Relative to The Universe?

[kaos]

Everybody always says you have to measure things such as speed and distance relative to something else, which is true. But what if everything has an absolute position within the universe. Like the universe was a big 3D grid, with the center being the point of the Big Bang. It is expanding infinatly, on all sides, but the center is still the same. That means that everything could be measured to have an absolute position within the universe. How to measure the dead center of the universe? Equal distances from all sides, because it is expanding at an equal rate on all sides right? Also you could argue that things are moving so fast and even expanding, so they are never in the same place.

[Martin]

...But what if everything has an absolute position within the universe. Like the universe was a big 3D grid, with the center being the point of the Big Bang. It is expanding infinatly, on all sides, but the center is still the same.

 

 

the usual view is that space is not expanding from a point that you can locate in present-day space

space is simply expanding (it isnt possible to define point of origin from which it expands, it just expands)

 

 

there are a lot of popular misconceptions about the "big bang" and there was an article in SciAm by Lineweaver and Davis that tried to fight these misconceptions. It is online, maybe you can find a link.

 

the words "big bang" give people the wrong idea. astronomers dont think about the beginning of expansion as an explosion that occurred at some point in pre-existing empty space. If it were that then it would merely be an expansion of MATTER, flying out from some central place, and that is not what they have in mind. In the classical picture, they are talking about an expansion of SPACE and what expands is all of space with no other space surrounding it. It doesnt expand "into" any surroundings, it just expands. One of the ideas in the mathematical description is to keep it as simple as possible---get rid of all unnecessary detail.

 

there is no way I know of that your idea of establishing an absolute reference for position could be made to work

 

However, to an excellent approximation, one can use the CMB to establish an absolute idea of rest and speed.

 

One can say, for example, that the sun with its planets is moving 370 km/second with respect to the universal rest frame. that is determined by the CMB dipole and astronomers refer things to the universe rest frame all the time----they talk about motion "with respect to CMB" or motion "with respect to the Hubble flow", which means essentially the same thing.

 

Do a google search for Lineweaver and Davis and see if you can find that SciAm article----it was from 2005 or maybe 2004---great article

 

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BTW some researchers have worked out an improved model of General Relativity which doesnt have a singularity at the beginning of expansion. Time extends back before. Gravity becomes repulsive at very high density. There is a prior collapse and a BOUNCE. It is not a bounce of matter, but a bounce of SPACE. Space collapses, reaching near Planck density, and rebounds into our present expansion phase. Numerical simulation studies of this are being carried on at penn state led by Abhay Ashtekar, Martin Bojowald and others. They are trying to find effects of the model that they can look for in the CMB as a way of testing the model. It predicts almost exactly the same observed phenomena as classic Gen Rel, only differing during a brief interval right around the bounce, so it is going to be hard to test. But i feel fairly confident they will bring the new quantum cosmology model to the point of being testable (I've been watching these guys for 3 years or more and I can gauge their determination and momentum.)

[Sisyphus]

The problem with that is that there is no single point where the big bang took place, and there are no edges to measure from. We don't know for sure what the "shape" of the universe is yet, but we do know that it is NOT just a simple, 3-dimensional sphere. For one thing, "the universe" includes all space, not just space where there is "stuff." Hence there is no "edge," because an edge means there would be universe on one side and "something else" on the other. But there is no "something else." Also, all our observations show that everything is moving away equally from everything else, not from a single point, and it appears that Big Bang took place everywhere at once. So the center of the universe is everywhere, and its edge is nowhere.

[Kyle]

Was the Universe the size it is now at the time of the big bang?

[Martin]

Was the Universe the size it is now at the time of the big bang?

 

if the universe is spatial infinite then the answer is yes.

 

if the universe is positive spatial curved and spatial finite then the answer is no.

 

infinite is the same size as infinite, infinite can expand a whole lot and still be the same size.

 

For simplicity, most cosmologists have been assuming for several years that the U is spatial flat and infinite

the data is CONSISTENT with this, the data does not disprove it, and mathematically it is a lot simpler

 

but the data also permits one to think of the universe as NEARLY but not perfectly spatial flat, and to have a slight average largescale POSITIVE curvature, so it could be analogous to a sphere, and be spatial finite.

 

the data is still not good enough to distinguish between a slightly curved (finite) case and a zero-curvature case.

 

So we can't tell yet what is the answer to your question! Frustrating.

Sorry about that.

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just keep watching the estimates for Omega

if it is exactly = 1, then she is flat

if it is even 1.01, then she is positive curved and finite!

 

the last confidence interval I saw was from the WMAP 3-year data and it was a 68 percent confidence interval and it was, amazingly,

[1.008, 1.013]. that looks awfully awfully like she might be positive curvature finite! but still too close to call.

=================

 

if, in fact, Omega turns out > 1, and we finally accept that the whole universe (not just the currently observed part but everything) is finite, like analogous to a sphere,

then AT THE TIME OF THE BOUNCE IT WAS VERY SMALL!

so then the answer to your question is no, it wasnt always this size, it used to be very small----briefly even quite small.

But we can't say that yet because the nice simple Omega = exactly 1 flat case has not yet been excluded by the CMB data.

[kaos]

Ok this is all very interesting to me. I understand that space is not surrounded by anything, or inside of anything. So if not, then can it even have a shape?

And if it DOES have a shape, wouldn't you have to find an edge?

I mean, what if it's shape was a U for example. If that was true, there could be a galaxy on one tip of the U and another on the other tip. Then the fastest distance should be between the tips, but that would be impossible. This sounds very very much like the whole wormhole theory where taking a shortcut between places by bending space, except that this is without bending. This is the same thing but space would already naturally be like that.

Also, how is it possible for everything to be moving farther away from everything unless there is a specific point in the center? There is no possible way that I can imagine for any number of things to move so that they are all moving away from eachother at the same time without there being a point.

I'm not trying to disagree, just trying to understand. Thanks

[CPL.Luke]

think of it like this, every star in the universe is moving away from us. the rate at which they move away is proportional to there distance from us. This is because space itself is expanding.

[Royston]

kaos, you should also look into the hubble constant...the further away a galaxy is, the faster it is moving.

 

v=H0r

 

v is velocity, r is the distance and H0 is the hubble constant...which is approx 60 kms-1 Mpc-1. So for every megaparsec a galaxy's distance is from the observer, the speed increases by 60 km s-1.

[kaos]

Understood.

[Martin]

Ok this is all very interesting to me. I understand that space is not surrounded by anything' date=' or inside of anything. So if not, then can it even have a shape?

[/quote']

 

I am beginning to like you. you keep thinking away at stuff.

where do you live BTW, if it is OK for me to ask?

 

A guy named Bernhard Riemann back around 1850 developed a way to define the curvature INTERNALLY. Carl Gauss helped. he was at Göttingen and he was like a mentor to Riemann.

 

suddenly it was possible to test the shape from within. to FEEL IT OUT darkly from the inside, and you didnt have to imagine going outside.

 

Riemann was unusually creative, he invented math tools that had not been seen before to do geometry intrinsically from within spaces of any dimension and any shape.

 

So you can talk about curvature of a space even though IT HAS NO EDGES OR BOUNDARY where it meets the "outside".

 

Astronomers today are riding on Riemann's back.

einstein used Riemann's toolbox to construct Gen Rel.

Nobody in cosmology could do anything if they didnt have Riemann's help because THAT IS WHERE WE ARE---we are in somewhere where there is no outside, and yet our space is CURVED, by the stars and massive objects in it. It is made hilly-valley.

 

Usually you learn a little differential geometry the SECOND YEAR of a math major. "differential geometry" is what they call doing it Riemann style.

Then if you like it, you can go back and learn more in the THIRD year, or later in grad school.

 

It is actually easier than that, you COULD learn some in your first year or in highschool but that is not what is usually done, at least where I live.

 

differential geometry has a lot of potential for beauty and for puzzles. HAVE YOU HEARD ABOUT GRISHA PERELMAN? he is famous for solving one of the riddles in differential geometry. Tomorrow we find out if he will be awarded the Fields medal in Madrid.

 

Riemann died young. they didnt have good medicine in those days.

 

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just a thought, kaos. why dont you try Wikipedia some?

see what they have about Carl Gauss and Bernhard Riemann (I may have the names or spelling slightly wrong) and see what they have about differential geometry.

[raves475]

Kaos said something about the universe being a 3D grid. I was just wondering if the universe is 3D or if it exceeds that?

[bascule]

This is because space itself is expanding.

 

Bingo: the Big Bang was not an explosion in space, it is an explosion of space

 

Kaos said something about the universe being a 3D grid. I was just wondering if the universe is 3D or if it exceeds that?

 

Well, according to GR, the universe consists of four-dimensional spacetime.

 

According to string theory, Kaluza-Klein theory, and others, there are higher dimensions beyond four dimensional space time, wrapped up so small that they are apparently nonexistent to us. There's no experimental evidence that any of these higher dimensions exist, however I recently read about experiments underway to test for their possible existence.

[Amos Schuman]

the words "big bang" give people the wrong idea. astronomers dont think about the beginning of expansion as an explosion that occurred at some point in pre-existing empty space. If it were that then it would merely be an expansion of MATTER, flying out from some central place, and that is not what they have in mind. In the classical picture, they are talking about an expansion of SPACE and what expands is all of space with no other space surrounding it. It doesnt expand "into" any surroundings, it just expands. One of the ideas in the mathematical description is to keep it as simple as possible---get rid of all unnecessary detail.

 

It's not exactly a wrong idea. Expansion in spacetime implies the same of its mass-energy content. That's pretty dead on for the idea of an explosion. The wall people run into frequently is covariance. The difference is that an explosion is usually described as content within in reference to content without in some universal embedding. You could describe the same explosion by simply discussing the relation of some subset of the content within to some other subset (I'm guessing that wouldn't be particularly useful to pyros, though).

 

EDIT: I should also add that another significant problem posed by GR's covariant approach is the problem with defining the total mass-energy content of the "universe." You can't say much of anything about the distribution of content in a spacetime without knowing something about how much content you have.

[Adrogans]

Hey Martin, I was wondering if you, or anyone else, could give a basic rundown of differential geometry. In simple terms please.

 

Thanks

[Ragib]

How much math do you know? And how simple? Do you want the concepts, or the mathematics as well? I know it sounds stupid to ask that about a math subject, but it is possible to know the concepts and not the math...

[Amos Schuman]

Hey Martin, I was wondering if you, or anyone else, could give a basic rundown of differential geometry. In simple terms please.

 

Thanks

 

If you have a calculus background, you can think of of differential geometry (or, interchangeably, topology), as the study of differentiation on n-dimensional surfaces. If not, then a suitable analogy is the study of changes in geometry on the infinitesimally small surface sections of some interesting set of objects. The key thing to remember is that the focus is on changes in small increments from some coordinate (differentiation) rather than global structure (integration), much in the same way you can treat Maxwell's equations locally (in differential form) or globally (in integral form).

[Amos Schuman]

How much math do you know? And how simple? Do you want the concepts, or the mathematics as well? I know it sounds stupid to ask that about a math subject, but it is possible to know the concepts and not the math...

 

Calculus is a must, but diff. geo. will take you beyond the canonical introduction into the world of forms (hint, all those dx's and dy's are actually algebraic objects in themselves, which you can operate on and map to other objects). A strong grasp of linear algebra helps build the basic notions of topology you'll need to navigate diff. geo. Analysis, particularly that of functions (maps), is also pretty foundational, but you don't need much more than the basics before you diverge into the topology-specific stuff.

[jackson33]

think of it like this, every star in the universe is moving away from us. the rate at which they move away is proportional to there distance from us. This is because space itself is expanding.

 

i suggest you tell this to the ones controlling the galaxy "Andromeda" or the two dwarf galaxy heading our way.

 

if every thing were moving "outward", what ever that is, how is it we see other galaxy already tangling. everything in your idea indicates we are a center and this very unlikely....

[jackson33]

if you have two or more unknowns in a math formula, how accurate could it be to reality???

[Royston]

i suggest you tell this to the ones controlling the galaxy "Andromeda" or the two dwarf galaxy heading our way.

 

if every thing were moving "outward", what ever that is, how is it we see other galaxy already tangling. everything in your idea indicates we are a center and this very unlikely....

 

The expansion of space is an observed phenomenon, there's no center to the universe, galaxies are moving away from each other. The idea would only indicate that our position is central, if our position was static, which it is not.

 

I'd read up on some basic principles of cosmology and relativity. Wiki is a good starting point.

[Amos Schuman]

if every thing were moving "outward", what ever that is, how is it we see other galaxy already tangling. everything in your idea indicates we are a center and this very unlikely....

 

Because the universe is only isotropic up to a point.