and altering the dark-energy equation-of-state measurement (w) by <0.01 if included. Thirdly, we demonstrate that the claimed 5Gyr progenitor age difference between nearby and distant supernovae is overstated by factors of three to five largely due to a conflation of host galaxy age with supernova progenitor age. We conclude that type Ia supernova cosmology remains robust for current measurements of dark energy.

full paper at...​

 
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4 CONCLUSIONS​

We have tested whether the correlation between SN Ia Hubble residuals and host galaxy ages claimed by S25 can induce an apparent redshift evolution in SN Ia standardization that biases cosmological inferences. While Y.-W. Lee et al. (2022) and S25 suggested that such an effect could remove the need for dark energy or otherwise significantly modify cosmological inferences, our analysis shows that these effects are both already accounted for, and exaggerated. We find:

  • Applying mass-standardization and bias-corrections in line with state-of-the-art cosmological analyses reduces the strength of the Hubble-residual–age relationship and renders it insignificant;
  • An effect driven by galaxy age predicts a redshift evolution in the size of the mass step; we show that measurements of the evolving mass step are inconsistent with the S25 prediction at ⁠;
  • We identify a number of inaccurate assumptions present in the S25 analysis, namely the direct application of a galaxy-age measurement to an SN-age redshift evolution, and an overestimation of the redshift evolution of SN ages due to assumptions about the SN Ia delay-time distribution;
  • We reiterate the need for careful modelling of galaxy star-formation histories and survey selection effects when predicting the evolution of SN Ia luminosities and host galaxy parameters.
SN Ia cosmology is a mature field in which great care is taken to address complex and often hidden systematic effects. Testing for the robustness of the methods is a critical undertaking, but one that requires equal measures of care. We look forward to the future of SN Ia cosmology as we enter the era of extreme data sets from the Zwicky Transient Facility (E. C. Bellm et al. 2019; M. Rigault et al. 2025), the Vera C. Rubin Observatory (Z. Ivezic et al. 2019; M. Lochner et al. 2022; P. Gris et al. 2023), the TiDES survey on the 4-m Multi-Object Spectroscopic Telescope (C. Frohmaier et al. 2025), and the Nancy Grace Roman Space Telescope (R. Kessler et al. 2025), where such care will be of more importance than ever before.
 
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