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Does a finite universe make sense to you?

by epkid08
Tags: finite, sense, universe
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sketchtrack
#145
Jul11-08, 01:17 PM
P: 62
I have always been told the universe is expanding faster than C. Isn't the expanding space rather than moving galaxies thing just to satisfy the hypothesis that mass cannot move faster than C?
epkid08
#146
Jul11-08, 01:29 PM
P: 267
Yes.

If the universe wasn't expanding at or faster than c, wouldn't we be able to the edge of the universe?
yuiop
#147
Jul11-08, 02:55 PM
P: 3,967
Quote Quote by mysearch View Post
Hi Chronos: Could I ask what you would briefly list as the top observational evidence that supports a finite universe and what % confidence you have in this evidence?
The latest observations combining supernova, cluster and WMAP CMB data centres on Omega = 1.02 with a error of about +/- 0.02 which means a flat universe (Omega =1.00) is not excluded but the data tends towards a closed universe, but it is a close call.

Quote Quote by mysearch View Post
o As I understand it, dark matter has been speculated based on the observation of a number of gravitational anomalies, i.e. rotation anomalies in spiral galaxies to gravitational lensing around unseen objects?
Yes, gravitational lensing is more important evidense for dark matter. The bullet cluster is a famous example that points towards evidense of dark matter of the WIMP variety.


Quote Quote by mysearch View Post
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o The first candidate were called MAssive Compact Halo Objects (MACHOs) which consisted of Jupiter-sized planets, brown dwarf stars, faint low-mass stars, white dwarf stars and even black holes. However, this idea would only account for a fraction of the dark matter required to explain all the previous anomalies?
Yes.
Quote Quote by mysearch View Post
o I understand that Weakly Interacting Massive Particles (WIMPs) is now the front-runner for the missing matter, which corresponds to the description of Cold Dark Matter? However, the existence of WIMPs has not yet been verified
See above comment about the bullet cluster.

Quote Quote by mysearch View Post
Hi
On a quite expansive note, although still within the scope of the question about the universe being finite or infinite, there was a poll in this forum about the universe being a black hole. I believe the basis of this speculation is linked to the fact that current mass-density of the universe is approximately of the right order to create an event horizon at about the same size of the ‘visible universe’. Note, as I understand it, this mass-density corresponds to normal mass, not dark matter or dark energy, so some questions:

o Would dark matter or dark energy affect the radius of the speculated event horizon?
Yes it would. All mormal matter and dark matter contribute to the density and gravitational effect. Dark energy contributes an antigravity effect. Certainly significant dark energy would exclude the concept of the universe being a black hole. One thing that should be considered in discussions of the universe as a black hole is that in a dynamic situation where all the mass is moving outward then the Schwarzchild radius is not simply R=2GM/c^2. That is the simplistic statc solution. How gravity acts on moving objects is more complicated. This is clearly seen in the coordinate spedd of light of a photon falling towards a black hole. The photon decelerates as it falls c'=c*(1-2GM/R/c^2) while a static object released near the black accelerates. Clearly there is velocity component to how gravity acts on a falling object. If the velovity at a given radius is below a certain critical value gravity accelerates the object and above the critical velocity it decelerates the object. (I am talking about coordinate measurements and not proper measurements).

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o Would this conceptual black hole universe have a centre of gravity?
Depends on whether you want to think in 4D or good old 3D ;)

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o Would your caveat to Newton Shells apply, i.e. do we really understand the meaning of time within a black hole to evaluate the scope of time dilation?
Some people say we cannot understand the physics inside a black hole because when we look at time and distance in proper measurements they are all imaginary numbers for R<Rs and at the singularity at the centre of a black hole the energy/mass density is infinite and doing math with infinite values is very difficult. The conventional interpretation is that the problems go away if we look at things in terms of proper measurements for a falling observer. The proper time of a falling observer is always one second per second. A fallng observer using his own clock can not detect time dilation and would not notice if time stopped or even reversed his own clock always ticks at one second per second as far as he is concerned. Personally, I think too much emphasis is placed on proper measureents as they tell us very little. For example in SR the proper length of a moving object is always it s rest length and the proper clock rate of a moving observer is always it rest clock rate. In other words if we only look at proper time in SR there is no such thing as time dilation and length contraction. Time dilation and length contraction only come about by comparing measurements of observers with different reference frames. That is a coordinate measurement. In GR and analysis of black holes, for some reason they choose to ignore coordinate measurements and concentrate only on proper measurements. I believe that is the wrong approac. I have shown in other posts that while the proper measurements below the event horizon are imaginary the coordinate measurements are real and so the coordinate measurements are valid. From that viewpoint, coordinate measurements show that a singularity can not form at the centre of a black hole, but I should add that is not the conventional viewpoint. Some papers that look at black holes from a quantum point of view tend to agree that singularities do not form in black holes and while that is not yet the accepted view, I detect a momentum of cutting edge opinion in that direction.
epkid08
#148
Jul11-08, 04:39 PM
P: 267
I thought I'd add this, to me, the most intriguing thing about the universe is that you can look up into the sky and know that there lies only one solution.
mysearch
#149
Jul12-08, 04:00 AM
PF Gold
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P: 522
Kev, thanks for your response, I appreciate the clarifications on the dark matter issues. I am not really questioning the data, but wanted to see where people are currently defining the ‘limits of inference’.

Yes it would. All normal matter and dark matter contribute to the density and gravitational effect. Dark energy contributes an antigravity effect. Certainly significant dark energy would exclude the concept of the universe being a black hole.
On the assumption that dark energy accounts for 75% of the energy density, I guess the consensus rejects any sort of black hole speculation?

Depends on whether you want to think in 4D or good old 3D ;)
It is not always clear to me what people are exactly inferring when they use the term 4D, e.g. 3-D space plus time or some form of curved finite - but infinite universe etc.

The conventional interpretation is that the problems go away if we look at things in terms of proper measurements for a falling observer. The proper time of a falling observer is always one second per second.
Time dilation and length contraction only come about by comparing measurements of observers with different reference frames. That is a coordinate measurement. In GR and analysis of black holes, for some reason they choose to ignore coordinate measurements and concentrate only on proper measurements.
I also have some problems in this area as well, but they may differ from yours. When I looked at the arguments relating to coordinate singularity at an event hole, e.g. Gullstrand-Painleve, I couldn’t help feel that it solved the problem only by not looking at it. What I mean is that it just appears to look at local proper time and ignores the implication on relative time in all other frames of reference. However, we are probably digressing into another topic.

Anyway, I shall now do some more reading based on all the information supplied. Thanks for the help.
yuiop
#150
Jul12-08, 09:48 AM
P: 3,967
Quote Quote by epkid08 View Post
I thought I'd add this, to me, the most intriguing thing about the universe is that you can look up into the sky and know that there lies only one solution.
Gödel's incompleteness theorems can be paraphrased as no non trivial theory (solution) can be both consistent and complete. In other words you can look up at the sky and know it is not possible to have a single solution for the universe that is both consistent and complete


See http://en.wikipedia.org/wiki/G%C3%B6...eness_theorems
epkid08
#151
Jul12-08, 11:01 AM
P: 267
Do you really think there isn't a single solution? I don't think it will ever be complete, or consistent as far as human knowledge goes, but I think there does lie a single solution. That very fact is what intrigues me.
robheus
#152
Jul13-08, 07:03 AM
P: 143
Quote Quote by sketchtrack View Post
I have always been told the universe is expanding faster than C. Isn't the expanding space rather than moving galaxies thing just to satisfy the hypothesis that mass cannot move faster than C?
It is the gravitational expansion of the space time metric, not movement of mass in space.


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