# Where is the center of the universe?

by thetexan
Tags: universe
Astronomy
PF Gold
P: 23,218
 Quote by Drakkith They don't say there isn't an edge. They say that the model doesn't require there to be an edge for it to work. As far as I know at least.
There is some truth to that! You can put it in terms of Occam Razor commandment---Thou shalt not make thy models unnecessarily complicated.

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 "Un-Copernican"

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 geometry---what 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 had---it 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 no-brainer 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 thing--the ancient tradition in mathematical sciences of keeping the model simple.
PF Gold
P: 28
 Quote by marcus 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?
Precisely. "Bubble theory" as a model for the uber-universe, does not require any membranes around the bubbles but rather they may simply be viewed as cosmological villages in the vast darkness, collections of local activity activated by some localized phenomenon such as a big bang and separated from any other such areas of activity by lots and lots of nothing, or something or what have you. And, of course, the mass and energy making up the village are reorganized every 14 billion years or so, at least in this mind model. It's always been there and always will be there following the various conservation laws.

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
P: 38
 Quote by marcus 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.
 Astronomy Sci Advisor PF Gold P: 23,218 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 usage---universe 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/astro-ph.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.
 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', ...
Mentor
P: 11,844
 Quote by voxilla 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', ...
What about it? Even if it's possible, there's no way for us to know at the moment. Our current theory is difficult enough as it is without trying to complicate it with unknowable stuff.
P: 53
 Quote by Chalnoth The problem is that explosions are messy. Really messy.
I think that sometimes are not that messy :)
P: 4,800
 Quote by minio I think that sometimes are not that messy :)
I believe that you only get this nice picture in a few, very specific wavelengths.
P: 38
 Quote by Drakkith What about it? Even if it's possible, there's no way for us to know at the moment. Our current theory is difficult enough as it is without trying to complicate it with unknowable stuff.
At least it can explain the acceleration of expansion, because of attraction to surrounding A2, A3, ... without need for dark stuff, isn't that a simplification ?
 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?
Mentor
P: 11,844
 Quote by voxilla At least it can explain the acceleration of expansion, because of attraction to surrounding A2, A3, ... without need for dark stuff, isn't that a simplification ?
Absolutely not. And it doesn't even explain the accelerating expansion.
 P: 38 One day I may make a GPU simulation out of it, to show how it can work.
P: 38
 Quote by marcus 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 usage---universe 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/astro-ph.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.
You might as well say that the cosmos revolve around the earth. That kind of perspective amounts to the very same thing. It could well be true but without proof in either favor, it is speculation and only serves to hinder progress. Nobody will ever find a way to prove it if we refuse to consider the possibilities.

Existing models are useful for practical applications but contemplating what is beyond our knowledge domain is paramount to discovery.
 Astronomy Sci Advisor PF Gold P: 23,218 http://edge.org/response-detail/805/...annot-prove-it or google "Steinhardt annual question 2005"
 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 pre-existing 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
P: 4,800
 Quote by marcus http://edge.org/response-detail/805/...annot-prove-it or google "Steinhardt annual question 2005"
I've always found that response to anthropic arguments to be rather pathetic.
Mentor
P: 11,844
 Quote by Fuzzy Logic You might as well say that the cosmos revolve around the earth. That kind of perspective amounts to the very same thing. It could well be true but without proof in either favor, it is speculation and only serves to hinder progress. Nobody will ever find a way to prove it if we refuse to consider the possibilities. Existing models are useful for practical applications but contemplating what is beyond our knowledge domain is paramount to discovery.
We have overwhelming evidence of at least a single universe and zero evidence of more than a single universe. I think in this case we should stick to a single universe model until something tells us otherwise.