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

[Sundance]

G'day from the land of ozzzzz

Is the universe Expanding or accelerating?

Is the Universe Contracting or deccelerating?

Simple observation of how the parts within the universe work gives us an idea.

Also a few scientific papers add weight.


http://arxiv.org/abs/astro-ph/0408383
The Fueling and Evolution of AGN: Internal and External Triggers

Authors: Shardha Jogee (Space Telescope Science Institute)
(Submitted on 20 Aug 2004 (v1), last revised 10 Jul 2008 (this version, v2))

Abstract: In this chapter, I review the fueling and evolution of active galactic nuclei (AGN) under the influence of internal and external triggers, namely intrinsic properties of host galaxies (morphological or Hubble type, color, presence of bars and other non-axisymmetric features, etc) and external factors such as environment and interactions. The most daunting challenge in fueling AGN is arguably the angular momentum problem as even matter located at a radius of a few hundred pc must lose more than 99.99 % of its specific angular momentum before it is fit for consumption by a BH. I review mass accretion rates, angular momentum requirements, the effectiveness of different fueling mechanisms, and the growth and mass density of black BHs at different epochs. I discuss connections between the nuclear and larger-scale properties of AGN, both locally and at intermediate redshifts, outlining some recent results from the GEMS and GOODS HST surveys.

and


http://arxiv.org/abs/0809.0537
[0809.0537] Cosmographic Hubble fits to the supernova data
Cosmographic Hubble fits to the supernova data

Authors: Celine Cattoen (Victoria University of Wellington), Matt Visser (Victoria University of Wellington)
(Submitted on 3 Sep 2008)

Abstract: The Hubble relation between distance and redshift is a purely cosmographic relation that depends only on the symmetries of a FLRW spacetime, but does not intrinsically make any dynamical assumptions. This suggests that it should be possible to estimate the parameters defining the Hubble relation without making any dynamical assumptions. To test this idea, we perform a number of inter-related cosmographic fits to the legacy05 and gold06 supernova datasets. Based on this supernova data, the "preponderance of evidence" certainly suggests an accelerating universe. However we would argue that (unless one uses additional dynamical and observational information) this conclusion is not currently supported "beyond reasonable doubt". As part of the analysis we develop two particularly transparent graphical representations of the redshift-distance relation -- representations in which acceleration versus deceleration reduces to the question of whether the relevant graph slopes up or down. Turning to the details of the cosmographic fits, three issues in particular concern us: First, the fitted value for the deceleration parameter changes significantly depending on whether one performs a chi^2 fit to the luminosity distance, proper motion distance or other suitable distance surrogate. Second, the fitted value for the deceleration parameter changes significantly depending on whether one uses the traditional redshift variable z, or what we shall argue is on theoretical grounds an improved parameterization y=z/(1+z). Third, the published estimates for systematic uncertainties are sufficiently large that they certainly impact on, and to a large extent undermine, the usual purely statistical tests of significance. We conclude that the supernova data should be treated with some caution.

 

[heldervelez]

Now that almost everyone are saying that most of the universe (~75%) is Dark Energy theese guys are robbing all that content ?
As the model says nothing on the contents of DE (>0,=0,<0) any value will be ok.
But a good model should say something.

 

[Sundance]

G'day

Heldervelez said

Now that almost everyone are saying that most of the universe (~75%) is Dark Energy theese guys are robbing all that content ?
As the model says nothing on the contents of DE (>0,=0,<0) any value will be ok.
But a good model should say something.

What is dark energy and what is dark matter?

Is there a difference?

In what context are they referring to.

Most of the matter/energy are found as degenerate matter/energy in and around compact objects and since we cannot see this matter , but for the gravity influence we call these dark matter/energy.

 

[Nereid]

[...]

Most of the matter/energy are found as degenerate matter/energy in and around compact objects and since we cannot see this matter , but for the gravity influence we call these dark matter/energy.

(bold added)

This (bold part) seems to be a common theme in your posts, Sundance.

However, as stated it also seems quite inconsistent with relevant astronomical observations.

For example, the mass of a typical rich cluster of galaxies is estimated to be distributed widely throughout the cluster, over ~several Mpc, and almost none of this mass is "degenerate", much less "in and around compact objects".

But perhaps it's just that your words are insufficiently precise; would you mind clarifying please?

Specifically:

* how is the "degenerate matter/energy" in a typical rich cluster of galaxies distributed?

* what do you mean by "in and around" (compact objects)? as in what distance is a typical "around" one?

 

[Chalnoth]

Most of the matter/energy are found as degenerate matter/energy in and around compact objects and since we cannot see this matter , but for the gravity influence we call these dark matter/energy.

Much of the time this is true. All we had to do was look hard enough until we found a situation for which it wasn't. Behold the Bullet Cluster. Scientific paper http://www.iop.org/EJ/abstract/1538-4357/648/2/L109/.

The basic idea here is that since dark matter doesn't interact, or interacts very weakly, during a collision it should pass straight through. If we're talking about a pair of galaxy clusters that are colliding, the galaxies should do the same, as they will mostly just miss one another.

But most of the normal matter in a galaxy cluster is not in its galaxies. Most of the normal matter is in this very hot, diffuse gas that is visible in x-rays. So what Clowe et. al. have done here is they compared the x-ray image of the cluster (where you can clearly see the results of the collision: one cluster has "punched through" the other) to the gravity lensing mass reconstruction of the cluster. They find that the mass is around the galaxies (which means around the dark matter), and not around the hot x-ray gas that got stuck in the middle after the collision.

Dark energy is a different beast entirely, and we don't expect it to be associated with normal matter by much, if at all.

 

[mehul ahir]

how our universe startsa expanding ? what force cause it to expand?

 

[Chalnoth]

how our universe startsa expanding ? what force cause it to expand?

Answering that question would require understanding of cosmic inflation began. Since we don't yet even know the specifics of what cosmic inflation was, it's a bit premature to talk about how it began. So the short answer is: we don't yet know.

 

[Chalnoth]

What makes people think that the universe had a start?

A start from what?

Well, our region of the universe had a start. But the entire universe? We don't know. We also don't know what our own region started from.

If there is an expansion in the universe, than why do the images show a clustering nature, such as Star cluster, galaxy unit, cluster of galaxies, super cluster of clusters of galaxies and so it goes on.

The Jeans' Length describes this. Basically, the expansion is a large-scale phenomenon. For stuff that is close together, the local density outweighs the overall expansion, and stuff collapses. The length scale between collapse and expansion is called the Jeans' Length, and it depends upon just how overdense the region is.

Normal matter makes up about 4% and the rest can be called Plasma or degenerate matter that is found in compact objects such as Stars, Neutron stars excotic stars such as Quark and Neutrino and the so called black holes, stellar black holes and super massive ultra compact matter.

Er, plasma and matter in stars and other compact objects is considered normal matter. This is much of the stuff that makes up that 4%.

 

[Nereid]

Hello all

This is for Nereid

http://arxiv.org/abs/0812.4813
Angular Energy Distribution of Collapsar-Jets

Authors: Akira Mizuta, Miguel A. Aloy
(Submitted on 28 Dec 2008)

Interesting work, thanks.

What, may I ask, does this paper have to do with the origin of the universe?

And irrespective of that answer, what does it have to do with degenerate matter?

What makes people think that the universe had a start?

A start from what?

There are several answers to these questions, in addition to those provided by Chalnoth.

If you make models from GR and the Standard Model (of particle physics; SM), add CDM and DE and inflation, you can show these models make predictions that are consistent with almost all relevant astronomical observations*.

In these models the currently observable universe 'starts' with physical conditions at the extreme edge of the domain of applicability of the SM; the physical state is one of high temperature and density, with fairly well constrained composition (quarks, anti-quarks, gluons, photons, leptons, etc).

If there is an expansion in the universe, than why do the images show a clustering nature, such as Star cluster, galaxy unit, cluster of galaxies, super cluster of clusters of galaxies and so it goes on.


Normal matter makes up about 4% and the rest can be called Plasma or degenerate matter that is found in compact objects such as Stars, Neutron stars excotic stars such as Quark and Neutrino and the so called black holes, stellar black holes and super massive ultra compact matter.

The estimated total mass of all electron degenerate objects (white dwarfs), nuclear degenerate objects (including possibly exotic hadronic degenerate states; neutron stars), and black holes (of stellar mass and above) in the observable universe is trivial compared with the estimated mass of baryons in the IGM (and inter-cluster medium) in the form of low density, high temperature plasma (elemental composition mostly H with some He; metals make up a tiny fraction).

* consistency with 'lab physics' - GR, QED, QCD, etc - is built-in

 

[Nereid]

Hello Nereid

It seems you read different papers than me on astropysics.

Maybe you can refer me to the properties of jets from your reading and the different types of jets that are formed from various locations.

and maybe what actually triggers the jets, and how the main jet keeps a stable position for millins of years.

The main trigger for jets is a via a magnetic entanglement and when this happens degenerate matter is formed.
Are you familiar with the process.

Maybe a mentor could split this post, and maybe the previous one of yours, from this thread and attach it to https://www.physicsforums.com/showthread.php?t=284178"?

It seems a discussion of the astrophysics of jets is quite out of place in a thread entitled "Origins of the Universe" in the Cosmology section!

 

[Janus]

Maybe a mentor could split this post, and maybe the previous one of yours, from this thread and attach it to https://www.physicsforums.com/showthread.php?t=284178"?

It seems a discussion of the astrophysics of jets is quite out of place in a thread entitled "Origins of the Universe" in the Cosmology section!

Moved.

 

[heldervelez]

about the OP question:

What have been said is that space is expanding.

Just because we are measuring an increase on the ratio space/matter, we can not say 'ad hoc' that it is an expansion. Other scenarios are possible.

Not to mention the limiting philosophical concern, put in evidence in the http://en.wikipedia.org/wiki/Sphere-world" , that ultimately limits our knowledge.

 

[Chalnoth]

Just because we are measuring an increase on the ratio space/matter, we can not say 'ad hoc' that it is an expansion. Other scenarios are possible.

Er, no. Other scenarios are not possible. It is expansion. Now, it is conceivably possible that one may use some different coordinate system to describe what is going on in a different way, but this does not invalidate the picture of an expanding universe. The expanding universe picture is still going to be accurate, because it's been demonstrated to be accurate.

Not to mention the limiting philosophical concern, put in evidence in the http://en.wikipedia.org/wiki/Sphere-world" , that ultimately limits our knowledge.

Er, that wikipedia article shows precisely how you'd examine whether or not we were living in such a world: by measuring the curvature of space.

 

[heldervelez]

Er, no. Other scenarios are not possible. It is expansion. Now, it is conceivably possible that one may use some different coordinate system to describe what is going on in a different way, but this does not invalidate the picture of an expanding universe. The expanding universe picture is still going to be accurate, because it's been demonstrated to be accurate.


Er, that wikipedia article shows precisely how you'd examine whether or not we were living in such a world: by measuring the curvature of space.

I did not invalidate the scenario of expansion, neither took an Emphasis on coordinate system.

Inherently to the act of measuring, is the acceptance of a measuring rod as standard. Well, this is allways a material piece that we take from our local world, and assume to be constant. We must state all the assumptions and be aware of them.

It seems to me that the assertion of expansion has an hidden assumption: matter is a constant.

What I've said, not being biased and removing assumptions, is that we are "measuring an increase on the ratio space/matter".

"measuring the curvature of space[" -- AFAIK it is globally flat, as we measure it. And I do not put GR in question.
The experience of Poincaré is another way of saying (like a member of this forum remember us) "A fish is not aware of the water"

 

[Chalnoth]

Inherently to the act of measuring, is the acceptance of a measuring rod as standard. Well, this is allways a material piece that we take from our local world, and assume to be constant. We must state all the assumptions and be aware of them.

It seems to me that the assertion of expansion has an hidden assumption: matter is a constant.

Well, no. It isn't a hidden assumption. It's a very explicit assumption: that of conservation of the stress-energy tensor. The conservation of this tensor, in fact, is required in General Relativity.

 

[heldervelez]

Well, no. It isn't a hidden assumption. It's a very explicit assumption: that of conservation of the stress-energy tensor. The conservation of this tensor, in fact, is required in General Relativity.

GR was written by Einstein as we know, and as of the moment of writing we was not considering an expanding universe. So the "The conservation of this tensor, in fact, is required in General Relativity" is much more natural and easy to apply in an universe as the way Einstein thought. The expanding universe came later. And so, we can not advocate against Einstein easily.
What you mean is, I think, that the overall mass-energy (or energy-momentum) content is conservative. In a expanding universe we have non conservative possibilities: photons loose energy, and a redefinition of 'energy' or 'conservation' is eventually needed. The overall 'temperature' of the universe is also lowering. 'Temperature' definition as a energy equivalent is in trouble.
You say "...conservation of the stress-energy tensor..." but I do preffer to think about "..overall mass-energy content is conservative.." with the meaning that no matter-energy has been created/destroyed after the initial moment of the universe. I think that we have the same concept in mind but expressed differently.

 

[Chalnoth]

While Einstein was the originator of General Relativity, the development of GR did not stop with him, and nor does the status of the theory rest upon Einstein's formulation. There is no reason whatsoever to spent one iota of time worrying about what sorts of things Einstein considered likely or not. What is important is what the evidence says.

And you can prefer saying that overall mass-energy content is conservative all you like, but it's still wrong.

 

[Sundance]

Hello All

Can you actually see exapansion of the universe?

Chalnoth said

Er, no. Other scenarios are not possible. It is expansion. Now, it is conceivably possible that one may use some different coordinate system to describe what is going on in a different way, but this does not invalidate the picture of an expanding universe. The expanding universe picture is still going to be accurate, because it's been demonstrated to be accurate.

Can yu supply an image that actually shows expansion?

I know they speak of space/time expansion based on ad hoc ideas.

 

[Chalnoth]

Hello All

Can you actually see exapansion of the universe?

Can you see atoms? No? How do you know they are there?

Can yu supply an image that actually shows expansion?

I know they speak of space/time expansion based on ad hoc ideas.

Saying that and backing it up are two entirely different things.

 

[Sundance]

Hello


An image from out there, would be nice.

Maths and graphs are down here.

 

[Chalnoth]

HelloAn image from out there, would be nice.

Maths and graphs are down here.

And as "expansion" relates velocity to position, it's not something you can simply view in a telescope image. It's something you infer from telescope images, but not simply by looking at them with your eyes. What you are asking for is impossible. You know it's impossible, but somehow you think that is important. It isn't.

 

[matt.o]

And as "expansion" relates velocity to position, it's not something you can simply view in a telescope image. It's something you infer from telescope images, but not simply by looking at them with your eyes. What you are asking for is impossible. You know it's impossible, but somehow you think that is important. It isn't.

It may be possible if and when the new generation of 30m plus telescopes come on board. Observing a high redshift source over a few decades using high resolution spectroscopy should be able to show that the redshift changes with time due to expansion. The spectra need to have centimetre per second resolution, but apparently this will be achievable in the future, given the instruments (e.g. http://adsabs.harvard.edu/abs/2008Sci...321.1335S") get made for the telescopes (and, indeed the ELTs go ahead).

See http://adsabs.harvard.edu/abs/2008MNRAS.386.1192L".

 

[marcus]

... centimetre per second resolution, but apparently this will be achievable ...

Beautiful, would/will be an astonishing achievement. Thanks for the links.

 

[Sundance]

Hello all


Chalnoth said

And as "expansion" relates velocity to position, it's not something you can simply view in a telescope image. It's something you infer from telescope images, but not simply by looking at them with your eyes. What you are asking for is impossible. You know it's impossible, but somehow you think that is important. It isn't.

Is that a trick question?

Observations are the key to understanding.

We assume that redshift is a measure of velocity?

We asssum that the King wears invisible robes.

The bottom line is:

How can space expand?

It cannot be answered by science, but for by ad hoc ideas.

This is quite interesting.
The dark side of gravity: Modified theories of gravity
http://adsabs.harvard.edu/abs/2008arXiv0807.1640L


and

Long-tailed distributions and non-uniform expansion in the analysis of large-scale structure.
http://adsabs.harvard.edu/abs/1997PhyA..245..253S


and

http://th-www.if.uj.edu.pl/acta/vol39/pdf/v39p1501.pdf

EXPANSION OF THE UNIVERSE — MISTAKE OF
EDWIN HUBBLE? COSMOLOGICAL REDSHIFT AND
RELATED ELECTROMAGNETIC PHENOMENA IN
STATIC LOBACHEVSKIAN (HYPERBOLIC) UNIVERSE

J. Georg von Brzeski
Helios Labs., 945 Hoxett St., Gilroy 95020 CA, USA
jgvb@helioslabs.com
(Received November 5, 2007)

As an alternative to the Big Bang (the standard model), we present
a mathematical theory of cosmological redshift. We show that a fundamental
formula of Lobachevskian (hyperbolic) geometry describes cosmological
redshift and the Doppler effect as well. As presented here, the
cosmological redshift preserves wavelength ratios (it shifts uniformly the
whole electromagnetic spectrum), it is scale invariant, it is a monotonically
increasing function of distance, and it is source independent. It agrees
with all experimental data. The distortion introduced by imaging from
hyperbolic into Euclidean space and the limitations of Special Relativity
are discussed. Physical observations in Lobachevskian space are discussed
and the new formula relating redshift and/or Doppler shift to aberration is
given. An analysis is presented of an erroneous origin of Hubble’s so called
velocity distance law.
PACS numbers: 98.70.Vc, 04.80.Nn, 98.80.–k, 98.80.Es

and

http://arxiv.org/abs/astro-ph?papernum=0509630
Research on candidates for non-cosmological redshifts

Authors: M. Lopez-Corredoira, C. M. Gutierrez
(Submitted on 21 Sep 2005 (v1), last revised 26 Sep 2005 (this version, v2))

Abstract: (Abridged:) The paradox of apparent optical associations of galaxies with very different redshifts, the so-called anomalous redshift problem, is around 35 years old, but is still without a clear solution and is surprisingly ignored by most of the astronomical community. Statistical correlations among the positions of these galaxies have been pointed out by several authors. Gravitational lensing by dark matter has been proposed as the cause of these correlations, although this seems to be insufficient to explain them and does not work at all for correlations with the brightest and nearest galaxies. Some of these cases may be just fortuitous associations in which background objects are close in the sky to a foreground galaxy, although the statistical mean correlations remain to be explained and some lone objects have very small probabilities of being a projection of background objects.
The sample of discordant redshift associations given in Arp's atlas is indeed quite large, and most of the objects remain to be analysed thoroughly. For about 5 years, we have been running a project to observe some of these cases in detail, and some new anomalies have been added to those already known; For instance, in some exotic configurations such as NGC 7603 or NEQ3, which can even show bridges connecting four object with very different redshifts. Not only QSOs but also emission-line galaxies in general are found to take part in this kind of event. Other cases are analyzed: MCG 7-25-46, GC 0248+430, B2 1637+29, VV172 and Stephan's Quintet.

 

[Nereid]

Hello all


Chalnoth said



Is that a trick question?

Observations are the key to understanding.

We assume that redshift is a measure of velocity?

No ... science, especially physics, is built on consistency.

For example: "Here's a theory (GR, say); from this the following may be derived/predicted: {list}. The observations and experiments we've done, to test this theory, all show (quantitative) consistency with the predictions/derivations from that theory."

If you, in translating the theory into terms you are comfortable with, choose to paraphrase that consistency as "we assume redshift is a measure of velocity", you are perfectly free to do so (obviously).

However, if you wish to keep full track of the details - as you seem to - then you would be able to fill in all the steps between the (vast numbers of) observations and the shorthand conclusion from application of an extremely successful theory.

In reading your many posts, Sundance, it seems that these vital connections may not be fully appreciated.

We asssum that the King wears invisible robes.

If we are discussing physics - cosmology in this case - this kind of comment is neither helpful nor pertinent.

The bottom line is:

How can space expand?

It cannot be answered by science, but for by ad hoc ideas.

Er ... no.

And several PF members have spent some time already trying to show you how your understanding is askew.

Perhaps a simple way to get a glimpse of this is to ask you, Sundance, what you think "space" means? When you use the term - as above - what do you mean?

This is quite interesting.
The dark side of gravity: Modified theories of gravity
http://adsabs.harvard.edu/abs/2008arXiv0807.1640L


and

Long-tailed distributions and non-uniform expansion in the analysis of large-scale structure.
http://adsabs.harvard.edu/abs/1997PhyA..245..253S


and

http://th-www.if.uj.edu.pl/acta/vol39/pdf/v39p1501.pdf

EXPANSION OF THE UNIVERSE — MISTAKE OF
EDWIN HUBBLE? COSMOLOGICAL REDSHIFT AND
RELATED ELECTROMAGNETIC PHENOMENA IN
STATIC LOBACHEVSKIAN (HYPERBOLIC) UNIVERSE

J. Georg von Brzeski
Helios Labs., 945 Hoxett St., Gilroy 95020 CA, USA
jgvb@helioslabs.com
(Received November 5, 2007)




and

http://arxiv.org/abs/astro-ph?papernum=0509630
Research on candidates for non-cosmological redshifts

Authors: M. Lopez-Corredoira, C. M. Gutierrez
(Submitted on 21 Sep 2005 (v1), last revised 26 Sep 2005 (this version, v2))

I guess that depends on what you find interesting, doesn't it?

Here, in this part of PF, perhaps a more pertinent question might be something like "to what extent do you, Sundance, regard these as observations ("the key to understanding")?" and, wrt the last, "in science, how should anomalies be addressed?"

 

[mehul ahir]

first what egg or hen? first what universe or law governing it?

 

[Sundance]

Hello Nereid

What is space?

Space without matter? Is infinity

Space with matter? This is the space between particles.

Expansion of space is the actual distance between particles or wave centres being an intrinic property. This is spacetime and not actual motion of the outward movement into space.


Actual images cannot show the expansion of the universe. They do show a clustering effect.

Accoding to the BBT the galaxies receed from each other. This is not observed by any images.

 

[Nereid]

Hello Nereid

What is space?

Space without matter? Is infinity

Space with matter? This is the space between particles.

Expansion of space is the actual distance between particles or wave centres being an intrinic property. This is spacetime and not actual motion of the outward movement into space.

[...]

PF has a section called https://www.physicsforums.com/forumdisplay.php?f=70", and there are many excellent books and website on these parts of modern physics.

I suggest that you take some time to become familiar with SR and GR, because I think your lack of understanding of them is hampering your attempts to understand modern cosmology.

For example you use of "the actual distance" in the post I'm quoting implies (to me) that you are working from the idea that distances are somehow absolute and fixed. Yet you follow this with an implication that distances are what rulers measure. I don't think this section of PF is the place to go over SR and GR.

I'll address the rest of your post later.

 

[Nereid]

first what egg or hen? first what universe or law governing it?

Does it matter, scientifically speaking?

How could you, or anyone, go about designing an experiment or doing an observation that would test hypotheses derived from this question?

 

[Nereid]

(continued)

[...]

Actual images cannot show the expansion of the universe. They do show a clustering effect.

Accoding to the BBT the galaxies receed from each other. This is not observed by any images.

First, let's address the 'expansion of the universe', and 'galaxies recede from each other'.

These are conclusions from applying a particular interpretation to astronomical data, interpretations based on the theory of General Relativity (GR).

A good question to ask might be: to what extent is the 'expanding/galaxies receding' conclusion unique? For example, is it possible to conclude that the universe is shrinking?

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. http://www.bautforum.com/space-astr...3475-other-way-look-universes-expansion.html" is an example of a discussion reflecting this.

And this illustrates a broader point, one that it seems to me you keep tripping up over, Sundance. Namely, that 'observations' - whether 'actual images' or not - are thoroughly intertwined with huge amounts of (physics) theory. One corollary is that in accepting that a particular .jpg file produced from something beamed down from the Hubble Space Telescope (for example) is an 'image', you are also - implicitly - accepting that the great many theories in physics embedded in the production of that .jpg file are faithful summaries of how the universe works (or something like this).

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<.

 

[heldervelez]

(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'.

 

[Nereid]

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?

 

[Sundance]

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.

 

[heldervelez]

...

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.

 

[Nereid]

...

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?