How can we accurately determine the expansion rate of the Universe?

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The discussion centers on the complexities of determining the expansion rate of the Universe, questioning whether it is expanding or accelerating. Observations and scientific papers are referenced to support various viewpoints, particularly regarding dark energy's role, which is suggested to constitute about 75% of the Universe. The conversation also touches on the distribution of dark matter and energy, emphasizing the need for precise definitions and models in cosmology. Additionally, the impact of cosmic inflation on the Universe's expansion is highlighted, with a consensus that further understanding is required. Overall, the thread underscores the ongoing debate and uncertainty in cosmological measurements and theories.
  • #31
Nereid said:
(continued)

wered in a reasonably straight-forward way: both 'expanding' and 'shrinking' conclusions are equivalent.

To those who have some familiarity with GR this equivalence is not surprising, but to most people it is really, really astonishing. http://www.bautforum.com/space-astr...3475-other-way-look-universes-expansion.html" is an example of a discussion reflecting this.

...
(my bold)

It seems so at first sight but in fact they are not equivalent at all. Both can explain the observed data, but achive a complete different conclusions. Sometimes what appears obvious is just that: 'appears'.
 
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  • #32
heldervelez said:
Nereid said:
This can be answered in a reasonably straight-forward way: both 'expanding' and 'shrinking' conclusions are equivalent.

To those who have some familiarity with GR this equivalence is not surprising, but to most people it is really, really astonishing. Here is an example of a discussion reflecting this.
(my bold)

It seems so at first sight but in fact they are not equivalent at all. Both can explain the observed data, but achive a complete different conclusions. Sometimes what appears obvious is just that: 'appears'.
(I quoted a bit more from the post of mine you quoted, to get more of the context)

I am using "equivalent" in a specific sense: there is no experiment or observation, or any combination of such, which could be done to distinguish the two conclusions (or interpretations), even in principle.

Many people have come across this concept with regard to quantum theory, the "Copenhagen interpretation", or the "Many Worlds Interpretation", for example.

Of course, from a philosophical perspective, two conclusions (or interpretations) that are equivalent (in this sense) may be quite different, and not at all equivalent, philosophically!
However, in this part of PF, our scope is limited to physics.

Did you read the discussion, in my link? What aspects do you think were not addressed, in terms of science?
 
  • #33
Hello All

Nereid said

So, crudely, if to you the assumption that >redshift is a measure of (line of sight) velocity< is equivalent to >the king is wearing invisible robes<, then the assumption that >you have in your hands (on your computer monitor) 'actual images'< is equally equivalent to >the king is wearing invisible robes<.

I agree.

But!

Space * Time = Spacetime (4D)

Is this an identity?

Can this without matter be expanded?

Can time itself be changed?

If the answer is yes than the speed of light can have no limit so to speak.
This has to be applied to the bigger picture and not to the local observed universe or else we would observe the fossilized remains.


I can live with the idea that expansion and contraction are part of an observable process. We notice this in matter being pulled into and ejected from stars and black holes big and small.
 
  • #34
Nereid said:
...

I am using "equivalent" in a specific sense: there is no experiment or observation, or any combination of such, which could be done to distinguish the two conclusions (or interpretations), even in principle.
...
Did you read the discussion, in my link? What aspects do you think were not addressed, in terms of science?

D'acord.. 'Occan's razor' has something to say.
Both Ptolemy and Galileu addressed equally data but only one is better than the other.
By nature of that interpretation it is an experiment that we can not do in our local lab (our finite time at Earth) but Universe at large scale keeps saying other things that we are unable to see.
As I can not address all the interrogations and answers in this forum please check there my answers. That thread is more evolved within this issue.
Perhaps your true assertion that we can not distinguish is much more important that we can think. It means that finally Relativity is puting us in the correct way of 'jump of' a special moment of time and space that actual view dictates.

It is the consequences of each distinct view that are important.
We have to look for the consequences.
 
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  • #35
heldervelez said:
Nereid said:
...

I am using "equivalent" in a specific sense: there is no experiment or observation, or any combination of such, which could be done to distinguish the two conclusions (or interpretations), even in principle.
...
Did you read the discussion, in my link? What aspects do you think were not addressed, in terms of science?
D'acord.. 'Occan's razor' has something to say.
Both Ptolemy and Galileu addressed equally data but only one is better than the other.
Perhaps you mean Kepler, or Copernicus, rather than Galileo?

After all, Galileo reported observations of the phases of Venus (and Mercury?) and the four largest moons of Jupiter (which we now call the Galilean moons), among other things. They are results which clearly distinguish between the two models.

Further, there is a critical rider in my post: "even in principle".

Those three words glide over much that we can, and perhaps should, discuss.

For example, there have been many attempts to develop a theory, or theories, which reduce to QM (the Standard Model of particle physics) and GR in the appropriate limits. It may well be that, in any - or even all - of these theories it is possible, in principle, to distinguish between an expanding and a shrinking universe. When the universe is viewed through the glasses of those theories, the two are not equivalent.

IOW, equivalence (in the narrow sense I am using) is theory-dependent; change the theory, and the equivalence may well disappear.

If you haven't already done so, I'd recommend that you read some of the excellent posts by marcus, in this section of PF; he has done a wonderful job, over several years, of presenting, and in many cases explaining, some of these 'beyond GR' theories, and how interpretations that are equivalent in GR may be distinguishable (though he may not have set out to present this explicitly).

By nature of that interpretation it is an experiment that we can not do in our local lab (our finite time at Earth) but Universe at large scale keeps saying other things that we are unable to see.
Parts of the universe that we are unable to see, even in principle, are not within the purview of science, by definition. They may be fascinating to speculate about, and may have deep philosophical implications, but they are not, and cannot ever be, science.

As I can not address all the interrogations and answers in this forum please check there my answers. That thread is more evolved within this issue.
Perhaps your true assertion that we can not distinguish is much more important that we can think. It means that finally Relativity is puting us in the correct way of 'jump of' a special moment of time and space that actual view dictates.

It is the consequences of each distinct view that are important.
We have to look for the consequences.
Indeed.

On the other hand, QM and GR are mutually incompatible; if (when?) we are able to explore regions of parameter space close to (or in!) the Planck regime, who knows what we may discover?
 
  • #36
Nereid,

Do you have a link where I can read up on this equivalence in more detail? I'd be rather surprised if the contracting solution would, for example, produce a cosmic microwave background or an accurate prediction of nucleosynthesis.
 
  • #37
Hello

I cannot remember if I posted this link before.

I like to have a remark on this paper.

http://arxiv.org/abs/0810.0153
Expanding Space: The Root of Conceptual Problems of the Cosmological Physics

Authors: Yu. V. Baryshev (Astron.Inst.St.-Petersburg Univ.)
(Submitted on 1 Oct 2008)

Abstract: The space expansion physics contains several paradoxes which were clearly demonstrated by Edward Harrison (1981, 1995, 2000), who emphasized that the cooling of homogeneous hot gas (including photon gas of CBR) in the standard cosmological model based on the violation of energy conservation by the expanding space. In modern version of SCM the term "space expansion" actually means continuous creation of vacuum, something that leads to conceptual problems. Recent discussion by Francis, Barnes, James, and Lewis (2007) on the physical sense of the increasing distance to a receding galaxy without motion of the galaxy is just a particular consequence of the arising paradoxes. Here we present an analysis of the following conceptual problems of the SCM: the violation of energy conservation for local comoving volumes, the exact Newtonian form of the Friedmann equation, the absence of an upper limit on the receding velocity of galaxies which can be greater than the speed of light, and the presence of the linear Hubble law deeply inside inhomogeneous galaxy distribution. The common cause of these paradoxes is the geometrical description of gravity, where there is no a well defined concept of the energy-momentum tensor for the gravitational field, no energy quanta - gravitons, and no energy-momentum conservation for matter plus gravity because gravity is not a material field.
 
  • #39
Nereid said:
Perhaps you mean Kepler, or Copernicus, rather than Galileo?

Galileo was the first to introduce Relativity to us. I think we can remember the boat conceptual experience.

Nereid said:
Further, there is a critical rider in my post: "even in principle".

Those three words glide over much that we can, and perhaps should, discuss.
We have towsands of pappers about things we can not directly perceive. We do not question when there exist an indirect possibility of reach knowledge.

Nereid said:
For example, there have been many attempts to develop a theory, or theories, which reduce to QM (the Standard Model of particle physics) and GR in the appropriate limits. It may well be that, in any - or even all - of these theories it is possible, in principle, to distinguish between an expanding and a shrinking universe. When the universe is viewed through the glasses of those theories, the two are not equivalent.

Continue to explore that link you provide and you can see that it is not a big TOE that is offered. There is no need to go to QM, nor GR is on stress.

Nereid said:
IOW, equivalence (in the narrow sense I am using) is theory-dependent; change the theory, and the equivalence may well disappear.
They are not equivalent at all. Only apparently they are equivalent. The assumptions at beginning, the non seen 'objects' and 'artifacts' we get rid, only one single parameter in the theory taken from data.

Nereid said:
If you haven't already done so, I'd recommend that you read some of the excellent posts by marcus, in this section of PF; he has done a wonderful job, over several years, of presenting, and in many cases explaining, some of these 'beyond GR' theories, and how interpretations that are equivalent in GR may be distinguishable (though he may not have set out to present this explicitly).

I know that marcus has been devoted to the comunity.

Nereid said:
Parts of the universe that we are unable to see, even in principle, are not within the purview of science, by definition. They may be fascinating to speculate about, and may have deep philosophical implications, but they are not, and cannot ever be, science.

What I intended to say was "we can not see that interpretation with our mind". The word see was misleading. Sorry.

If what you say here is to be accounted, then anything that mentions time before the CMB should be avoided (inflation, BB,Dark matter, Dark Energy, now even pre-BB)
 
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  • #40
Sundance said:
Hello

I cannot remember if I posted this link before.

I like to have a remark on this paper.

http://arxiv.org/abs/0810.0153
Expanding Space: The Root of Conceptual Problems of the Cosmological Physics

Authors: Yu. V. Baryshev (Astron.Inst.St.-Petersburg Univ.)
(Submitted on 1 Oct 2008)
Honestly, I don't know who Baryshev is, but these objections are just non-issues.

the violation of energy conservation for local comoving volumes
This isn't an issue because General Relativity has no requirement of such energy conservation. It's really easy to understand why this is the case: Newtonian gravity behaves in the exact same way, after all. Just take two test masses initially far away from one another. If you release them, what happens? Naturally they pick up speed and move towards one another.

But where did that energy come from? The standard answer is that it came from gravitational potential energy, which became more negative as the two masses got closer together and picked up speed.

Here's the problem, however: in the normal formulation of general relativity, gravitational potential energy is not considered. If you ignore gravitational potential energy, of course energy isn't going to be conserved.

But there is an alternative: the Hamiltonian formalism. And for a closed FRW universe, it turns out that the total energy is always identically zero: all of the positive energy in matter fields is made up by negative gravitational potential energy.

So that objection is meaningless.

the exact Newtonian form of the Friedmann equation
Well, this is interesting, but I don't see how it's a serious objection. Or even surprising. We know, after all, that whatever equations we make up from Newtonian gravity, those equations must be the same as the prediction for General Relativity as long as the relative speeds remain low and the gravitational fields relatively weak. Since the Friedmann equation assumes a homogeneous, isotropic universe, the gravitational fields are by definition very, very weak. And since the two theories will necessarily have the same prediction locally, where the relative velocities (naively estimated) remain low, while at the same time the assumption of homogeneity is used, the same equation used locally will necessarily also be usable globally. So no, I think we should be more disturbed if this weren't the case, given the particular assumptions used in the derivation.

the absence of an upper limit on the receding velocity of galaxies which can be greater than the speed of light
Given that velocity is only well-defined locally in General Relativity, this is also not an objection. It is slightly unsettling to many people, but it is fully within the predictions of General Relativity, which only states that no massive object can ever outrun a light ray. General Relativity does not guarantee that light rays will be capable of reaching from anywhere to anywhere. In fact, if this were a serious objection, it would also indicate that the prediction of black holes should be considered equally as objectionable.

and the presence of the linear Hubble law deeply inside inhomogeneous galaxy distribution
All this means is that the assumptions of homogeneity and isotropy remain good approximations to the universe on large scales. I have seen nothing to indicate that our current knowledge of structure formation predicts that we should somehow see this Hubble law strongly violated.

So yeah, I am completely unimpressed by these objections.
 
  • #41
Chalnoth said:
All this means is that the assumptions of homogeneity and isotropy remain good approximations to the universe on large scales. I have seen nothing to indicate that our current knowledge of structure formation predicts that we should somehow see this Hubble law strongly violated.
Nonsense. The Hubble law is seen right down to the doorstep of the local group of galaxies, i.e.,
below 3 Mpc. The GR prediction is that the Hubble law should become important at the scale of
clusters of galaxies, i.e., about 40 Mpc. Moreover, the local Hubble flow is very cold; the velocity
dispersion is much smaller than that obtained in (straightforward) computer simulations of the local group and its nearby surroundings. This is a problem for GR, and it has been known for some time.

A recent, relevant preprint is arXiv:0811.4610. Please study this paper (and references therein)
before commenting on issues whereof you obviously are ignorant.
 
  • #42
Old Smuggler said:
Nonsense. The Hubble law is seen right down to the doorstep of the local group of galaxies, i.e.,
below 3 Mpc. The GR prediction is that the Hubble law should become important at the scale of
clusters of galaxies, i.e., about 40 Mpc.
As the Hubble constant is near of 72km/s/Mpc, at 3Mpc we would be talking about over 200km/s/Mpc, which is starting to get into the velocity dispersion of less massive regions. Why shouldn't it be apparent?

Furthermore, the Hubble flow is very coherent, while velocity dispersion is much less so.

Old Smuggler said:
Moreover, the local Hubble flow is very cold; the velocity
dispersion is much smaller than that obtained in (straightforward) computer simulations of the local group and its nearby surroundings. This is a problem for GR, and it has been known for some time.

A recent, relevant preprint is arXiv:0811.4610. Please study this paper (and references therein)
before commenting on issues whereof you obviously are ignorant.
There is no reason yet to suspect that any problem here is a result of General Relativity. The assumptions that lead to the greater than expected dispersions use linear gravity, and linear gravity is clearly not applicable on these scales. They attempt to correct for this, but these are really small scales. I don't see how we can trust that correction. There are further problems with baryonic physics that are ignored in these calculations.

So no, I see no reason from what you have given me here to suspect there is a genuine problem. Perhaps it will tell us something interesting when more careful work is performed, such as detailed hydrodynamic simulations of structure formation, but it's always going to be really, really difficult to say that we have a genuine misunderstanding of physics here and not just a miscalculation in applying said physics. This is why most of the best data we have in cosmology is on large scales: it's not because we have so much more data, but because the theoretical interpretation of the results is much cleaner.

That said, I fully admit it's possible that this is telling us something interesting. It might, for example, be a hint as to the temperature or decay rate of dark matter. But I'm just not convinced it's genuinely due to new physics, and not just misinterpreting our current theories.
 
  • #43
what came first chiken or egg? in the other sense universe appears firstly or law governing it?
 
  • #44
I'm a little bit puzzled all of a sudden. I don't want to be rude or anything but i just wanted to ask a question to you, Mr. Ahir, how is your post relevant to the origin of the universe? have a good day.
 
  • #45
Mr. Ahir's comment seems justified though perhaps deserving of its own thread.

Why is our Universe composed of spacetime, matter/energy, and a bunch of forces linking it all together? Why wasn't it just a monochromatic beam of Schrodinger cats passing through two giant superstring slits to create an interference pattern of feline superpositions?

Presumably originally all physical laws and constructs were possible and it was the breaking of symmetries which lead to the dispensation which we are privileged to inhabit.

Purest speculation, of course.
 
  • #46
Carid said:
Mr. Ahir's comment seems justified though perhaps deserving of its own thread.

Why is our Universe composed of spacetime, matter/energy, and a bunch of forces linking it all together? Why wasn't it just a monochromatic beam of Schrodinger cats passing through two giant superstring slits to create an interference pattern of feline superpositions?

Presumably originally all physical laws and constructs were possible and it was the breaking of symmetries which lead to the dispensation which we are privileged to inhabit.

Purest speculation, of course.
For this sort of speculation, I rather like Max Tegmark's ideas on the subject. You can read about it on his website here:
http://space.mit.edu/home/tegmark/toe.html
(inside is a link to his paper for a more in-depth description)

The basic, basic idea is that his hypothesis is that perhaps all mathematical structures have real existence. The laws of physics in our universe would be one realization of one of these mathematical structures. This idea can be thought of as a more explicit writing down of the statement, "all fully-consistent sets of physical laws exist." In order to bring this idea forward into the realm of reality, one would have to use some sort of anthropic argument.
 
  • #47
Math as to serve Physics, and not the contrary.
Math is a mental construction not a reality of its own.
 
  • #48
heldervelez said:
Math as to serve Physics, and not the contrary.
Math is a mental construction not a reality of its own.
Why? I'd say Tegmark's argument is very effective here. We know that as long as the laws which govern our universe are fully self-consistent, then it is necessary that those laws are represented by some mathematical structure. We therefore know that at least one mathematical structure has real existence.

Why can't others?
 
  • #49
Chalnoth said:
... We therefore know that at least one mathematical structure has real existence.

Why can't others?

I do not understand which one you have in your mind.
 
  • #50
I also agree with the Mark Tegmarks arguments.

I tried to ask several times: if you believe that our universe is not completely equivalent to a purely mathematical structure then please provide an example of a purely *physical* axiom which can not be expressed in mathematical terms.

Nobody succeded...
 
  • #51
Dmitry67 said:
I also agree with the Mark Tegmarks arguments.

I tried to ask several times: if you believe that our universe is not completely equivalent to a purely mathematical structure then please provide an example of a purely *physical* axiom which can not be expressed in mathematical terms.

Nobody succeded...

This is a philosophical question and the answer must also be like like this:

We can ommit the observers (we) from the universe and it will continue (or go back in our past 10000 years).
Subtract the Universe and we (the owners of math concepts) will not survive.
Clearly one is not 'equivalent' to the other.

Idem reasoning I apply to the hipotetical 'space is topology'.
Topology is a math construct.

We humans use math concepts to make an understable framework of our knowledge.
The egyptians started to measuring (and triangulating...) to say "this is my land" but the land itself is there in the first hand.
 
  • #52
heldervelez said:
I do not understand which one you have in your mind.
None. I don't know which mathematical structure is isomorphic to our universe. Nobody does, yet. But it is enough to know that our universe must be isomorphic to some mathematical structure, because if it weren't, then it wouldn't be consistent. And we obviously can't have that.
 
  • #53
Math as to serve Physics, and not the contrary.
Math is a mental construction not a reality of its own.


Physics is also a mental construct; in fact everything we know is a mental construct.
Yes, there is a reality out there, and we are part of it, and so are our mental constructs. Plato told us we see shadows on the wall; Newton told us he was picking up pretty shells in front of an ocean of knowledge, etc..

However, the real point of this post is so that I can lay claim to inventing the term "Snowflake Universe" to describe the unique mathematical form that each of Professor Tegmark's universes will adopt.
 
  • #54
Hello

What do the actual images tell us?

a) Of all the images that we see the common behaviour is clustering effect.

Matter is drawn into a gravity sink, such as stars and black holes.

Dwarf galaxies cluster around larger galaxies, these cluster and form local cluster of galaxies, these local clusters cluster to form super clusters of clusters of galaxies.


b) That compact objects such as Stars, neutron stars, the theoretical quark stars and black holes form jets that reform their surroundings even effecting the formation of stars and galaxies.

So what we actually see is at random a process that expands and contracts. This is simple enough and yet we try to complicate the observations.
 
  • #55
Sundance said:
So what we actually see is at random a process that expands and contracts. This is simple enough and yet we try to complicate the observations.
It's not complicating the observations. It's just attention to detail.
 
  • #56
Hello

Chalnoth said

It's not complicating the observations. It's just attention to detail.


Detail has been the issue for hundreds of years.

Some create a model and try to fit the data to the model and in many cases with ad hoc ideas. EG: Big Bang Theory
 
  • #57
Sundance said:
Hello
..
Some create a model and try to fit the data to the model and in many cases with ad hoc ideas. EG: Big Bang Theory
(my bold)

'Had Hoc' is very appropriate, even in the case of BB beeing correct.
It was born after the 'discovery that galaxies are moving apart'. Then all matter had to be in some point of space at the same instant.
If Hubble findings (measure) can be interpreted differently than BB is out.
 
  • #58
Sundance said:
Hello

Chalnoth said

Detail has been the issue for hundreds of years.

Some create a model and try to fit the data to the model and in many cases with ad hoc ideas. EG: Big Bang Theory
So when one single added parameter to the theory not only explains the original observation it was originally used for, but also half a dozen other observations made since, we gain some confidence that it is at least approximately accurate.
 
  • #59
Hello


heldervelez said:


'Had Hoc' is very appropriate, even in the case of BB beeing correct.
It was born after the 'discovery that galaxies are moving apart'. Then all matter had to be in some point of space at the same instant.
If Hubble findings (measure) can be interpreted differently than BB is out.

Did I miss something. Actual images show a clustering effect and that galaxies move towards an attractor, a gravity sink so to speak. We see this in the Milky way little group of dwarf galaxies rotaing and gravity bound to the Milky Way. This little group is part of the local group of galaxies with M87 as the centre and this local group is moving towards the great attractor.

If you see galaxies moving away from each other its because they belong to a different group with a different attractor.
 
  • #60
Sundance said:
Did I miss something. Actual images show a clustering effect and that galaxies move towards an attractor, a gravity sink so to speak. We see this in the Milky way little group of dwarf galaxies rotaing and gravity bound to the Milky Way. This little group is part of the local group of galaxies with M87 as the centre and this local group is moving towards the great attractor.

If you see galaxies moving away from each other its because they belong to a different group with a different attractor.
If your supposition here were correct, then galaxies on opposite sides of both attractors would appear to be moving toward one another.
 

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