Marginal evidence for cosmic acceleration from Type Ia SNe

[JuanCasado]

You'd better avoid adjectives such as dreck to disqualify, reject or dismiss the peer reviewed work of other authors.
The paper does not use Newtonian arguments to suggest that matter should not impact the expansion rate, because matter really does.

 

[Garth]

The CMB is the earliest data point from radiation (and has the lowest systematic uncertainties). Nucleosynthesis, though it does have more systematic uncertainties, is from an even earlier time. I think you've just made my point for me.

The regime changes from normal to linear expansion as matter becomes dominant.

The BBN epoch was well down in the radiation dominated epoch so $R(t) \propto t^\frac{1}{2}$, thus leaving BBN as in the standard model.

Indeed ages were different at the surface of last scattering, which was in the matter dominated epoch, therefore in the 'Milne/Coasting/SCC/Steady-Flow' model fluctuation cell sizes are consequently larger than in the $\Lambda$CDM model.

However the hyperbolic space of the Milne model increases angular distance and that compensates for this increased cell size.

Oh, wow. That paper is complete dreck.

JuanCasado has already commented on that - I would add that as an author of this refereed paper is actively participating in this thread such a comment is against the PF civility guidelines.

He claims to say that Newtonian arguments suggest that matter should not impact the expansion rate. That's just wildly incorrect.

No! The key word here is "suggest" - there are different pointers towards a Milne model, I began my research on the topic for my MSc project (almost 40 years ago (!)) using the old LNH - and so I was interested to read that this paper used a modern version of that hypothesis, but it does so only to suggest why such a model might be considered. The linear expansion is actually delivered by DE with an EoS of $p = -\frac{1}{3}\rho$, as I know you realized.

It might be possible to come up with some elaborate model where you have a dark energy-like fluid which interacts with matter in such a way to produce a combined $w=-1/3$. But that would be extremely elaborate. And it's not likely to work for the early universe anyway.

In my (published) work on Self Creation Cosmology there is a non-minimally connected Brans-Dicke-type scalar field coupled to the trace of $T_{\mu \nu}$ that does just that. But I won't go on any further here because to do so would violate PF policy as there is as yet unpublished research.

To get us back to the OP paper I would just add that this discussion arises from its comment "that the data are still quite consistent with a constant rate of expansion", perhaps people will want to discuss other aspects of it...

Garth

 

[Chalnoth]

You'd better avoid adjectives such as dreck to disqualify, reject or dismiss the peer reviewed work of other authors.
The paper does not use Newtonian arguments to suggest that matter should not impact the expansion rate, because matter really does.

It really is that bad. Here is the paper where he actually makes the argument:
http://philpapers.org/rec/GIMMTE

First thing to note: this paper was published in Apeiron, a journal for ancient Greek and Roman philosophy. That alone should raise alarm bells all over the place. I stand by my characterization. He's arguing that because the universe is isotropic, gravity cancels on large scales. There is no excuse for anybody who knows a good amount about physics to make a mistake this horrendously wrong.

Finally, you'd be surprised how much terrible stuff makes it through the peer review process. The proliferation of low-quality journals doesn't make things much better.

 

[Chalnoth]

The regime changes from normal to linear expansion as matter becomes dominant.

The BBN epoch was well down in the radiation dominated epoch so $R(t) \propto t^\frac{1}{2}$, thus leaving BBN as in the standard model.

You'd be surprised how much relatively subtle changes in the expansion rate can modify BBN. I'd like to see a proper analysis of this expansion regime before concluding that it "just works".

However the hyperbolic space of the Milne model increases angular distance and that compensates for this increased cell size. JuanCasado has already commented on that - I would add that as an author of this refereed paper is actively participating in this thread such a comment is against the PF civility guidelines.

Very well. Edited to remove that word. But it's still a terrible paper.

No! The key word here is "suggest" - there are different pointers towards a Milne model, I began my research on the topic for my MSc project (almost 40 years ago (!)) using of the old LNH - and so I was interested to read that this paper used a modern version of that hypothesis, but it does so only to suggest why such a model might be considered. The linear expansion is actually delivered by DE with an EoS of $p = -\frac{1}{3}\rho$, as I know you realized.

Except $w=-1/3$ for dark energy is completely ruled out by the available evidence. The only way this model within striking distance of the data is because it also eliminates matter from the Friedmann equations for no good reason.

In my (published) work on Self Creation Cosmology there is a non-minimally connected Brans-Dicke-type scalar field coupled to the trace of $T_{\mu \nu}$ that does just that. But I won't go on any further here because to do so would violate PF policy as there is as yet unpublished research.

To get us back to the OP paper I would just add that this discussion arises from its comment "that the data are still quite consistent with a constant rate of expansion", perhaps people will want to discuss other aspects of it...

That would be a bit more interesting. It's conceivable that a more elaborate model would actually fit the data while retaining some of the general features of linear expansion. But it's just sloppy to throw linear expansion up there when it has no reasonable theoretical foundation and does not fit the available data. And it's also worth noting that the more things you add to the theory, the more likely it is to be incorrect.

 

[Garth]

Very well. Edited to remove that word. But it's still a terrible paper.

Thank you; but we were not talking about that 'popular' article http://philpapers.org/rec/GIMMTE, that I had not see before, but the refereed published Steady Flow cosmological model.

Except $w=-1/3$ for dark energy is completely ruled out by the available evidence.

But surely only by the type of evidence that we have been discussing in this thread? Evidence which may be ambivalent?

Garth

 

[Chalnoth]

Thank you; but we were not talking about that 'popular' article http://philpapers.org/rec/GIMMTE, that I had not see before, but the refereed published Steady Flow cosmological model.

Hmm. I see he added $\rho_m$ back into that model, but then gave it a value that is 1/5th the observed value. That's pretty much just as bad.

But surely only by the type of evidence that we have been discussing in this thread? Evidence which may be ambivalent?

It really isn't. In order for dark energy with $w=-1/3$ to not be wildly ruled out, you need to get rid of the gravitational effect of the matter (or perform some other similar manipulation that makes no sense).

 

[Garth]

It really isn't. In order for dark energy with $w=-1/3$ to not be wildly ruled out, you need to get rid of the gravitational effect of the matter (or perform some other similar manipulation that makes no sense).

Thank you for pointing that out, I have realized I have been unclear. For linear expansion the EoS $p = -\frac{1}{3} \rho$ is the total EoS for all species within the universe.

As $\ddot{R} =-\frac{4\pi GR}{3}(\rho + \frac{3p}{c^2})$ then with total $p = -\frac{1}{3}$total $\rho$ then $\ddot{R} = 0$ and R = ct.

Sorry about the confusion, my bad.

Garth

Edit: Of course I have also been mixing unit systems - the statement $p = -\frac{1}{3} \rho$ implies c is unity, otherwise $p = -\frac{1}{3} \rho c^2$

 

[Chalnoth]

Thank you for pointing that out, I have realized I have been unclear. For linear expansion the EoS $p = -\frac{1}{3} \rho$ is the total EoS for all species within the universe.

As $\ddot{R} =-\frac{4\pi GR}{3}(\rho + \frac{3p}{c^2})$ then with total $p = -\frac{1}{3}$total $\rho$ then $\ddot{R} = 0$ and R = ct.

Sorry about the confusion, my bad.

Garth

Right, so if you had some sort of fluid that was tightly coupled to matter, this might work. But I'm pretty sure that including the full range of cosmological observations will still rule it out, especially at early times.

 

[Garth]

Right, so if you had some sort of fluid that was tightly coupled to matter, this might work. But I'm pretty sure that including the full range of cosmological observations will still rule it out, especially at early times.

Agreed; it is still "a work in progress"...

Garth

 

[PeterDonis]

the hyperbolic space of the Milne model

The Milne model describes a universe with zero stress-energy and zero cosmological constant--it's just Minkowski spacetime in a different coordinate chart. (More precisely, it's the "upper wedge" of Minkowski spacetime--the interior of the future light cone of the origin in a standard inertial coordinate chart--in a hyperbolic coordinate chart, where the "surfaces of constant time" are hyperbolas of constant $t^2 - x^2$ in the standard inertial chart.) So it is not a viable model for describing our actual universe.

 

[Chalnoth]

The Milne model describes a universe with zero stress-energy and zero cosmological constant--it's just Minkowski spacetime in a different coordinate chart. (More precisely, it's the "upper wedge" of Minkowski spacetime--the interior of the future light cone of the origin in a standard inertial coordinate chart--in a hyperbolic coordinate chart, where the "surfaces of constant time" are hyperbolas of constant $t^2 - x^2$ in the standard inertial chart.) So it is not a viable model for describing our actual universe.

It's possible if you have a sort of dark energy which has an interaction with matter such that the combined fluid has $w=-1/3$, but I am extraordinarily skeptical that there is any such model that isn't highly contrived, and even more skeptical that there is a model like this that fits the data.

 

[PeterDonis]

It's possible if you have a sort of dark energy which has an interaction with matter such that the combined fluid has w=−1/3

No, it isn't. Dark energy and matter both have positive energy density, so even if you get the pressures to exactly cancel, which is what $w = -1/3$ does, you still haven't canceled the energy density, and the stress-energy tensor is not zero. The Milne model literally requires a zero stress-energy tensor and a zero cosmological constant, i.e., it requires the only possible EFE solution to be Minkowski spacetime. (To see this, compute the Riemann curvature tensor of the Milne metric; you will find that it is identically zero.)

 

[Chalnoth]

No, it isn't. Dark energy and matter both have positive energy density, so even if you get the pressures to exactly cancel, which is what $w = -1/3$ does, you still haven't canceled the energy density, and the stress-energy tensor is not zero. The Milne model literally requires a zero stress-energy tensor and a zero cosmological constant, i.e., it requires the only possible EFE solution to be Minkowski spacetime. (To see this, compute the Riemann curvature tensor of the Milne metric; you will find that it is identically zero.)

Right, to get Milne. The idea here is to get a Milne-like expansion rate. I do agree that it's pretty absurd, but it isn't quite that bad.

 

[PeterDonis]

The idea here is to get a Milne-like expansion rate.

But even if you get that, it will only be for an instant, strictly speaking, correct? As the universe expands, the dark energy density stays constant while the matter density decreases, so the $w = -1/3$ balance won't be preserved. Or is the "interaction" between the two supposed to continuously adjust the densities to maintain $w = -1/3$? If so, I agree that such a model would be highly contrived.

 

[Chalnoth]

But even if you get that, it will only be for an instant, strictly speaking, correct? As the universe expands, the dark energy density stays constant while the matter density decreases, so the $w = -1/3$ balance won't be preserved. Or is the "interaction" between the two supposed to continuously adjust the densities to maintain $w = -1/3$? If so, I agree that such a model would be highly contrived.

Yes, the interaction is supposed to continuously adjust the densities.

Models with similar ideas have been proposed many times, most of them going under the umbrella of tracking quintessence models (see https://en.wikipedia.org/wiki/Quintessence_(physics)). The purpose of those models is to present a type of dark energy that tracks the total matter/radiation energy density until the density gets low enough that it freezes out and becomes nearly constant (typically occurring around matter/radiation equality). One of the motivations for this kind of model is to produce a model which unifies inflation and the current accelerated expansion. The biggest constraints on these models tend to come from BBN.

 

[Garth]

Or is the "interaction" between the two supposed to continuously adjust the densities to maintain $w = -1/3$? If so, I agree that such a model would be highly contrived.

Thank you PeterDonis and Chalnoth for this part of the discussion and clarifying the matter.

Any such linear expansion (sometimes called 'Milne' as shorthand) would seem to have to be highly contrived, unless there is some new physics here.

That is why an extraordinary 'Milne' claim will only be seriously considered if there is extraordinary evidence to support it.

However with DM and the Inflaton still not identified (LHC tomorrow?), and knowing GR will have to be integrated into some hitherto undiscovered QG theory at Planck scales, it may be apposite to be on the look out for indications to possible alternative theories and associated observed anomalies.

Some time ago I started a thread Critique Of Mainstream Cosmology where I asked the following questions: (that thread #19)
1. Are SNe Ia Standard Candles?
2. Is there an Age Problem in the Mainstream Model?
3. Are the Cosmological Coincidences just coincidences?
4. The Axis of Evil, is there a low-l mode deficiency in the WMAP power spectrum?

Eight years later these questions are still issues, as recent threads based on recent papers have shown. Of course the answer may simply be 'All is well with the standard model', however any heterodox answers, such as the tentative 'Milne' consistency in the two papers that have been discussed in this thread, may be indicators of something new.

Garth

 

[wabbit]

As a layman, I find several of these questions interesting even though some are of the type "the evidence isn't clearcut, there's a hint of an issue but only more data will confirm" - and I see no reason to believe the current standard model is the last word, certainly not until the nature of inflation (and dark matter) is better established.

But I don't understand what the argument is in favor of linear expansion specifically, among all possible alternatives. It does not fit the data better, and it does not come with a better explanation either, rather it seems to requires unspecified physics to work. What makes it special?

 

[Garth]

But I don't understand what the argument is in favor of linear expansion specifically, among all possible alternatives. It does not fit the data better, and it does not come with a better explanation either, rather it seems to requires unspecified physics to work. What makes it special?

Only that it keeps 'popping up' in the analysis of some data plots, such as:

1. Perlmutter et al.'s classic Measurements of Omega and Lambda from 42 High-Redshift Supernovae Figure 2 where the text includes, "The middle solid curve is for (Ω_M,Ω_Λ) = (0,0). (Milne) Note that this plot is practically identical to the magnitude residual plot for the best-fit unconstrained cosmology of Fit C," (Italics mine) (data out to z = 0.83)
   
2. The OP paper Marginal evidence for cosmic acceleration from Type Ia supernovae comment "that the data are still quite consistent with a constant rate of expansion." (see fig 3) (data out to z = 1.24) and
   
3. data from IS THERE EVIDENCE FOR DARK ENERGY EVOLUTION? my plot #50. (data out to z = 2.34)

No firm, let alone 'extraordinary', evidence here - just 'indications' of "quite consistent".

A 'linear' model would also ease any age problem in the early universe and readily explain why, from observed values, Hubble time is coincidentally the same as the age of the universe.

Added comment: Finally any cosmology with an overall linear or accelerating expansion rate would not require Inflation as they would not suffer from the horizon, smoothness and density problems of GR that Inflation was devised to resolve.

Garth

 

[Chalnoth]

Thank you PeterDonis and Chalnoth for this part of the discussion and clarifying the matter.

Any such linear expansion (sometimes called 'Milne' as shorthand) would seem to have to be highly contrived, unless there is some new physics here.

That is why an extraordinary 'Milne' claim will only be seriously considered if there is extraordinary evidence to support it.

However with DM and the Inflaton still not identified (LHC tomorrow?), and knowing GR will have to be integrated into some hitherto undiscovered QG theory at Planck scales, it may be apposite to be on the look out for indications to possible alternative theories and associated observed anomalies.

Some time ago I started a thread Critique Of Mainstream Cosmology where I asked the following questions: (that thread #19)
1. Are SNe Ia Standard Candles?
2. Is there an Age Problem in the Mainstream Model?
3. Are the Cosmological Coincidences just coincidences?
4. The Axis of Evil, is there a low-l mode deficiency in the WMAP power spectrum?

Eight years later these questions are still issues, as recent threads based on recent papers have shown. Of course the answer may simply be 'All is well with the standard model', however any heterodox answers, such as the tentative 'Milne' consistency in the two papers that have been discussed in this thread, may be indicators of something new.

Garth

1. Yes. They're not wonderful, but they're decent standard candles.
2. Nothing solid.
3. The cosmological coincidence issue is a lot less interesting if you simply plot density fraction as a function of time.
4. No. The statistics aren't strong enough to say anything is going on here.

 

[JuanCasado]

It really is that bad. Here is the paper where he actually makes the argument:
http://philpapers.org/rec/GIMMTE

First thing to note: this paper was published in Apeiron, a journal for ancient Greek and Roman philosophy. That alone should raise alarm bells all over the place. I stand by my characterization. He's arguing that because the universe is isotropic, gravity cancels on large scales. There is no excuse for anybody who knows a good amount about physics to make a mistake this horrendously wrong.

Finally, you'd be surprised how much terrible stuff makes it through the peer review process. The proliferation of low-quality journals doesn't make things much better.

This is neither the paper we were talking about, nor the same model. You are deliberatelly confusing different things and ideas just to desprestige a model that fits the data as well as LCDM.

 

[wabbit]

This is neither the paper we were talking about, nor the same model. You are deliberatelly confusing different things and ideas just to desprestige a model that fits the data as well as LCDM.

You cannot say it fits the data as well, that is not what any of the papers quoted here show.

 

[Chalnoth]

This is neither the paper we were talking about, nor the same model. You are deliberatelly confusing different things and ideas just to desprestige a model that fits the data as well as LCDM.

When the model describes the CMB data well (meaning a prediction of the full power spectrum out to $\ell=3000$ or so), then it would make sense to say it fits the data. But right now, this model is tens if not hundreds of standard deviations away from fitting the CMB data without adding some dynamics to make the late universe approximately linear in growth but the early universe following CDM + inflation.

 

[JuanCasado]

When the model describes the CMB data well (meaning a prediction of the full power spectrum out to $\ell=3000$ or so), then it would make sense to say it fits the data. But right now, this model is tens if not hundreds of standard deviations away from fitting the CMB data without adding some dynamics to make the late universe approximately linear in growth but the early universe following CDM + inflation.

But this is the point: The Steady Flow model is linear in growth in recent times, while it follows standard dynamics for the early universe.

 

[Chalnoth]

But this is the point: The Steady Flow model is linear in growth in recent times, while it follows standard dynamics for the early universe.

Except with a very different matter density. I just don't think that's going to work. I'll believe it when I see it.

The issue here is that the CMB constrains the matter density very tightly. The baryon density is the tightest constraint as the baryon density is largely determined by the magnitude of the first acoustic peak. The ratio of dark matter to normal matter is then determined by the ratio of the heights of the even an odd acoustic peaks*.

The CMB itself doesn't actually constrain the dynamics of the expansion since it was emitted, but changing those dynamics has very little impact on the estimated matter and dark matter density from the CMB. For example, compare these parameters, which are WMAP 9-year only using $\Lambda$CDM with no spatial curvature:
http://lambda.gsfc.nasa.gov/product/map/dr5/params/lcdm_wmap9.cfm

To these parameters, which use the same data and assumptions except for relaxing the assumption of flat space:
http://lambda.gsfc.nasa.gov/product/map/dr5/params/olcdm_wmap9.cfm

In particular, the $\Omega_\Lambda$ and other density fraction parameters are extremely poorly constrained in the second case: $\Omega_\Lambda$ has 95% confidence limits between 0.22 and 0.79. When flat space is assumed, this tightens to $0.732 \pm 0.025$ (68% confidence limits, making this somewhat confusing).

But if you compare this to the measures of the cold dark matter and baryon density ($\Omega_ch^2$ and $\Omega_bh^2$, respectively), those remain very tightly constrained and are largely unaffected by the assumption of flatness. In fact, the errors on the density parameters barely budge.

* This isn't how it's done when people are doing CMB parameter estimates, of course. But it does illustrate why the constraints on these parameters are so tight.