A Strong Progenitor Age Bias in Supernova Cosmology

[PeterDonis]

TL;DR

A new analysis of Type 1a supernovae to correct for the effects of the age of the progenitor stars suggests that the universe's expansion might not be accelerating

The thread title is the first part of the title of a paper published just today [1]. Here is the abstract:

Supernova (SN) cosmology is based on the key assumption that the luminosity standardization process of Type Ia SNe remains invariant with progenitor age. However, direct and extensive age measurements of SN host galaxies reveal a significant ($5.5 \sigma$⁠⁠) correlation between standardized SN magnitude and progenitor age, which is expected to introduce a serious systematic bias with redshift in SN cosmology. This systematic bias is largely uncorrected by the commonly used mass-step correction, as progenitor age and host galaxy mass evolve very differently with redshift. After correcting for this age bias as a function of redshift, the SN data set aligns more closely with the $w_0 w_acommonly used mass-step correction, as progenitor age and host galaxy mass evolve very differently with redshift. After correcting for this age bias as a function of redshift, the SN data set aligns more closely with the$ cold dark matter (CDM) model recently suggested by the Dark Energy Spectroscopic Instrument (DESI) baryon acoustic oscillations (BAO) project from a combined analysis using only BAO and cosmic microwave background (CMB) data. This result is further supported by an evolution-free test that uses only SNe from young, coeval host galaxies across the full redshift range. When the three cosmological probes (SNe, BAO, and CMB) are combined, we find a significantly stronger (⁠⁠$> 9 \sigma$) tension with the CDM model than that reported in the DESI papers, suggesting a time-varying dark energy equation of state in a currently non-accelerating universe.

[1] https://academic.oup.com/mnras/article/544/1/975/8281988?login=false

 

[Cerenkov]

Hello Peter

I'm interested in the issue of the universe's apparent accelerated expansion and would like to learn more.

Could you please bring the abstract down a level or two so that I can understand what it says more readily.

Thank you,

Cerenkov.

 

[pinball1970]

Hello Peter

I'm interested in the issue of the universe's apparent accelerated expansion and would like to learn more.

Could you please bring the abstract down a level or two so that I can understand what it says more readily.

Thank you,

Cerenkov.

They measured supernovas took into account the star system/galaxy (edit) that produced the supernovas, then cross checked with other data from CMBR and BAO. conclusion is it is slowing down not speeding up.

I only know all this because I have reading about it for the last two days and was going to post a thread on it!

Perlmutter won the Nobel for coming to a different conclusion with a lot of the same data from what I have read. He did not standardize the SN the same way Lee did, "standardized SN magnitude and progenitor age."

Disclosure - I used copilot to compare the papers as a lot of the technical stuff is is too difficult.

The refs it gave were. Oxford Academic - Lees paper above

https://arxiv.org/abs/astro-ph/9812473

https://iopscience.iop.org/article/10.1086/306106/fulltext/37558.text.html

https://www.stsci.edu/~ariess/documents/1998.pdf

I hope this acceptable.

 

[Ibix]

We use Type 1a supernovae to estimate distances because how bright they are is well correlated with various other characteristics like their colour. That makes them excellent standard candles for measuring cosmological distances, and hence for estimating the history of expansion.

However, the expansion history implied by these supernovae is different from that derived from CMB measures. This paper suggests that this is because the model of SN 1a brightness is wrong. They say it actually also depends on galaxy age. I haven't quite got to the bottom of why - they have observational evidence, and I think the theoretical reason is that they think the characteristics of the population of white dwarfs (that generate SN 1a explosions) change with galaxy age.

Of course, there were more young galaxies in the early universe because they hadn't had time to become old galaxies. So changing to their model of SN 1a brightness fixes (so they say) a systematic bias in the estimated distance of supernovae. Then the supernova-based distance estimates align better with the CMB based estimates. Furthermore, that has a knock-on effect on the best fit model of the scale factor as a function of time, and hence our estimate of dark energy density as a function of time. That turns out to be a now-decelerating universe.

 

[Cerenkov]

Thanks very much for your help pinball1970 and Ibix.

So, let me see if I understand this.

Perlmutter et al worked on the premise that type 1a supernovae could be treated as 'standard candles' and on the back of this assumption, in 1998, they found that the expansion of the universe was speeding up. But if the results of the paper PeterDonis cited hold up, then the standardization of type 1a's is called into doubt.

Which means that certain other things are also called into doubt. Such as the presence of a small but positive cosmological constant and the need to invoke Dark Energy as the mechanism for this accelerated expansion.

Going further, Hawking and Penrose made two basic assumptions in this 1970 paper.

https://royalsocietypublishing.org/doi/10.1098/rspa.1970.0021

First, that the gravitational attraction of the mass of the universe would eventually overcome the expansion, reverse it and then lead to a collapse. Second, that the universe had a negative or null cosmological constant.

I was informed, here in this forum, that since their work on the initial cosmological singularity required a negative or null cosmological constant, the 1998 discovery of a positive value for it invalidated their theorem. They had made an assumption about the universe that was later found to be false. Which meant that the conclusion of their paper, that space-time emerged from an initial singularity, couldn't hold up.

But now, with the advent, of this new finding about type 1a supernovae, is it possible that the cosmological constant will be assigned a null or negative value?

Which would seem to put Hawking and Penrose's singularity theorem back into play. Or not?

Ok, these are very early days and I'm probably guilty of jumping the gun and making mistakes in my speculations. But if anyone who understands these things could please comment on and/or correct my thoughts I'd very much appreciate the help. I've been fascinated by the subject of dark energy, the cosmological constant and the accelerating expansion of the universe for many years now and I'm keen to know and understand more.


Thank you,

Cerenkov.

 

[PeterDonis]

Could you please bring the abstract down a level or two so that I can understand what it says more readily.

I marked this thread as "A" level for a reason. You've gotten some good general responses, but be aware that you might not be able to follow the details of what's in this paper if you don't have the background for an "A" level thread.

 

[PeterDonis]

I was informed, here in this forum

Where? Please give a link to the specific thread.

 

[PeterDonis]

Which would seem to put Hawking and Penrose's singularity theorem back into play. Or not?

Not. Even if it turns out that there is no dark energy and the expansion of the universe is not accelerating, that doesn't mean the universe will recollapse.

You are leaving out one crucial premise of the Hawking-Penrose singularity theorems. In the context of whether the universe will recollapse or not, that premise works out to this: that the universe must be contracting. The theorem tells you that, if the universe is contracting at some instant of time, and the other premises are met (one of which is that there is a zero or negative cosmological constant), then there will be a "Big Crunch" singularity at some point in the universe's future.

But the universe is not contracting now; it's expanding. Even if the expansion is not accelerating, it's still expanding. And the Hawking-Penrose theorem tells you nothing at all about whether it will cease expanding at some point in the future and start contracting.

 

[Cerenkov]

Where? Please give a link to the specific thread.

B Post in thread 'The singularities of gravitational collapse and cosmology'

Mar 19, 2019

Hello again Peter.

Thank you for your patience.

But please understand that I do not properly understand the language you are using. Not modern English, but the language of mathematics. If you wanted a parallel example to illustrate what I mean, please consider the following. As a teenager I read books on astronomy and taught myself to read the Greek symbols used in designating the magnitude of the stars - Alpha, Beta, Gamma, Delta, etc. Nowadays I can look at a Greek Interlinear New Testament, which is written in Koine (1st century Greek symbols) and see words like Pneuma and...

• Cerenkov

It seems that I didn't recall things correctly Peter. We did discuss the energy conditions of that paper by Hawking and Penrose. But the key point is found in the abstract.

The theorem applies if the following four physical assumptions are made: (i) Einstein’s equations hold (with zero or negative cosmological constant),

What seems to have happened is that on my own I've concluded that because the universe appears to have a small, but positive cosmological constant, Einstein's equations do not hold. Because this paper was based upon the assumption of a zero or negative cosmological constant it didn't agree with our observations of the universe. So, the paper and its proof were invalid.

I've probably drawn the wrong conclusion... and now await for you to tell if that's so.

Thank you,

Cerenkov.

 

[Cerenkov]

Not. Even if it turns out that there is no dark energy and the expansion of the universe is not accelerating, that doesn't mean the universe will recollapse.

You are leaving out one crucial premise of the Hawking-Penrose singularity theorems. In the context of whether the universe will recollapse or not, that premise works out to this: that the universe must be contracting. The theorem tells you that, if the universe is contracting at some instant of time, and the other premises are met (one of which is that there is a zero or negative cosmological constant), then there will be a "Big Crunch" singularity at some point in the universe's future.

But the universe is not contracting now; it's expanding. Even if the expansion is not accelerating, it's still expanding. And the Hawking-Penrose theorem tells you nothing at all about whether it will cease expanding at some point in the future and start contracting.

Ok, thanks for clarifying and explaining Peter.

Now, just off the top of my head I seem to recall that there's a theorized scenario that's neither accelerated expansion or collapse. Where the universe expands at a slower and slower rate but never actually comes to a halt. Is that right?

If so, does this new finding line up with that scenario?


Thank you,

Cerenkov.

 

[PeterDonis]

because the universe appears to have a small, but positive cosmological constant, Einstein's equations do not hold

Of course that's wrong. The Einstein Field Equation works just fine with a positive, zero, or negative cosmological constant. All models of our universe in cosmology use the Einstein Field Equation.

Because this paper was based upon the assumption of a zero or negative cosmological constant it didn't agree with our observations of the universe.

At the time it was written, this part did; everyone thought the cosmological constant was zero.

In the late 1990s, when the (apparent) discovery of accelerated expansion was made, then the assumption of a zero or negative cosmological constant was (believed to be) no longer in agreement with observation. However:

So, the paper and its proof were invalid.

Wrong. The paper proves a mathematical theorem based on certain premises. That proof was, is, and remains valid. Mathematical theorems don't become invalid just because their premises don't necessarily hold in our actual universe.

 

[PeterDonis]

I seem to recall that there's a theorized scenario that's neither accelerated expansion or collapse. Where the universe expands at a slower and slower rate but never actually comes to a halt. Is that right?

Yes, there are theoretical models that do that. If there is no dark energy (zero cosmological constant), then the only significant component of stress-energy in our universe now and in the future would be ordinary matter, and in any model which is dominated by ordinary matter and has a density less than or equal to the critical density, the expansion will decelerate forever but never quite come to a halt.

If so, does this new finding line up with that scenario?

If this new finding turns out to be confirmed, yes, it would put scenarios like the ones I described above back into play.

 

[Cerenkov]

Of course that's wrong. The Einstein Field Equation works just fine with a positive, zero, or negative cosmological constant. All models of our universe in cosmology use the Einstein Field Equation.


At the time it was written, this part did; everyone thought the cosmological constant was zero.

In the late 1990s, when the (apparent) discovery of accelerated expansion was made, then the assumption of a zero or negative cosmological constant was (believed to be) no longer in agreement with observation. However:


Wrong. The paper proves a mathematical theorem based on certain premises. That proof was, is, and remains valid. Mathematical theorems don't become invalid just because their premises don't necessarily hold in our actual universe.

So I've tripped up on my use of terms.

What I should have said was that the proof is mathematically valid, but, in accordance with what Hawking and Penrose wrote, it does not apply to OUR universe. Because the measured value of our universe's cosmological constant appear to be positive and the theorem only applies if it is measured to be negative or zero.

Is that more accurate?

 

[PeterDonis]

What I should have said was that the proof is mathematically valid, but, in accordance with what Hawking and Penrose wrote, it does not apply to OUR universe. Because the measured value of our universe's cosmological constant appear to be positive and the theorem only applies if it is measured to be negative or zero.

Is that more accurate?

It is, provided that our universe's cosmological constant actually is positive.

 

[Cerenkov]

It is, provided that our universe's cosmological constant actually is positive.

Which may not be the case if the new analysis of type 1a supernovae holds up?

Would you please explain what it mean if the Perlmutter's 1998 finding were overturned by this new finding?

What would the implications be for the cosmological constant?

Would it's value need to be revised?


Thank you,

Cerenkov.

 

[PeterDonis]

Which may not be the case if the new analysis of type 1a supernovae holds up?

Would you please explain what it mean if the Perlmutter's 1998 finding were overturned by this new finding?

What would the implications be for the cosmological constant?

Would it's value need to be revised?

Yes, one possible result if this new analysis holds up is that cosmologists would revise their estimate of the value of the cosmological constant--probably to zero (since a negative cosmological constant has even more theoretical issues than a positive one, and I don't think any of the evidence looked at by this analysis, if it holds up, points to a negative cosmological constant, just to it not being positive). That would mean revising our best current model of the universe and its expansion history.

 

[Cerenkov]

https://science.nasa.gov/dark-energy/

If memory serves Peter, to account for the speeding up of cosmic expansion scientists invoked Dark Energy. But if it is found that Perlmutter et all made the wrong assumptions about the standard candles they were using and the universe isn't accelerating in it's expansion, what then? Is there any need to invoke the place holder of Dark Energy?

Or are there other lines of evidence for the existence of this mysterious stuff/energy/thingy/whatever?

Does this new finding change the Hubble Tension in any way?

Finally, what about the DESI results from earlier this year? As I understand it they seemed to indicate a slowing of cosmic expansion. How does this sit with these new findings about the type 1a's?


Thank you,

Cerenkov.

 

[PeterDonis]

If memory serves Peter, to account for the speeding up of cosmic expansion scientists invoked Dark Energy.

Basically, yes.

if it is found that Perlmutter et all made the wrong assumptions about the standard candles they were using and the universe isn't accelerating in it's expansion, what then? Is there any need to invoke the place holder of Dark Energy?

Not from cosmology, no.

Does this new finding change the Hubble Tension in any way?

If I understand the paper correctly, revising our model of the universe in the way the paper is saying would at the very least greatly reduce the Hubble Tension. Whether it would completely resolve it, I don't know.

what about the DESI results from earlier this year? As I understand it they seemed to indicate a slowing of cosmic expansion. How does this sit with these new findings about the type 1a's?

I haven't looked into this so I can't say. Maybe another poster in this thread will have more information.

 

[Ibix]

Finally, what about the DESI results from earlier this year? As I understand it they seemed to indicate a slowing of cosmic expansion. How does this sit with these new findings about the type 1a's?

The paper cited in the OP makes heavy use of the DESI 2025 release. Statistical analysis is in section 4.

 

[sbrothy]

Rather than go through the thread and like all @Cerenkov ’s questions I’d just thank him/her for having the courage to ask all the questions I also had. You’re obviously way better informed on the subject than me so in addition to bringing the subject a little down from it’s A level I avoided making a fool of myself (which I normally excel at ).

 

[Cerenkov]

Why thank you, sbothy.


But how can I be better informed than you if I've asked all the questions you had? We must be on a par to have thought of the same questions.


Cerenkov (who is a he, btw)

 

[sbrothy]

Yes, Cerenkov-radiation at its most beautiful and up close deadly. Let's not make this into a humbleness competition. Your questions was more informed than mine would have been. Learn to take a compliment!

 

[Cerenkov]

I humbly accept your compliment.