Multiverse cosmology


by Otherkin
Tags: cosmology, multiverse
Chalnoth
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#19
Nov22-11, 07:39 PM
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Quote Quote by Cosmo Novice View Post
I thought you were reffering to things smaller than this!

I thought this was the point though - isotropy and homogoneity exclude local variance - so yes individual galaxies may look different, clusters and superclusters etc. Overall as a whole though the OU complies with the principle of isotropy and the geometric expansion due to the scale factor being the mechanism that moderates said isotropy. As long as the scale factor is uniform which we know it is, then I am failing to see how larger scales will be anisotropic, in fact I would expect the MORE Universe we take into consideration then the more isotropic U would be. Evidence from this is seen in the 1/1000 uniformity of the CMB?

As always anything I have misunderstood I am welcome to correction.

Cosmo
Bear in mind that the differences from homogeneity and isotropy exist on all scales. The statement that our universe is homogeneous and isotropic is a statistical statement, which is another way of saying it's an approximation.

But just fyi, once you remove the dipole of the CMB (which is mostly due to our motion), the CMB is uniform to about one part in 100,000.
Mind Bender
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#20
Nov22-11, 11:17 PM
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I have always found Multiverse theories to be fascinating! Personally I think there is some truth to the many-worlds interpretation...every event has an infinite number of probable outcomes. Each of these outcomes really do manifest...just not in a single Universe. For example, suppose the Nazis had developed nuclear weapons before the United States did? In an alternate/parallel Universe the Nazis, did in fact, develop nuclear weapons first and conquered most of the world.
Chronos
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#21
Nov23-11, 12:23 AM
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There is no compelling observational evidence of other 'universes'. Most rational theories that propose such a thing admit the idea is inherently unprovable.
skydivephil
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#22
Nov23-11, 02:36 AM
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Its important to distinguish the mulitverse that follows from eternal inflation and the many worlds interpretation of Qm. These are generally considered (apart from some recent talk of a link through holography) seperate ideas.
The multiverse of eternal inflation arises from the idea that the universe found itself in a state that exponentially expands , as it does so it decays creating a "pocket universe". But the remaining material is exponentially expanding, so that in the next half life there is not less of the original material as in a normal decay process. Hence the process is eternal conitnually creating "pocket universes". As has been pointed there has been the possibility of testing this idea with observations of the CMB. Read here for a laymans guide to how this might be done:
http://blogs.discovermagazine.com/co...se-guest-post/

Im not aware of any suggestion of how one might test the many worlds interpretation of QM.

Another important point, the multiverse of eternal inflation arose as a consequence of normal inflation. Normal inflation according to Guth was a consequence of trying to solve the monopole problem. So ,no, it was not created as a way of solving any issue with fine tuning of various constants. Although ironically inflation was immediatlely shown to solve the fine tuning of one constant, Omega, but not via creating a multiverse, this wasnt realised until later. Most of the supporters of inflation: Guth, Vilnekin, Linde etc and its detractors Turok, Steindhart etc agree that inflation produces a multiverse. In this sense the mutliverse is not a theory its a proposed consequecne of a theory: inflation. I would highly reccomend this article:
http://arxiv.org/abs/hep-th/0702178
or reading Guths book for a historical account of how the theory came about .
http://www.amazon.com/Inflationary-U.../dp/0201328402
Cosmo Novice
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#23
Nov23-11, 06:37 AM
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Quote Quote by Chalnoth View Post
Bear in mind that the differences from homogeneity and isotropy exist on all scales. The statement that our universe is homogeneous and isotropic is a statistical statement, which is another way of saying it's an approximation.

But just fyi, once you remove the dipole of the CMB (which is mostly due to our motion), the CMB is uniform to about one part in 100,000.
Ok I see what you are saying.

Thanks for the information Chalnoth.
Chalnoth
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#24
Nov24-11, 06:14 AM
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Quote Quote by PeterKinnon View Post
A model which, at the expense of some of our inherited anthropocentric conceits, conforms to the principle of parsimony and also circumvents the notions of "intelligent design" which derive from the hearsay of superstitious mythology.
Such a model would be neat, but there is no evidence whatsoever that any such model exists. Until we have good evidence in support of such a model, the default assumption should absolutely be a model which predicts a proliferation of universes.
PeterKinnon
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#25
Nov24-11, 12:30 PM
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Quote Quote by Chalnoth View Post
Such a model would be neat, but there is no evidence whatsoever that any such model exists. Until we have good evidence in support of such a model, the default assumption should absolutely be a model which predicts a proliferation of universes.
Before affirming that no such model exists you should perhaps take the trouble to read the book that presents it. Or are you saying the book does not exist?

The model, and its evidential basis is outlined therein. It exists.

Whether it corresponds to a plausible interpretation of reality is a different matter.

That is for the reader to form his/her own conclusions.

It does not, by the way, preclude a multiverse scenario. Indeed, I happen to intuitively favour a particular version. But like all the others, it is pure speculation
Chalnoth
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#26
Nov24-11, 04:17 PM
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Quote Quote by PeterKinnon View Post
Before affirming that no such model exists you should perhaps take the trouble to read the book that presents it. Or are you saying the book does not exist?

The model, and its evidential basis is outlined therein. It exists.
Books are not where scientific results are published.
PeterKinnon
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#27
Nov24-11, 04:37 PM
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The multiverse concept and dark energy/matter are certainly NOT scientific results.

Along with the model presented in my book they are interpretations.

Except that, for my model , I use well-established principles from fields such biology and chemistry rather than the more tenuous observations of cosmology.
Chalnoth
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#28
Nov24-11, 05:33 PM
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Quote Quote by PeterKinnon View Post
The multiverse concept and dark energy/matter are certainly NOT scientific results.

Along with the model presented in my book they are interpretations.
*sigh*

Every result in science is an interpretation. We accept a model, such as the electron, as likely to be true when it matches with a wide body of diverse evidence.

Quote Quote by PeterKinnon View Post
Except that, for my model , I use well-established principles from fields such biology and chemistry rather than the more tenuous observations of cosmology.
That's just sad.
marcus
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#29
Nov25-11, 02:22 PM
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I glanced at this thread just briefly, not enough to join discussion. But want to make a comment.
Skydivephil makes a lot of good points in his cosmology forum posts but I think he is only partially right in something here.

He says "multiverse is a logical consequence of inflation". That's not true for inflation itself, but for a lame idea of an inflation mechanism.

Inflation happens normally in Loop cosmology without assuming any "quantum fluctuations" or other leaps of faith that are then hard to turn off or get rid of.

In that context there is no reason for inflation to be eternal or chaotic or happen repeatedly. It is just something that happens (under fairly mild assumptions) in connection with a particular quantum relativistic collapse and rebound.

It can be treated as a one-time process that is part of a one-time bounce. No need to go beyond that into grandiose realms of untestable fantasy. In Loop context, adequate one-time inflation is mundane and robust. The physics of the bounce itself sets it up and triggers it.

It's when you don't have a simple straightforward explanation for adequate inflation at the start of expansion, that you have to invoke leap-of-faith explanations that then will not stop producing universes. This then becomes an embarrassment, as in the Sorcerer's Apprentice story where the magic keeps on fetching water far beyond what is wanted or relevant to the problem.
Chronos
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#30
Nov27-11, 02:09 AM
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I prefer models that model observation . . . Such as this universe. That is not speculative, it is observationally supported.
skydivephil
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#31
Nov28-11, 05:01 AM
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Just to clarify what I said was :

"In this sense the mutliverse is not a theory its a proposed consequecne of a theory: inflation. I would highly reccomend this article:
http://arxiv.org/abs/hep-th/0702178
or reading Guths book for a historical account of how the theory came about .
http://www.amazon.com/Inflationary-U.../dp/0201328402"

I also said eternal inflation arises from normal inflation. I didnt mean to imply this was the only the logical cosequence of inflation and if I gave that impression I apologise. Thast why I like the phrase "a proposed consequecne of a theory". What I meant was the idea of a mutlvierse arises from a certain approach to analysing what inflation implies. Its not something that was simply invented to get rid of fine tuning problems which was what was being claimed. That approach is outlined by Guth in the links above. Whether hes right, I have no opinion.

As fas as LQC being incompatible with eternal inflation. I didnt find any papers on this. I did write to Martin Bojowald (for those that dont know, he wrote one of the first papers on the LQC bounce and the popular article that made the front cover of Scientific American )about it and you may find his reply ineteresting.
He said "LQC is consistent with eternal infaltion...combining eternal inflation with LQC has not been done in detail yet becuase it is technically complicated, but conceptually you would get a picture in which there is a bounce leading from collapse to expansion, followed by several phases of inflation in the expanding branch".

Of course thats just one man's opinion. So if you could link to anything that opposes that view, I would welcome any material you may have.
marcus
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#32
Nov28-11, 02:40 PM
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Phil, I have trouble with some of the language. So I will quote some of the language in your post and try to sort out and respond.
Quote Quote by skydivephil View Post
Just to clarify what I said was :

"In this sense the mutliverse is not a theory its a proposed consequecne of a theory: inflation...
I also said eternal inflation arises from normal inflation. I didnt mean to imply this was the only the logical cosequence of inflation ...What I meant was the idea of a mutlvierse arises from a certain approach to analysing what inflation implies...

As fas as LQC being incompatible with eternal inflation. ...
He said "LQC is consistent with eternal infaltion...
...
I hope you realize I did not say LQC was incompatible with some inflation scenario.

We have to distinguish carefully between being consistent with and implying.

If the LQC bounce implies some X as a consequence then if have the bounce then you have to have X.

If LQC bounce is merely consistent with some Z it merely means it doesn't rule out the possi bility. If you have the bounce it doesn't prevent the possibility that, under whatever additional assumptions are necessary, Z might happen. In other words it is not incompatible with Z. That does not mean that LQC implies Z.
George Jones
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#33
Nov28-11, 03:37 PM
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Please keep religious discussion, either pro or con, out of all posts in this thread, and out of all posts in the science forums at Physics Forums.
Tanelorn
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#34
Nov28-11, 09:48 PM
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Quote Quote by George Jones View Post
Please keep religious discussion, either pro or con, out of all posts in this thread, and out of all posts in the science forums at Physics Forums.

George, sorry I will comply with the rules. In haste I was trying to recall Hawking's arguments with reference to reasons why a Multiverse might be a possibility.
Just to be clear does this also include terms like "creator"? etc.

I think you might have to delete alot more posts though - about 118!
http://www.physicsforums.com/search....archid=2986718
skydivephil
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#35
Nov29-11, 04:42 AM
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[QUOTE=marcus;3639220]Phil, I have trouble with some of the language. So I will quote some of the language in your post and try to sort out and respond.


I hope you realize I did not say LQC was incompatible with some inflation scenario.

We have to distinguish carefully between being consistent with and implying."
-----------
Yes thats make a lot of sense. I relaise that being compatible with and implying are not the same, very good point as always.
My personal worry would be that if eternal inflation and LQC are both true, and I recognise this is a big if, that might remove the bounce from detectability. Do you agree this is correct or have I misunderstood?
marcus
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#36
Nov29-11, 11:49 AM
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Quote Quote by skydivephil View Post
...
My personal worry would be that if eternal inflation and LQC are both true, and I recognise this is a big if, that might remove the bounce from detectability. Do you agree this is correct or have I misunderstood?
I think you are right about that. Can't speak with much confidence about the such inflation scenarios so I have to emphasize that I just suspect that's right, without feeling sure.

In practical research community terms the main issue is what was the immediate cause of the big bang or as I prefer to say the immediate cause of the start of expansion.

Regarding that question "a random quantum fluctuation in some unknown physics" and "LQG bounce" are two competing answers. In a practical, immediate problem sense, I mean.

LQG is comparatively mundane unexotic and I expect in the near/medium term they'll be looking for signs of it having happened. If they see signs then research time and money will go more into studying that, with less interest in eternal inflation.

On the other hand if they don't see signs of bounce having happened then I would expect interest in Loop cosmology and perhaps LQG as a whole to wane. With more active interest in some random quantum fluctuation as a trigger for expansion.


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