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Dark energy vs "Timescape" cosmology

Mike Helland

Philosopher
Joined
Nov 29, 2020
Messages
5,184

Both the Hubble tension and the surprises revealed by DESI are difficult to resolve in models which use a simplified 100-year-old cosmic expansion law—Friedmann's equation.

This assumes that, on average, the universe expands uniformly—as if all cosmic structures could be put through a blender to make a featureless soup, with no complicating structure. However, the present universe actually contains a complex cosmic web of galaxy clusters in sheets and filaments that surround and thread vast empty voids.
When the proposed timescape model was last tested in 2017, the analysis suggested it was only a slightly better fit than the ΛCDM as an explanation for cosmic expansion, so the Christchurch team worked closely with the Pantheon+ collaboration team who had painstakingly produced a catalog of 1,535 distinct supernovae.

They say the new data now provides "very strong evidence" for timescape. It may also point to a compelling resolution of the Hubble tension and other anomalies related to the expansion of the universe.
 
The statement below is critical. If it is wrong then so is the analysis.

The model suggests that a clock in the Milky Way would be about 35 percent slower than the same one at an average position in large cosmic voids, meaning billions more years would have passed in voids.
 
Haven't yet read the paper, but I hope it isn't along the lines of Sarkar's nonsense that only looks at SNe 1a time dilation data. One would hope it would also attempt to deal with the integrated Sachs-Wolfe effect observed in the CMB, as well as the baryon acoustic oscillation data.
 
Okay, I have now heard a bit more about this, and my initial fears were pretty much confirmed. See, for instance;


To quote the last paragraph from the article;

I expect timescape cosmology to remain an area of interest for a few select researchers, but not to gain a broader following based on this research. It’s exciting that a cosmological test has been concocted, but the truth is that dark energy’s existence is now based on a wide, robust suite of evidence that’s so comprehensive that even if we ignored all of the type Ia supernova data entirely, we would still be compelled to conclude that dark energy exists. It’s important to keep your mind open to new ideas, but to always let reality itself rein you back in. Like many new ideas, the timescape cosmology simply withers when faced with the full suite of cosmological evidence.
 
the truth is that dark energy’s existence is now based on a wide, robust suite of evidence that’s so comprehensive that even if we ignored all of the type Ia supernova data entirely, we would still be compelled to conclude that dark energy exists.

Yeah, right.

Anyways.

The guts of this timescape idea seem to rely on something called a backreaction. Ethan mentions this:

and three recent papers arguing the same concept: that dark energy isn’t real, and only appears as an apparent effect due to the backreaction of cosmic inhomogeneities.

But doesn't really give any detail on what that is. I take it the easiest way to "debunk" this cosmology (and defend LCDM's honor) is to just claim backreaction isn't real.
 
Yeah, right.

Anyways.

The guts of this timescape idea seem to rely on something called a backreaction. Ethan mentions this:



But doesn't really give any detail on what that is. I take it the easiest way to "debunk" this cosmology (and defend LCDM's honor) is to just claim backreaction isn't real.
Nope, the best way for the new 'model' to make any traction is for it to explain not only the SNe 1a data, but also the BAO and ISW effect observations. It doesn't. Therefore it is not a player at the moment. Eh?
 
In the interest of saving you a click: The linked page tells us absolutely nothing about the topic here, or what "Steve and Bob" have to say about it. Indeed, the linked page doesn't even consider the topic.
The linked page contains a podcast. You can click the big Play button ( ▶️ ) in order to listen to it. I assume you know what a podcast is.

The relevant segment starts at slightly before the 53 minute mark if you'd like to skip straight to it.
 
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Nope, the best way for the new 'model' to make any traction is for it to explain not only the SNe 1a data, but also the BAO and ISW effect observations. It doesn't. Therefore it is not a player at the moment. Eh?

LCDM can't explain the supernovae data along with the CMB data and the BAO data. Only if you let it have different parameters at different times.

LCDM
1998 - 2024

So it has to be replaced by something. The wCDM seems like the most natural successor. It simply says that the cosmological constant (the L in LCDM) isn't constant. Which, mathematically, basically works out to, there's still a cosmological constant. It just has an exponent with some variable(s) in it where it appears.


So maybe it's wCDM. You still get that homogeneous universe with that model.

Or maybe the inhomogeneities are something that need to be there. I'm not saying the timescape model is a better alternative. But the mainstream seems to be considering more than one thing.

Here's an article from April:

World’s top cosmologists convene to question conventional view of the universe​

Meeting at London’s Royal Society will scrutinise basic model first formulated in 1922 that universe is a vast, even expanse with no notable features

So, it's more than a couple rouge researchers wondering what the next evolution of our notion of the universe will look like.
 
LCDM can't explain the supernovae data along with the CMB data and the BAO data
Errrm, yes it can. The proposition is that the universe is expanding at an accelerated rate. The supernovae data were the first, back in the late 90s, to show us that this was happening. Previously, some rather smart scientists had predicted something that came to be called the integrated Sachs-Wolfe effect, that may be discernible with fututre observations (this was in the 60s-70s) if the universe were expanding in such a way. Nobody really thought it was at the time. It was, it turns out. Combining WMAP, Planck and numerous other smaller scale surveys, it was detected to ~ 4 sigma. And then there is the BAO data.
Nobody is saying it perfectly explains everything, and there could be things that were different in the past. However, it explains it better than any other model at the moment. The hype over the Timescape thing was just that - hype. It is a bit like the guy who suggested the universe was twice as old as current estimates. Lots of headlines, but not particularly solid when you get down to the nitty gritty.
 
I don’t think it is a question of finding flaws in LCDM but rather in finding another theory that does not have flaws right from the beginning.
 
And you can say "4 sigma" or whatever, but the ISW evidence is very underwhelming when you actually look at it.
But not according to the scientists who have published the results. Nor, as far as I know, according to anyone who has replied to those papers.
 
But not according to the scientists who have published the results. Nor, as far as I know, according to anyone who has replied to those papers.
We've seen it before, the BICEP2, they calculated a sigma 5 and popped some champagne. Turned out to be dust.
 
I don’t think it is a question of finding flaws in LCDM but rather in finding another theory that does not have flaws right from the beginning.
Sure, but the flaw as far as Ethan and jonesdave seem to see it, is that dark energy is real, therefore timescape cosmology is wrong.

(I'm not advocating timescape cosmology in any way. Just pointing out the logic being applied here.)
 
Sure, but the flaw as far as Ethan and jonesdave seem to see it, is that dark energy is real, therefore timescape cosmology is wrong.
I’m sure jonesdave116 can answer for himself, but it really is not what he said. In a recent post he said “Nobody is saying it perfectly explains everything, and there could be things that were different in the past. However, it explains it better than any other model at the moment.” So the argument is not that because timescape does’t have dark energy, it is wrong.
Alternative theories simply have a difficult job to present something that fit as many data as LCDM.
 
We've seen it before, the BICEP2, they calculated a sigma 5 and popped some champagne. Turned out to be dust.
And that was jumped on rather quickly by others to show it was wrong. That is not the case with the ISW combined observations going back some years from various instruments.
 
And that was jumped on rather quickly by others to show it was wrong. That is not the case with the ISW combined observations going back some years from various instruments.
One of the issues with the ISW is called the "ISW void anomaly", eg:


This seems like something timescape cosmology might have something to say about, because it's based on how much of the universe is stuff and how much is voids. It's time running faster in the voids that gives the illusion of dark energy effects.

If timescape cosmology not only resolves the Hubble tension and S8 tension, but explains the ISW void anomaly too, it might just pull away as the leading cosmology. Big "if" of course.

It seems a little odd me though how the ISW effect is supposed to arise in LCDM.

Suppose a photon enters a gravitational well today, and that Hubble's parameter is 70 km/s/Mpc.

What will Hubble's parameter be when the photon leaves that gravitational well?

It won't be more than 70 km/s/Mpc. It'll be less.

The "acceleration" of the expansion of the universe doesn't mean it's expanding faster. It means the rate at which it is slowing down is decreasing. Instead of going to 0 and then negative (indicating a collapse) it's going to be asymptotic around 50 km/s/Mpc.
 
One of the issues with the ISW is called the "ISW void anomaly", eg:

That is only a claim, one that has failed to be replicated. Other papers have looked at voids, and seen no excess. And the most robust analyses, simply cross correlating all galaxies with the CMB, shows no significant excess at all in many studies and datasets.


Note that the existence of the ISW effect at all, was a prediction of Lambda. There is no ISW effect without acceleration.

And you can say "4 sigma" or whatever, but the ISW evidence is very underwhelming when you actually look at it.

That's not the ISW effect at all. Those are SZ effect imprints.
 
Note that the existence of the ISW effect at all, was a prediction of Lambda. There is no ISW effect without acceleration.

So... why would that be?

It seems like the ISW effect is a result of the expansion rate being different between light entering the well and leaving it.

If we used FLRW with OmegaM = 1 and OmegaLambda = 0 (presumably what we were using prior to 1998?) then H at the time when light leaves the well will be lower than H at the time light enters the well.

In the concordance model where OmegaM = 0.3 and OmegaLambda = 0.7, the same thing happens. H will still be lower when the light leaves, just not as much.

It wouldn't actually be higher at the time of exit unless we were in an open universe.

Right?

That's not the ISW effect at all. Those are SZ effect imprints.

That's correct. The CMB data is publicly available, and with a little Python, not that hard to get at.
 
It seems like the ISW effect is a result of the expansion rate being different between light entering the well and leaving it.
It is not. In homogeneous cosmologies, like LCDM, the over- and underdensities are all still expanding, at the same rate. The ISW effect is due to the decay of potentials over time.

 
And that was jumped on rather quickly by others to show it was wrong. That is not the case with the ISW combined observations going back some years from various instruments.

Unveiling the largest structures in the nearby Universe: Discovery of the Quipu superstructure​

We show that the superstructures should produce a modification on the cosmic microwave background through the integrated Sachs-Wolf effect. Searching for this effect in the Planck data we found a signal of the expected strength, however, with low significance.
 
I don’t think it is a question of finding flaws in LCDM but rather in finding another theory that does not have flaws right from the beginning.

Hello steenkh! Say G'day to jd116 for me.

:lolsign:


Problems with mainstream models? Nah...

Entertaining read... Pops back into the PLASMA Universe...
 
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