Positions of distant supernova explosions false?

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I'm googling and reading about Shape Dynamics and I came across this site about Julian Barbour research:
http://discovermagazine.com/2012/mar/09-is-einsteins-greatest-work-wrong-didnt-go-far

Can you find flaw in the following arguments about possible error in the computations of the accelerating universe?

"David Wiltshire, a physicist at the University of Canterbury in New Zealand and a visitor to Barbour’s College Farm, thinks the reason dark energy is so mysterious is that it is an illusion. Wiltshire’s argument is that most physicists essentially ignore one of the major principles at the heart of general relativity: that clocks in different parts of the universe can run at different rates. Einstein held that there is no such thing as universal time and that matter affects the rate at which clocks tick, such that time slows near massive objects. Accordingly, Wiltshire notes, the flow of time near galaxies could be slower than the flow of time in empty space. “In a truly relativistic view, the age of the universe differs from place to place,” he says. “In empty space, over 18 billion years have elapsed since the Big Bang, but within galaxies only about 15 billion years have passed.” (Because Wiltshire starts from a separate set of physical assumptions, his numbers are different from the now canonical 13.7 billion years for the age of the universe.)

By ignoring those nuances, Wiltshire claims, cosmologists have misinterpreted the positions of the distant supernova explosions used to determine how quickly the universe is expanding. Light from a supernova travels to Earth’s telescopes after passing through both patches of empty space (where the universe expands more rapidly) and through intervening galaxies filled with matter (where the expansion slows). As a result, Wiltshire says, cosmologists expect supernovas to be closer than they appear, creating the illusion that the expansion of the universe is speeding up. Supernova measurements are the key evidence for dark energy. But Wiltshire thinks physicists may have been chasing shadows rather than zeroing in on reality for years."
 

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  • #2
Drakkith
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Can you find flaw in the following arguments about possible error in the computations of the accelerating universe?
I don't know about a flaw, but if his assumptions (as apposed to the mainstream assumptions mentioned in the articles) are wrong then his results will be wrong as well. That's mostly likely what it boils down to. A different set of assumptions. Note that the assumptions used by mainstream cosmologists are very well supported by observational experimental evidence, so it seems unlikely to me that they are that inaccurate.
 
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But in basic relativity.. isn't it that the flow of time near galaxies could be slower than the flow of time in empty space? Or is it similar? But doesn't it being similar violate SR which dictates that near mass time slows down?
 
  • #4
Drakkith
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But in basic relativity.. isn't it that the flow of time near galaxies could be slower than the flow of time in empty space?
It is slower. But not by much. If we compare the passage of time here on Earth to the passage of time near the center of an intergalactic void the difference in time comes out to be something of the order of a few days or so if I remember correctly. So, over 13+ billion years, the difference is minuscule. The difference in the passage of time near the galaxy but not within the sphere of influence of any stars compared to an intergalactic void is even less since you're not close to a massive object.

The amount of mass needed to cause a significant amount of time dilation near/inside galaxies is much larger than observed.

But doesn't it being similar violate SR which dictates that near mass time slows down?
That's GR, not SR. And yes, it would violate GR if time dilation did not occur near massive objects.
 
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It is slower. But not by much. If we compare the passage of time here on Earth to the passage of time near the center of an intergalactic void the difference in time comes out to be something of the order of a few days or so if I remember correctly. So, over 13+ billion years, the difference is minuscule. The difference in the passage of time near the galaxy but not within the sphere of influence of any stars compared to an intergalactic void is even less since you're not close to a massive object.

The amount of mass needed to cause a significant amount of time dilation near/inside galaxies is much larger than observed.



That's GR, not SR. And yes, it would violate GR if time dilation did not occur near massive objects.
So what separate set of physical assumption was Wiltshire using that made him state that "In a truly relativistic view, the age of the universe differs from place to place,” he says. “In empty space, over 18 billion years have elapsed since the Big Bang, but within galaxies only about 15 billion years have passed"?
 
  • #6
Drakkith
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So what separate set of physical assumption was Wiltshire using that made him state that "In a truly relativistic view, the age of the universe differs from place to place,” he says. “In empty space, over 18 billion years have elapsed since the Big Bang, but within galaxies only about 15 billion years have passed"?
Per the article on page 2:

With his newly minted doctorate, young Barbour pressed on where Einstein had feared to tread, coming closer to Mach by dispensing not just with Newton’s rigid grid but with the very concept of space-time. In general relativity, time is a dimension interwoven with the dimensions of space. In Barbour’s universe, on the other hand, time is emergent: It is a measure of how space changes but not a fundamental component of it.
Barbour (and thus Wiltshire) basically stops using GR in favor of his own theory. Somehow he makes a theory in which time is emergent instead of fundamental. Is he correct? We don't know. I doubt there's any clear experimental evidence that supports Barbour and Wiltshire over standard cosmological theories and models. Otherwise it would be big news.

The most likely event is that Barbour's theory is incorrect. That's not to say that he doesn't know what he's doing, only that the great majority of new theories and models turn out to be incorrect once more data is accumulated and we gain a better understanding of physics.
 
  • #7
kimbyd
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Here's the preprint for this model:
https://arxiv.org/abs/1706.07236

I haven't examined the details of the theory yet, but just looking at their distance/redshift ratio with their model (Fig. 1), the deviation between their model and ##\Lambda##CDM will diverge significantly at higher redshifts. This model is probably already ruled-out by high-redshift supernova observations.
 
  • #8
kimbyd
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Here's the preprint for this model:
https://arxiv.org/abs/1706.07236

I haven't examined the details of the theory yet, but just looking at their distance/redshift ratio with their model (Fig. 1), the deviation between their model and ##\Lambda##CDM will diverge significantly at higher redshifts. This model is probably already ruled-out by high-redshift supernova observations.
Looking into the paper in more detail, they show that they can fit the current large-scale supernova surveys (I don't know if the relatively small number of high-redshift supernova that have been observed to date are included). But they also predict model parameters which diverge pretty markedly from current observational estimates, which likely means that they can't fit a large combination of data.

Perhaps even worse, they "fit" the data by drastically increasing the error bars on the model parameters. Such an increase likely means that the fit is spurious: they have presented a model which can fit a far wider variety of supernova data than the ##\Lambda##CDM model can.

Finally, I'm pretty sure that their model would utterly fail to fit the Baryon Acoustic Oscillation data, especially if combined with the CMB data.

Overall, I do think the model is fairly interesting. And it's certainly not complete nonsense: it's a serious attempt at proposing a reasonable alternative to the standard cosmology. But I don't think it can fit the data that we have, not when you put it all together.
 
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  • #9
Drakkith
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Overall, I do think the model is fairly interesting. And it's certainly not complete nonsense: it's a serious attempt at proposing a reasonable alternative to the standard cosmology. But I don't think it can fit the data that we have, not when you put it all together.
I'd like to emphasize this last bit. A great number of proposed theories and models are supported by some evidence, but very few are supported by nearly all available evidence.
 
  • #10
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So they can make model where empty space has time running significantly faster than near galaxy or mass.
Has any physicist happened to put forward any model where dark matter or the dark sector has time running at different rate than our visible matter?

If there is none. What kind of model where there is another sector (not necessarily dark matter but could be others like other branes or multiverse) with time running at different rates than ours? This would make much more sense.
 
  • #11
Drakkith
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So they can make model where empty space has time running significantly faster than near galaxy or mass.
You can do whatever you like with a theory. But unless it reflects reality, then it isn't a useful tool.

Has any physicist happened to put forward any model where dark matter or the dark sector has time running at different rate than our visible matter?
I am not aware of any reputable theories that model time this way.

If there is none. What kind of model where there is another sector (not necessarily dark matter but could be others like other branes or multiverse) with time running at different rates than ours?
Any such theories would be very theoretical and would most likely not even make sense to anyone unfamiliar with the mathematical details of the theory.

This would make much more sense.
Take care when saying that certain things make more sense than others when it comes to science. Unless you are a scientist working specifically in that area then what makes sense to you is likely completely wrong, or, at best, is a lucky guess.
 
  • #12
kimbyd
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Has any physicist happened to put forward any model where dark matter or the dark sector has time running at different rate than our visible matter?
I don't think that's a meaningful statement to make. How are you measuring time, after all?

For normal matter, we can measure time using the amount of time various physical processes take, such as nuclear decays. But it's impossible to compare those time scales to dark matter: even if we knew what the dark matter particle was, the physics that govern its behavior would be so completely different from normal matter that there would be no way to compare, say, the half-life of a cesium atom to anything that dark matter does, because there is no such thing as a cesium atom made out of dark matter.
 
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I don't think that's a meaningful statement to make. How are you measuring time, after all?

For normal matter, we can measure time using the amount of time various physical processes take, such as nuclear decays. But it's impossible to compare those time scales to dark matter: even if we knew what the dark matter particle was, the physics that govern its behavior would be so completely different from normal matter that there would be no way to compare, say, the half-life of a cesium atom to anything that dark matter does, because there is no such thing as a cesium atom made out of dark matter.
If a galaxy is made of partly matter and partly dark matter and they rotate at the same revolution (the typical galaxy where the edge has same speed as near the center). Would this mean time (how ever it is measured in either sector) is nearly similar between them? Or can they vary (again however it is measured in either sector)?
 
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  • #14
Drakkith
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If a galaxy is made of partly matter and partly dark matter and they rotate at the same revolution (the typical galaxy where the edge has same speed as near the center).
Both normal and dark matter rotate under the influence of the same gravitational laws as far as we know. However, we've never seen dark matter move because we can only detect its gross (i.e. "widescale") gravitational influences on normal matter. We can't locate a 'clump' of dark matter and watch it for a few years as it moves around since it doesn't clump up like regular matter. But it is still believed to follow the same gravitational laws as normal matter. Thus the distribution and motion of dark matter and normal matter vary greatly and I am confident in saying that the rotational speed of normal and dark matter don't match up one bit. In fact, I think dark matter particles are believed to take highly elliptical trajectories, where they spend a great deal of time traveling slowly out in the 'halo' around the galaxy, before gravity pulls them inward near the core where they accelerate, pass through the disk and core, and then decelerate as they move back out of the galaxy towards the halo.

Would this mean time (how ever it is measured in either sector) is nearly similar between them?
The passage of time in a frame of reference moving with the dark matter particles and the passage of time in a frame moving with the normal matter would be extremely similar if both are moving at approximately the same speed.
 
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  • #15
kimbyd
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If a galaxy is made of partly matter and partly dark matter and they rotate at the same revolution (the typical galaxy where the edge has same speed as near the center). Would this mean time (how ever it is measured in either sector) is nearly similar between them? Or can they vary (again however it is measured in either sector)?
There's no reasonable mechanism for dark matter and normal matter to have an impact on the flow of time for other particles beyond their gravitational influence. The same mass (as measured by gravitational attraction) must have the exact same impact on time regardless of the source.

And by the way, gravitational time dilation is pretty much negligible for most anything that isn't right next to the horizon of a black hole.
 
  • #16
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If generic General Relativity is not exactly correct and time is really emergent and not fundamental.. and some kind of Barbour timeless state is possible.. then a realm where time is not flowing from past to present but all at once can occur. So dark matter can't be this timeless state? Does it mean Barbour timeless state occurs outside of space and time or within it. How do you understand it?
 
  • #17
kimbyd
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If generic General Relativity is not exactly correct and time is really emergent and not fundamental.. and some kind of Barbour timeless state is possible.. then a realm where time is not flowing from past to present but all at once can occur. So dark matter can't be this timeless state? Does it mean Barbour timeless state occurs outside of space and time or within it. How do you understand it?
Barbour's model is completely independent of this discussion. His model is an alternative way of understanding time as an emergent property of matter fields. It's been quite a while since I looked at Shape Dynamics (he gave a talk at the school I worked at something like 8 years ago or so, and we had a stimulating discussion afterwards). But from what I remember it doesn't actually result in a theory which offers any theoretical predictions distinct from General Relativity that are measurable today. What it does provide is a fundamentally different way of understanding the relationship between space and time (with space being fundamental and time being an emergent property). This fundamentally different formulation might also provide a completely new way of quantizing gravity, one that avoids the difficulties of previous attempts to do so.

None of that suggests there's any possible way for Shape Dynamics to provide a mechanism for dark matter to influence time differently from normal matter.

BTW, for any readers of this thread who are interested in theory, I do highly recommend reading up on Shape Dynamics. It really is a clever and fascinating theory. This rather lengthy paper provides a good description of the theory and its motivations:
https://arxiv.org/abs/1409.0105

I don't know how to interpret the likelihood of Shape Dynamics, because it's (mostly) just a different way of conceptualizing General Relativity. But it's really neat how they can arrive at a theory with nearly identical dynamics starting from an utterly different physical basis. The small discrepancy, by the way, appears to be related to the fact that Shape Dynamics is invariant under Weyl transformations, while General Relativity is not. The lack of Weyl invariance in GR is a significant wart on the theory that makes it seem unphysical: Weyl transformations involve an overall rescaling of coordinates, and the dynamics of the theory really shouldn't depend upon coordinate choice. The fact that Shape Dynamics produces a nearly-identical theory without this wart is rather compelling.

But because it isn't really possible to fix this wart while retaining identical dynamics, Shape Dynamics and General Relativity are (subtly) different, in that solutions to one are not necessarily solutions to the other. Those differences haven't been fully explored yet.
 
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Barbour's model is completely independent of this discussion. His model is an alternative way of understanding time as an emergent property of matter fields. It's been quite a while since I looked at Shape Dynamics (he gave a talk at the school I worked at something like 8 years ago or so, and we had a stimulating discussion afterwards). But from what I remember it doesn't actually result in a theory which offers any theoretical predictions distinct from General Relativity that are measurable today. What it does provide is a fundamentally different way of understanding the relationship between space and time (with space being fundamental and time being an emergent property). This fundamentally different formulation might also provide a completely new way of quantizing gravity, one that avoids the difficulties of previous attempts to do so.
You stated that shape dynamics doesn't actually result in a theory which offers any theoretical predictions distinct from General Relativity that are measurable today.. but isn't it shape dynamics don't require dark matter nor dark energy so it has theoretical predictions that are distinct from GR already. Not?

None of that suggests there's any possible way for Shape Dynamics to provide a mechanism for dark matter to influence time differently from normal matter.

BTW, for any readers of this thread who are interested in theory, I do highly recommend reading up on Shape Dynamics. It really is a clever and fascinating theory. This rather lengthy paper provides a good description of the theory and its motivations:
https://arxiv.org/abs/1409.0105

I don't know how to interpret the likelihood of Shape Dynamics, because it's (mostly) just a different way of conceptualizing General Relativity. But it's really neat how they can arrive at a theory with nearly identical dynamics starting from an utterly different physical basis. The small discrepancy, by the way, appears to be related to the fact that Shape Dynamics is invariant under Weyl transformations, while General Relativity is not. The lack of Weyl invariance in GR is a significant wart on the theory that makes it seem unphysical: Weyl transformations involve an overall rescaling of coordinates, and the dynamics of the theory really shouldn't depend upon coordinate choice. The fact that Shape Dynamics produces a nearly-identical theory without this wart is rather compelling.

But because it isn't really possible to fix this wart while retaining identical dynamics, Shape Dynamics and General Relativity are (subtly) different, in that solutions to one are not necessarily solutions to the other. Those differences haven't been fully explored yet.
In Shape Dynamics.. space is fundamental while time is emergent property
In Loop Quantum Gravity... the fundamental entities are essentially pieces of spacetime
In String theory.. space and time is emergent from dynamics of strings (but against what background?)

Is there other theory (or model) besides them where both space and time are emergent or pieces of spacetime and you can interact with the fundamental degrees of freedom?
 
  • #19
kimbyd
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You stated that shape dynamics doesn't actually result in a theory which offers any theoretical predictions distinct from General Relativity that are measurable today.. but isn't it shape dynamics don't require dark matter nor dark energy so it has theoretical predictions that are distinct from GR already. Not?
I really, really doubt it. In order to achieve Weyl invariance, Shape Dynamics does need to introduce a few more degrees of freedom. Some have posited that these degrees of freedom might behave like dark matter or dark energy.

Given the evidence we have so far about dark matter, I really, really doubt that can be replaced by any alternative gravity theory.

With respect to dark energy, well, the simplest solution to dark energy is the cosmological constant, which is a part of the gravity theory already. Any alternative gravity theory ought to have something similar to the cosmological constant, regardless of its formulation. So explaining dark energy via gravity is hardly something surprising: GR already does this on its own.

Personally, I'm not sure that the extra degrees of freedom can be interpreted as anything real at all. For instance, in one conformal variation of General Relativity, there is an added field to the system, but that field is simply a representation of the conformal invariance which allows you to, through a change in coordinates, determine whether space-time curvature shows up in the conformal scalar term, or in the curvature term. It doesn't actually change anything about the dynamics, just how it's presented.

In Shape Dynamics.. space is fundamental while time is emergent property
In Loop Quantum Gravity... the fundamental entities are essentially pieces of spacetime
In String theory.. space and time is emergent from dynamics of strings (but against what background?)

Is there other theory (or model) besides them where both space and time are emergent or pieces of spacetime and you can interact with the fundamental degrees of freedom?
I'm not aware of any detailed attempts beyond those three.
 
  • #20
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I really, really doubt it. In order to achieve Weyl invariance, Shape Dynamics does need to introduce a few more degrees of freedom. Some have posited that these degrees of freedom might behave like dark matter or dark energy.

Given the evidence we have so far about dark matter, I really, really doubt that can be replaced by any alternative gravity theory.

With respect to dark energy, well, the simplest solution to dark energy is the cosmological constant, which is a part of the gravity theory already. Any alternative gravity theory ought to have something similar to the cosmological constant, regardless of its formulation. So explaining dark energy via gravity is hardly something surprising: GR already does this on its own.

Personally, I'm not sure that the extra degrees of freedom can be interpreted as anything real at all. For instance, in one conformal variation of General Relativity, there is an added field to the system, but that field is simply a representation of the conformal invariance which allows you to, through a change in coordinates, determine whether space-time curvature shows up in the conformal scalar term, or in the curvature term. It doesn't actually change anything about the dynamics, just how it's presented.
What description or criteria that decides whether the extra degrees of freedom can be interpreted as anything real at all? "Real" is a vague term. So I need to know the criteria you or they are using to distinguish it. Can you give an example what stuff in GR is real and not real.. so I can understand the context of you mean regarding the extra degrees of freedom being real or not real.. this is important and not a philosophical question but mathematically and dynamical.


I'm not aware of any detailed attempts beyond those three.
 
  • #21
kimbyd
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What description or criteria that decides whether the extra degrees of freedom can be interpreted as anything real at all? "Real" is a vague term. So I need to know the criteria you or they are using to distinguish it. Can you give an example what stuff in GR is real and not real.. so I can understand the context of you mean regarding the extra degrees of freedom being real or not real.. this is important and not a philosophical question but mathematically and dynamical.
If you can remove the degrees of freedom by selecting a certain gauge, then they can't really represent any physical process.
 
  • #22
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If you can remove the degrees of freedom by selecting a certain gauge, then they can't really represent any physical process.
I'm familiar about the gauge theory of the 3 forces where the rotations, etc. are unphysical. What is the analogy of this in Shape Dynamics? Is the shape or size invariant being like the gauging in gauge theory? Couldn't there be other degrees of freedom that is not like this?
 
  • #23
kimbyd
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I'm familiar about the gauge theory of the 3 forces where the rotations, etc. are unphysical. What is the analogy of this in Shape Dynamics? Is the shape or size invariant being like the gauging in gauge theory? Couldn't there be other degrees of freedom that is not like this?
The gauge transformation involved here is the Weyl transformation, which is a shape-preserving transformation (hence the name Shape Dynamics). Shape-preserving means that angles are unchanged, but overall scale can change.

Having a theory which is invariant under these transformations is attractive because it means that the theory is no longer sensitive to the choice of length or time coordinates.
 
  • #24
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The gauge transformation involved here is the Weyl transformation, which is a shape-preserving transformation (hence the name Shape Dynamics). Shape-preserving means that angles are unchanged, but overall scale can change.

Having a theory which is invariant under these transformations is attractive because it means that the theory is no longer sensitive to the choice of length or time coordinates.
What is other gauge transformation where even shape is not preserved so the theory is no longer sensitive to the choice of length or time coordinates or even the angles?
 
  • #25
kimbyd
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What is other gauge transformation where even shape is not preserved so the theory is no longer sensitive to the choice of length or time coordinates or even the angles?
I'm not aware of any such transformation. I'm skeptical that you could construct a meaningful theory that way. It would be equivalent to saying that you could rewrite the entire metric via a gauge transformation. How could you reproduce any meaningful dynamics out of a theory which has no constraints?
 

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