
#55
Jan1912, 07:21 PM

Astronomy
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PF Gold
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We see no evidence of a boundary, so why put it in the model? Plus it would be a mathematical headache. If there is a boundary forces would be unbalanced and expansion would appear asymmetrical unless you put the earth at center. But putting earth at center is "UnCopernican" The thing a lot of newcomers don't realize is that cosmology is a mathematical science that is primarily concerned with a math MODEL of the universe and that model runs according to the 1915 GR equation (Einstein Field Equation) which is our current law of gravity. If you have a model which must run according to an equation you don't have a lot of freedom to mess around. The GR equation describes the evolving geometry of all space. We know it changes and the law of gravity describes how. On page one of the book you are given a manifold which is all space thru all time, and a metric or distance function that describes the geometrywhat paths are straight and how distances relate to areas and volumes and how distances change thru time etc. Personally I find the idea of a boundary UNINTUITIVE. The universe is supposed to be ALL SPACE. So what would a boundary be separating space from? The GR equation is by far the most accurate law of gravity we've ever hadit gives more precise numbers than the older Newton law. It has been tested repeatedly and checks out every time. And it is a law both of gravity and evolving geometry because they are the same thing (how massive objects influence geometry). So there is a COMMON SENSE reason to base models of the universe's geometry on the GR equation. Its a nobrainer in fact. So if you accept to base your model cosmo on the accepted law of gravity/geometry, then the idea of a boundary has no standing. How to implement? Maybe by having the average density of galaxies gradually peter out so that our region is surrounded by a large "void". But then expansion would most likely be decelerating so let's fix that with a larger cosmological constant, and so on. It is not naturally a part of the picture. Just giving the idea of a boundary a meaning seems likely to lead to headaches and a more complex picture. The discussion gets over into talking about what's called the "cosmological principle". On large scale the universe seems on average uniform. Matter seems uniformly distributed throughout space, at any one time. So we infer looking back in time. At each epoch matter was uniformly distributed at some average density that prevailed at that time. So people say "homogeneous and isotropic" which basically just means evenly distributed on largescale average. That makes the model simple and we see no evidence to the contrary so since the job is to get the simplest model with the best fit, and no evidence to contrary, evenness is assumed. Maybe, as Drakkith suggests, it is at heart an Occam thingthe ancient tradition in mathematical sciences of keeping the model simple. 



#56
Jan1912, 10:47 PM

PF Gold
P: 28

But, no boundary or membrane encapsulating the universe or universes is needed since the vastness of infinite space pretty much takes care of the isolation question on its own. Best, RD 



#57
Jan2012, 01:09 AM

P: 38





#58
Jan2012, 01:28 AM

Astronomy
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P: 22,809

it gets into semantics and I lose interest. most "other universe" talk seems vacuous, devoid of empirical content. I try to keep language simple and consistent with professional usageuniverse is all space and all physical existence.
That seems to be how it is used in 99% of the cosmology research papers that come out daily on the preprint archive. Have a look for yourself. http://arxiv.org/list/astroph.CO/recent Essentially nothing about "multiverse" in the run of mill professional literature. More confined to popular media where they stimulate the imagination in order to sell books. Talk about string theory and God and multiverses and stuff. 



#59
Jan2012, 01:58 AM

P: 38

You obviously don't get my point.
Call our accelerated expaning bubble of galaxies 'A', picture it in space and surround it with 'A2', 'A3', ... 



#60
Jan2012, 07:01 AM

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#61
Jan2012, 07:29 AM

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#62
Jan2012, 07:43 AM

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#63
Jan2012, 08:31 AM

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#64
Jan2012, 09:55 AM

P: 366

How would you see an outer edge or for that matter an inner edge, from our view point we only see signals between objects?




#65
Jan2012, 05:49 PM

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#66
Jan2112, 01:19 PM

P: 38

One day I may make a GPU simulation out of it, to show how it can work.




#67
Jan2412, 07:45 AM

P: 38

Existing models are useful for practical applications but contemplating what is beyond our knowledge domain is paramount to discovery. 



#68
Jan2412, 08:42 AM

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#69
Jan2412, 09:03 AM

P: 366

Just to reinforce some previous comments.
The entire Universe has no center, for it to have a center would also preclude a leading edge. This would violate the Cosmological principle and also undermine relativity by applying different and preferential reference frames. The BB was not a ballistic explosion in a preexisting space and is entirely background independant. To try to assume external vantage points "outside" the Universe is pointless and does not provide any helpful understanding IMO. Now there are edges to the Universe, but these are not spatial; they are temporal. When I stand and look up into the sky I am on the temporal edge of the Universe. I hope this helps and am happy to discuss this further as sometimes it can help for a layperson to explain this. (My head still hurts if I think about it too much.) Cosmo 



#70
Jan2412, 09:16 AM

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#71
Jan2412, 09:48 AM

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#72
Jan2412, 10:42 AM

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