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spikey
2010-Apr-05, 12:22 PM
Hello,
As a relatively new member I'm a little unsure of whether this has been tackled before although I did try a search with nothing coming up as an exact match to the question.
My question is as follows: With the expanding Universe being able to be measured in terms of its age, expansion rate and known size is it not possible to extrapolate backwards to determine the location of it's geographic centre i.e the place where the big bang occurred 13.7 billion years ago?
Spikey.
p.s. I live in the Isle of Man (a small island about halfway between the UK and Ireland) and having only recently discovered the resource listen to the earlier episodes on my iPOD when out walking my retreiver on her daily walks - the walks are a lot more enjoyable now!:razz:

Swift
2010-Apr-05, 01:06 PM
Hi spikey, if it hasn't been said already, "welcome to BAUT".

In the sense that you describe, there is no "center of the Universe".

One analogy that I like (though it isn't perfect) is to think of the Universe as the surface of a big balloon. At the big bang it was a tiny dot of a balloon and it has expanded over time. But since we are only on the surface, no matter where in the Universe we look, we'll never find the center.

thoth II
2010-Apr-05, 04:58 PM
Boy, if there was a "center" to the universe, that would be news to me. The center is the big bang, so the center is EVERYWHERE

EDG
2010-Apr-05, 05:23 PM
It's down the road from me. No, really!

(the outreach centre for the local observatory is called "Centre of the Universe". I have a pic of a road sign saying "Centre of the Universe: 150m ahead" :) )

But seriously, swift's balloon analogy is the easiest way to visualise it - if spacetime is the surface of the balloon, and the centre of the universe is the centre of the balloon, then we won't see the centre. Though I'm not sure how literally the analogy should be taken - would that mean that the centre of the universe would actually be accessible if we could travel in a dimension perpendicular to spacetime?

TampaDude
2010-Apr-05, 06:57 PM
Boy, if there was a "center" to the universe, that would be news to me. The center is the big bang, so the center is EVERYWHERE

+1 Everywhere is the center of the universe. No matter where you are, the universe is expanding away from you in all directions.

Cougar
2010-Apr-05, 07:43 PM
+1 Everywhere is the center of the universe. No matter where you are, the universe is expanding away from you in all directions.

To expand on this correct answer a bit (no pun intended), we're not exactly sure about the conditions at the instant of the big bang, but we presume that the entire universe, including all the mass and space as well, was in effect all crammed together in one spot, and as TampaDude says, every "point" then expanded away from every other point. Therefore, every point in the now universe was, and still can be considered to be, the "center" of the universe. Or alternatively, there is no "center." Take your pick.

astromark
2010-Apr-05, 08:34 PM
And I too welcome you 'Spikey'.. The above I see as correct and can only add that to find a point of center we would need to know its whole mass. We are restricted in the fact that we can only know of the observable universe. There may be more to it.
I would further attempt to clear your thoughts as to expansion of the universe. All of those objects of mass that are not gravity bound to galaxies and solar systems... are eccelorating away from your point of reference. The space between objects is getting bigger, and doing it continually faster... Winding back the clock does not seem to show a center. Just a smaller space. That balloon idea works fine for me. Hope I have helped, mark.

dhd40
2010-Apr-05, 08:46 PM
But seriously, swift's balloon analogy is the easiest way to visualise it - if spacetime is the surface of the balloon, and the centre of the universe is the centre of the balloon, then we won't see the centre. Though I'm not sure how literally the analogy should be taken - would that mean that the centre of the universe would actually be accessible if we could travel in a dimension perpendicular to spacetime? (my bold)

I like that very much :clap: not least (but not only) because it shows the limits of analogies

Jeff Root
2010-Apr-06, 12:22 AM
One of the limitations of the balloon analogy is that the surface of the
balloon is two-dimensional, while spacetime is four dimensional. The
surface of the balloon is not so much analogous to the Universe as a
whole as it is analogous to a thin plane slice cut through the Universe.
That slice could be *anywhere* in the Universe and still look pretty
much the same.

Blowing up the balloon is supposed to be analogous to the expansion
of the Universe, so the center of the balloon is sometimes considered
as analogous to the point *IN TIME* where the Big Bang occurred.

-- Jeff, in Minneapolis

Ken G
2010-Apr-06, 12:45 AM
My answer to the OP would be that you first have to define what you mean by "the universe" before you can answer where its "center" is. With some of the most empirically natural definitions of "the universe" (say, the mass we can see, or the mass we can infer exists using physics), we are indeed at the "center of the universe", right here on Earth. However, there is no reason to expect that would not also be true for astronomers on some planet a billion light years away-- they would be at the center of their universe. That it would simply not be the same universe would be a direct ramification of the definition of "the universe" as having to do with what can be observed, and that in turn falls within the limitations of the observer, which in turn guarantees that the observer is at the center.

However, it is certainly the standard mindset to imagine that any matter that would be observed by any hypothetical observer anywhere "out there" should count in the definition of "the universe", so if we extend to such a hypothetical meaning of the universe, then we must say there is no center to it (or at least that our best scientific theory is unable to confer any meaning to such a center). Personally, I have no problem with saying that if my universe is that piece of existence that I have observational authority over, then I am at its center-- but that is probably not the standard way of thinking about "the universe", whatever it is.

WayneFrancis
2010-Apr-06, 02:02 AM
Boy, if there was a "center" to the universe, that would be news to me. The center is the big bang, so the center is EVERYWHERE

Welcome to the BAUT forums thoth.
in my world I'm the centre of the universe! JK

Swift's explanation is a good one. I'll expand on it. It is a 2D representation of an expanding universe. You might go "2d? The balloon is 3D!" The balloon is 3d to us but to anything that lives on the surface of the balloon it is only 2D. They can't get off the surface basically. Our universe might be similar in 4 dimensions. If you think of the balloon for a second as 2D with the radius of the balloon as a "time" dimension you will see that as time moves forward the universe (the 2d surface) gets larger. We are forever caught on the surface watching the universe expand as time goes by.

Hope my additional explanation doesn't confuse things in your head.

WayneFrancis
2010-Apr-06, 02:05 AM
It's down the road from me. No, really!

(the outreach centre for the local observatory is called "Centre of the Universe". I have a pic of a road sign saying "Centre of the Universe: 150m ahead" :) )

But seriously, swift's balloon analogy is the easiest way to visualise it - if spacetime is the surface of the balloon, and the centre of the universe is the centre of the balloon, then we won't see the centre. Though I'm not sure how literally the analogy should be taken - would that mean that the centre of the universe would actually be accessible if we could travel in a dimension perpendicular to spacetime?
I don't think we need to travel perpendicular to spacetime just well along the time dimension. Right now we are stuck on the time dimension along for the ride. We can alter our perception of the time but in the end we are still stuck on it like the bus in the movie speed.

EDG
2010-Apr-06, 04:00 AM
I don't think we need to travel perpendicular to spacetime just well along the time dimension. Right now we are stuck on the time dimension along for the ride. We can alter our perception of the time but in the end we are still stuck on it like the bus in the movie speed.

What, so if we ever manage to slow down time below a critical value, the universe may explode? ;)

WayneFrancis
2010-Apr-06, 05:16 AM
What, so if we ever manage to slow down time below a critical value, the universe may explode? ;)

thinking...
well lets look at time dilation

http://latex.codecogs.com/gif.latex?\Delta t'= \frac{\Delta t}{\sqrt{1-\frac{v^{2}}{c^{2}}}}
Well say we got to some CRAZY speed like 0.9999999999999975c our time dilation is only a bit about 14 million to 1. So even then the if you lived a nice long 100 years you'd still only see about 1/10th of the history of the universe. Maybe if you went to velocities of 1-1x10-20c<<v<c you may very well see the universe explode. As your whizzing through the universe at break neck speeds the universe is expanding at a rate that is independent of your speed.

What is happening is you are still on the surface of the balloon but your perception of time just has slowed. You can get close to 0 but never reach it and you can never cause time to reverse itself.

noncryptic
2010-Apr-06, 10:16 AM
The universe that we can measure is not so much "the universe", rather it is the observable universe; the part of the universe that we can observe.

Observations are limited because as we look further out we look back in time and at the furthest reaches we see the young universe as a hot plasma (http://en.wikipedia.org/wiki/Cosmic_microwave_background_radiation), which is opaque.
Given that there is no logical reason and no observations that suggest our place in the universe is special, the universe looks like that from every location in it.

The mechanism of expansion is such that to any observer anywhere in the universe their location appears to be at the center of expansion, just as for every raisin in a rising raisin bread all other raisins appear to be moving away from it.

Jeff Root
2010-Apr-06, 10:47 AM
The raisin bread analogy has a particularly big problem: A loaf of bread
has an outer boundary. A microscopic voyager travelling through a loaf of
raisin bread would pass by one raisin after another, until he reached the
surface of the bread, beyond which there would be no more bread and no
more raisins. A voyager limited to the surface of a balloon, or in the real
Universe, could travel forever and never reach an end -- even if the balloon
or the real Universe is finite in extent.

The balloon analogy and the raisin bread analogy are each useful for showing
various different aspects of the cosmic expansion and the shape of spacetime.

-- Jeff, in Minneapolis

WaxRubiks
2010-Apr-06, 12:32 PM
If you have a hyper-raisin bread, a 3-bread, the analogy works. :)

Cougar
2010-Apr-06, 01:27 PM
With the expanding Universe being able to be measured in terms of its age, expansion rate and known size is it not possible to extrapolate backwards to determine the location of it's geographic centre i.e the place where the big bang occurred 13.7 billion years ago?

Another way to look at it: A center would be a point that every other point would view as stationary. But everything in the universe is in motion, and motion can only be determined relative to something else that is in motion. There is no universal reference point from which all motion is measured. You'd think Einstein came up with this view first, but actually it was Galileo.

cosmocrazy
2010-Apr-06, 03:55 PM
things to consider when talking about the centre of the universe are :- If we consider the BB as the accepted theory of how the universe began then as we go back in time to the initial state then things get a little weird. If you consider the initial BB to be born from a singularity then at this point all time and space doesn't exist, there is no centre or any co-ordinates of any physical form, when for some reason this singularity begins to expand it creates time and space, this space-time is the centre just expanding larger from our perspective and we and all things we observe are part of it. Also if you consider the universe to have no boundary then its impossible to co-ordinate to any centre point in space, so the centre is defined as being everywhere.

Balstrome
2010-Apr-08, 08:05 AM
So dumb ideas like buy huge amounts of computer time and calculating galaxy travel vectors and drawing lines to cross each other, is not a good idea?

rodin
2010-Apr-09, 05:29 PM
If the universe is expanding away from Earth in all directions to the same extent that would imply Earth WAS at the centre of the Big Bang

If as seems more likely Earth is but a part of the material that erupted from the singularity more likely it is some ways out from the centre. In which case we should see a distribution of recession velocities - maximum looking back through the void presumably left by the Big Bang?

slang
2010-Apr-09, 05:42 PM
So dumb ideas like buy huge amounts of computer time and calculating galaxy travel vectors and drawing lines to cross each other, is not a good idea?

You just might find the answer to be 42. :)

Hornblower
2010-Apr-09, 07:55 PM
So dumb ideas like buy huge amounts of computer time and calculating galaxy travel vectors and drawing lines to cross each other, is not a good idea?

As far as my feeble mind can tell, that remark does not follow in any way from the previous discussion in this thread. Please explain.

cjameshuff
2010-Apr-09, 09:48 PM
So dumb ideas like buy huge amounts of computer time and calculating galaxy travel vectors and drawing lines to cross each other, is not a good idea?

It's not a bad idea, but their trajectories aren't going to point to any specific center. You'll find that the individual galaxies directions of motion are random, but that they are overall moving apart from each other, distances between galaxies increasing over time at rates proportional to their distances. No matter which galaxy you pick as an origin point, you will see the surrounding galaxies receding with velocities that increase with distance and depend little on direction. (aside from a handful of nearby galaxies that are close enough to move "upstream")

It's misleading to say that the universe started out as a point. Tracking its state back in time, its density rises toward infinity and the distance between any two objects approaches zero, but its spatial extent may very well have always been infinite. Considering this point might make it easier to understand that the expansion is expansion of the universe, not of stuff in the universe.

astromark
2010-Apr-09, 10:14 PM
So for the few of us that have not grasped the idea yet... 'If we observe that all of the observable universe is eccelorating away from us then that can be truly said that we are the center of the Universe.
UNFORTUNATELY... That is not how it looks from any place other than here. Regardless of your position in this observable universe the apparent view of every thing receding away is the same. Nothing has expansion velocity. It has the space between all unbound objects getting bigger. The center is every where. Do not think of the BB as a explosion. More of a space expansion. Continuing.

RAMS57
2010-Apr-09, 10:50 PM
We do it this way: 13.7 billion + 13.7 billion, divide by two. Do this in the NS Universe map as known. Then, do the same thing E and W. Approximate yes, but there are two variables we cannot know: time, and our location apparent. Thus, Universe center location is an unknown, even with backwards extrapolation. Why? Because everything we 'see' in all spectra, is the past. Example, the sun is 8 light minute distant. Thererfore, it could burn out, but we would be bathed in its spectra for 8 more minutes.......

Further, for data sake, that 13.7 billion year distance is also time, and what we are now "seeing" has not existed for a very long time. Something else may be there, out there, but we will not know for another untold quantum of time........beyond about one light week, time is the variable, not distance. Thus, age becomes more important than location.

Consider. Good post OP.

Spaceman Spiff
2010-Apr-10, 01:54 AM
To expand on this correct answer a bit (no pun intended), we're not exactly sure about the conditions at the instant of the big bang, but we presume that the entire universe, including all the mass and space as well, was in effect all crammed together in one spot, and as TampaDude says, every "point" then expanded away from every other point. Therefore, every point in the now universe was, and still can be considered to be, the "center" of the universe. Or alternatively, there is no "center." Take your pick.

Let's be more careful with our words. The Universe may be infinite, and therefore always was (whatever "always" actually means). All that we know is of the universe that lies (or perhaps has lain) within our cosmic horizon -- our backwards pointing light cone in space-time. The only mass-energy that was at one time found in a volume similar to a golf ball (or whatever) is that within our cosmic horizon. And one shouldn't take that "golf ball" picture literally, either. There is and has never been a "one spot" where all the matter sat inside. The big bang model merely finds that the energy density within at least our cosmic horizon at some very early time was enormous (understating it by many orders of magnitude), and equivalent to stuffing the mass-energy within our cosmic horizon (scaled by the scale factor R = (1+z) of the appropriate power) within such a volume.

To minimize confusion concerning the "meaning" of expanding space, I find it most useful to picture a changing energy density with time -- initially very, very large, now small, and at present still becoming smaller.

I am sure the above could be further sharpened in scientific precision, but there's a first cut. I don't mean to be picky, but it's good practice to try to be "choosey" in our choice of words as we explain a phenomenon with our best model. It's like using a razor again and again to shave off the extraneous crud, leaving behind an ever more accurate representation of what the scientific model actually addresses. Less precision leads to more misconceptions. And I am certainly not without fault.

Jeff Root
2010-Apr-10, 02:54 AM
The Universe may be infinite ...
If the Universe is infinite, and by "the Universe" we mean everything that
came out of the Big Bang, and is participating in the cosmic expansion,
then everywhere in the infinite Universe must have begun expanding
simultaneously, 13.7 billion years ago. If it hadn't all begun expanding
at the same instant, there would be a compression wave moving outward
from the point where the expansion started.

Do you believe that the expansion could have begun simultaneously
everywhere throughout an infinite Universe? If so, why? If not, how
can you think that the Universe may be infinite?

-- Jeff, in Minneapolis

Ken G
2010-Apr-10, 03:29 AM
If it hadn't all begun expanding
at the same instant, there would be a compression wave moving outward
from the point where the expansion started.Who says there isn't? Spacetime is contorted all over the show on galactic scales, and seems to settle down to the cosmological principle (perhaps owing to inflation) on the largest scales we see. Who says the cosmological principle is not abandoned on much larger scales still? We have no idea, and it really looks like we never will. That is just the hand we are dealt, the scientific thing to do is just live with it. We don't know if the universe is finite or infinite, probably never will, and no amount of telling the universe what it "should" be like will make a hill of beans of difference.

grav
2010-Apr-10, 06:26 AM
Mine's in the next room. :) I'd better not let her know I wrote this though. If she takes it the wrong way, it'll be the Big Bang all over again. ;)

Spaceman Spiff
2010-Apr-10, 09:35 PM
If the Universe is infinite, and by "the Universe" we mean everything that
came out of the Big Bang, and is participating in the cosmic expansion,
then everywhere in the infinite Universe must have begun expanding
simultaneously, 13.7 billion years ago. If it hadn't all begun expanding
at the same instant, there would be a compression wave moving outward
from the point where the expansion started.

Do you believe that the expansion could have begun simultaneously
everywhere throughout an infinite Universe? If so, why? If not, how
can you think that the Universe may be infinite?

-- Jeff, in Minneapolis

[statement in emphasis] Why do you presume this? Do you know that this is a specific prediction of General Relativity? If so, please explain. I do not pretend to understand the nature of space-time at a sufficiently deep level, especially on these sorts of grand scales, to draw the conclusions you did. And lastly, my statement was meant to imply that "Universe" (as opposed to my use of "universe") considers the possibility of space-time not only beyond our cosmic horizon, but "beyond" our little "big bang" and even beyond "our" inflationary bubble. I don't pretend to know much of anything about these things, although there are physicists who do take these sorts of questions seriously and can make intelligent statements about them.

So there are several different meanings when we say the word "U(u)niverse". Here is how I think of them:

1) All matter/energy that has been at least at one time (since the "bang") within our cosmic horizon.

2) All matter/energy that originated in whatever went "bang", the vast majority of which almost certainly lies far, far beyond our cosmic horizon -- perhaps our inflationary bubble.

3) Whatever lies "beyond", in a space-time sense, our inflationary bubble (multiverse, timeless inflation, whatever...)

To the extent that (2) and (3) leave a potentially measurable imprint on our measurable "universe", then they are questions within the purview of science.

Jeff Root
2010-Apr-10, 11:39 PM
If it hadn't all begun expanding at the same instant, there would be a
compression wave moving outward from the point where the expansion
started.
Why do you presume this?
Because it wouldn't be possible for there not to be one.



Do you know that this is a specific prediction of General Relativity?
No, but I wouldn't expect it to be. General relativity is a theory of space, time,
motion, and gravity, not of cosmology.



If so, please explain.

I'll explain anyway. If you have a lot of things bunched together, and some of the
things in the middle of the bunch start getting farther apart from each other, while
others around them don't, then there will be a boundary between the two regions
where the things are getting closer to each other, so the density is increasing.

I was using a definition of "the Universe" that matched your description of it in
post #27. Namely, one which is synonymous with everything that participates in
the Big Bang and cosmic expansion.

Do you believe that the expansion could have begun simultaneously
everywhere throughout an infinite Universe? If so, why? If not, how
can you think that the Universe may be infinite?

-- Jeff, in Minneapolis

DrRocket
2010-Apr-10, 11:45 PM
Blowing up the balloon is supposed to be analogous to the expansion
of the Universe, so the center of the balloon is sometimes considered
as analogous to the point *IN TIME* where the Big Bang occurred.

-- Jeff, in Minneapolis

This is just plain wrong.

The balloon is used to avoid using the mathematical concept of a manifold. But in fact the surface of the balloon is a 2-manifold. Spacetime, the universe, is a 4-manifold.

The balloon analogy is intended to be understood in the following way. The universe is the surface of the balloon. It is not embedded in anything and the surface of the balloon is the whole enchilada. It has no "center" for this illustration.

Now imagine that the balloon is inflated, through an infinitesimal hole. Then the sphere that is the surface expands, and all points on that surface move apart from one another. They not only move apart, but the distance (measured along a great circle joining them on the surface) increases at a rate that is proportional to the distance separating the points. All points see all other points as receding directly away from them. In that sense, any point is the "center". But in no case is the center to be construed as the center of the ball of which the sphere is the boundary.

RAMS57
2010-Apr-11, 01:51 AM
So for the few of us that have not grasped the idea yet... 'If we observe that all of the observable universe is eccelorating away from us then that can be truly said that we are the center of the Universe.
UNFORTUNATELY... That is not how it looks from any place other than here. Regardless of your position in this observable universe the apparent view of every thing receding away is the same. Nothing has expansion velocity. It has the space between all unbound objects getting bigger. The center is every where. Do not think of the BB as a explosion. More of a space expansion. Continuing.

This is the correct answer. Perfect. It is an expansion, and it is time related, not distance as to where. Very good.

Robert

Jeff Root
2010-Apr-11, 06:48 AM
Blowing up the balloon is supposed to be analogous to the expansion
of the Universe, so the center of the balloon is sometimes considered
as analogous to the point *IN TIME* where the Big Bang occurred.
This is just plain wrong.
I don't think so. I believe that I accurately described the analogy that is
sometimes made. The analogy between time and the radial direction of
the balloon certainly isn't made anywhere near as often as the analogy
between the cosmic expansion and the blowing up of the balloon, nor as
often as the analogy between the Universe as a whole and the balloon's
skin, but it is made often enough.

Note that I did *NOT* say that the analogy is between the center of the
balloon and a point in *space*. I said that the analogy is between the
center of the balloon and a point in *time*. The radial direction of motion
of the balloon's skin as it is being blown up is said to represent the time
dimension in the analogy. That seems okay to me.

You can say that the skin of the balloon represents the entire Universe,
just as a photograph of a person represents that person. But a photograph
is two-dimensional while a person is three-dimensional. On the other hand,
all we see of a person is what is on the surface, the same as a camera sees,
so the two-dimensional photograph also represents the two-dimensional
surface of the person.

The two-dimensional surface of a balloon can represent the entire Universe,
but only as completely as a two-dimensional photograph represents a
three-dimensional person. The two-dimensional surface of a balloon is much
more like a photograph of the sky than it is like the Universe. But rather than
representing a two-dimensional image of the entire sky visible from one place
in the Universe, the surface of the balloon represents a two-dimensional plane
slice through the Universe, like a CAT scan image.

Many balloons would be needed to represent the entire Universe in its full
three-dimensional depth, just as many 2-dimensional CAT scans are needed
to represent an entire three-dimensional human body. (Actually it is trickier,
taking a single spiral scan that goes around and around and around...)
The many balloons would be of all sizes and have their centers in all locations
throughout the Universe. The balloons would intersect and pass through one
another. A galaxy would appear on the surfaces of many different balloons,
just as a point in a human body is imaged from many different directions in
a CAT scan. All the balloons would have their centers located at the same
point in time, even though they would be located at different points in space.

It is just an analogy, of course. It is necessarily flawed. But that is my
understanding of an analogy that is used fairly commonly.

-- Jeff, in Minneapolis

loglo
2010-Apr-11, 07:14 AM
The many balloons would be of all sizes and have their centers in all locations
throughout the Universe. The balloons would intersect and pass through one
another. A galaxy would appear on the surfaces of many different balloons,
just as a point in a human body is imaged from many different directions in
a CAT scan. All the balloons would have their centers located at the same
point in time, even though they would be located at different points in space.

It is just an analogy, of course. It is necessarily flawed. But that is my
understanding of an analogy that is used fairly commonly.

-- Jeff, in Minneapolis


I once used an onion instead of a balloon to try and explain this. I thought it quite neatly showed that as you go back in time the universe is smaller just like the surface of each layer of the onion is smaller as you go towards the centre. (it works better with pickled onions as the layers come off easier.) The centre of the onion was the BB, once you peeled off the last layer of onion you couldn't go any further, just as when you look back in time you can't see any further than the BB.

The explanation didn't go down very well, but the sandwich did, which was one advantage I guess.

astromark
2010-Apr-11, 10:08 AM
This I would like to endorse... But imagining a pickled onion some 27 ly wide... WOW:o:

At this point I look about for the stack of turtles... while reaching into the jar for another..

The point being that we can not find a center. That there is not one is clear.

and the reality of multi Universes could render this question as a smaller fraction than was previously imagined. We can move on from this question and into much speculation... informed, and with knowledge of and always willing to expand our understanding.

As for the remnants of that singular point. Its just over there... under this rock... Oops !

Ken G
2010-Apr-11, 12:26 PM
The big "aha" comes when you realize that the balloon analogy is not just an analogy for the actual expansion of space itself, it is an analogy for the analogy that space is expanding. The fact is, physics is always about discovering quantitatively effective analogies, because it is a language, and all language can ever do is translate what is not understood into terms that are. We understand balloons and onions, so we use them to form flawed analogies, and we understand scale factors and differential equations, so we use them to form much more accurate analogies. It's analogies all the way down, such is the meaning of meaning.

WaxRubiks
2010-Apr-11, 12:32 PM
one thing I realised about the balloon analogy is that as a 2 dimensional object, embedded in 3 dimensions, its actual surfaces in the 3 dimensions could be reversed(ie the inside surface becoming the outside surface etc) without it making any difference to it as a 2 dimensional object; so the idea of there being a real 3 dimensional center, inside the balloon, becomes meaningless, I would suppose.

MediumRare
2010-Apr-11, 09:01 PM
I'm totally a non-scientist, but can I ask one of these possibly stupid questions?


Nothing has expansion velocity. It has the space between all unbound objects getting bigger. The center is every where. Do not think of the BB as a explosion. More of a space expansion. Continuing.

OK I think I understand. But what does "unbound objects" mean in this context? In the balloon analogy every point on the surface travels away from every other point, or? The space between myself and my computer keyboard isn't expanding, or is it?

Best,
MR

DrRocket
2010-Apr-11, 11:22 PM
I'm totally a non-scientist, but can I ask one of these possibly stupid questions?



OK I think I understand. But what does "unbound objects" mean in this context? In the balloon analogy every point on the surface travels away from every other point, or? The space between myself and my computer keyboard isn't expanding, or is it?

Best,
MR

The space between you and your keyboard wants to expand. The same forces that are causing the universe to expand and for the rate of that expanison to be increasing are, as far as we know, at work between you and your keyboard. But, unlike the situation in deep space, there are other forces also at work, gravity, the electromagnetic force, the stong force and the weak force, that tend to oppose the force of expansion (which is not well understood). The net result is that those opposing forces win, and the distance between you and your keyboard, and between atoms in your body, is not increasing. This also applies on a much larger scale, the solar system, the galaxy and even the local group -- these are examples of "gravitationally bound bodies" that are not apparently much affected by the expansion of the universe.

The balloon analogy is only meant to illustrate the expansion of the universe on the largest scales.

DrRocket
2010-Apr-11, 11:24 PM
I don't think so. I believe that I accurately described the analogy that is
sometimes made.

In that case you are describing the analogy when it is made incorrectly.

Your further explanationis also just plain wrong.

I know that you don't think so. That is part of the problem.

Occam
2010-Apr-12, 12:01 AM
All of these answers are far too non-committal. Anyone who's ever had a baby knows exactly where the centre of the universe is:lol:

Luckmeister
2010-Apr-12, 04:40 AM
The space between you and your keyboard wants to expand. The same forces that are causing the universe to expand and for the rate of that expanison to be increasing are, as far as we know, at work between you and your keyboard. But, unlike the situation in deep space, there are other forces also at work, gravity, the electromagnetic force, the stong force and the weak force, that tend to oppose the force of expansion (which is not well understood). The net result is that those opposing forces win, and the distance between you and your keyboard, and between atoms in your body, is not increasing. This also applies on a much larger scale, the solar system, the galaxy and even the local group -- these are examples of "gravitationally bound bodies" that are not apparently much affected by the expansion of the universe.

The balloon analogy is only meant to illustrate the expansion of the universe on the largest scales.

Is it possible that our entire universe is experiencing spatial expansion, and that nothing in it is gravitationally immune from that expansion, even within atoms? The argument I hear for local gravitational binding is the redshift we observe at distance with no apparent expansion locally. Local expansion would be so many many orders of magnitude smaller than expansion between galaxies that it wouldn't be measurable to us, and the physical yardsticks we use for measurement would be expanding with space anyway. But we can observe it at a galactic level, as redshift, because of the immense magnification of the effect at that scale.

If I'm off-base here, I'd be grateful if someone would point out what I'm missing.

Mike

01101001
2010-Apr-12, 05:31 AM
Local expansion would be so many many orders of magnitude smaller than expansion between galaxies that it wouldn't be measurable to us, and the physical yardsticks we use for measurement would be expanding with space anyway.

Ned Wright Cosmology FAQ (http://www.astro.ucla.edu/~wright/cosmology_faq.html)


Why doesn't the Solar System expand if the whole Universe is expanding?

[...] For the technically minded, Cooperstock et al. computes that the influence of the cosmological expansion on the Earth's orbit around the Sun amounts to a growth by only one part in a septillion over the age of the Solar System. This effect is caused by the cosmological background density within the Solar System going down as the Universe expands, which may or may not happen depending on the nature of the dark matter. [...] Even on the much larger (million light year) scale of clusters of galaxies, the effect of the expansion of the Universe is 10 million times smaller than the gravitational binding of the cluster.

WaxRubiks
2010-Apr-12, 05:33 AM
if the yard stick expanded too, then the measurement of the distant galaxies light frequencies would show no red shift.

Ken G
2010-Apr-12, 05:42 AM
Put differently, there is no such thing as a true scale-- the only meaningful concept is relative scale.

astromark
2010-Apr-12, 06:37 AM
Is it possible that our entire universe is experiencing spatial expansion, and that nothing in it is gravitationally immune from that expansion, even within atoms? The argument I hear for local gravitational binding is the redshift we observe at distance with no apparent expansion locally. Local expansion would be so many many orders of magnitude smaller than expansion between galaxies that it wouldn't be measurable to us, and the physical yardsticks we use for measurement would be expanding with space anyway. But we can observe it at a galactic level, as redshift, because of the immense magnification of the effect at that scale.

If I'm off-base here, I'd be grateful if someone would point out what I'm missing.

Mike

In this post of yours mike. You quoted Dr Rocket. You then ask if thats right. YES it is. Understanding that the great expansion is nullified by the local forces. Atomic, Electromagnetic, Gravitational... and what Dr Rocket said...
We all, it would seem can take from a simple statement many strange and different things... I can only advise some reading to better research and broaden your understanding.. Noting that I am often not well understood... As to the advancement of this expansion including subatomic particles. I know of no such proof... Its a interesting argument that all things are being torn asunder... but I have not found a convincing argument to support this.
This thread asked of the centers location... There is and was no center to find.

Jeff Root
2010-Apr-12, 07:27 AM
The space between you and your keyboard wants to expand. The same
forces that are causing the universe to expand and for the rate of
that expanison to be increasing are, as far as we know, at work
between you and your keyboard.
That is a possibility. It is equally possible that the force or
forces exist only between widely-separated galaxies or clusters
of galaxies. There is as yet no evidence to distinguish the two.

There might be a single force causing the original expansion,
Inflation, and the acceleration of the expansion, or there might
be two different forces, or three. Whatever caused the original
expansion might still be at work, now causing the acceleration,
or its effect might have been practically instantaneous, getting
the expansion started and doing nothing after that.

Whatever caused the original expansion might have pervaded all
of space at the time of the Big Bang, while whatever is causing
the current acceleration might be limited to the spaces between
widely-separated galaxies.

Nuclear, electromagnetic, and gravitational binding is certainly
sufficient to prevent expansion of things smaller than galaxy
clusters, but we have no direct evidence that any force is being
applied to them, trying to pull them apart, nor any evidence that
space itself is doing anything. All we know is that distances
between widely-separated galaxies is increasing, and the rate
of separation apparently has been increasing for the last several
billion years, after first decreasing for several billion years.

-- Jeff, in Minneapolis

astromark
2010-Apr-12, 07:58 AM
That all looks good for me too Jeff... Just the last line, Its the acceleration rate that has apparently slowed and has since speed up again. The acceleration rate of that expansion is what has accelerated., and still is.

mugaliens
2010-Apr-12, 08:00 AM
That is a possibility.

And it's an equal possibility that such expansion over the lifetime of a human being is measured in micrometers.

Do you (not you, Jeff, but DrRocket) have evidence to the contrary?

astromark
2010-Apr-12, 08:22 AM
We seem to have moved subjects here and as its part of the general understanding of expansion. It would seem that it is not unreasonable to extrapolate that... As the whole of the known universe is expanding at a rate increasing... that right down at a subatomic level it could be also increasing to expand. As much as we know, that is not happening.
Its not what observation tels us. I can only ask that if such were the case... could we detect it ? I think yes. and its not there.

Jeff Root
2010-Apr-12, 08:28 AM
Luckmeister,

If everything were expanding equally, no change would be detectible.
We wouldn't see any consistent redshift from distant galaxies.

We know that the strength of the nuclear, electromagnetic, and
gravitatonal bonds between things smaller than clusters of galaxies
is enough to hold them together, whether there is a small force or
forces trying to pull them apart or not. If there is such a force,
and it has a constant strength, then it produces a tiny tension on
all matter, making all things very slightly larger than they would be
if the force were not there. Since we can't turn the force off or
block it, we can't make a comparison measurement of the size of a
thing with the force and without the force. The difference would
likely be too small to measure, anyway, whether the thing being
measured is a platinum bar, the Earth, or the distance between
Earth and Pioneer 10.

-- Jeff, in Minneapolis

Ken G
2010-Apr-12, 08:28 AM
And it's an equal possibility that such expansion over the lifetime of a human being is measured in micrometers.

Do you (not you, Jeff, but DrRocket) have evidence to the contrary?Ah, the old "absence of evidence is not evidence of absence" problem. I think the safe way to navigate around that pesky question of burden of proof in science is to distinguish a statement that describes how things are in reality, from how things are in our best current theory. For the former, the burden of proof is on the person making the claim, but in the latter, the burden of proof is on the person disputing the claim. In other words, if I say "reality is A", you can ask, "how do you know", but if I say "our best theory asserts A", I need only establish that there is no better theory. As for the issue of which is "more likely" to be correct in reality, I doubt there's much of anything we can say that is likely to be correct in reality, except something like "we do not understand reality."

mmaayeh
2010-Apr-12, 10:42 AM
With all this space expansion then, is there anything happening with the time component in space-time? Or, is time unaffected by the Universe expanding?

Ken G
2010-Apr-12, 11:12 AM
With all this space expansion then, is there anything happening with the time component in space-time? Or, is time unaffected by the Universe expanding?The expansion is itself a relation between space and time-- the distances get larger witih time. Imagining that it is something "happening" to space as the universe ages is itself a kind of coordinatization of spacetime-- called "comoving frame" coordinates, because it means the clocks are "comoving" with the overall mass. In those coordinates, the expansion is a kind of curvature in the time direction, where the time direction follows the diverging world lines of these generically comoving clocks, ergo the "expansion."

Spaceman Spiff
2010-Apr-12, 01:59 PM
With all this space expansion then, is there anything happening with the time component in space-time? Or, is time unaffected by the Universe expanding?

Read the discussion surrounding the two figures (click on each for details) shown on this page (http://en.wikipedia.org/wiki/Metric_expansion_of_space#Understanding_the_expans ion_of_Universe).

mmaayeh
2010-Apr-12, 03:48 PM
I see it is a "comoving frame" that describes the space-time coordinate as a whole. Therefore, time curves along with space and expanding along with space in the "time direction" (it's own unit vector direction -- so to speak) but, if I understood the article by Spaceman Spiff (nice article thank you) then, time will move coplanar on a surface with space. Well, I was about to ask a question about time and why it exists but, I see it maybe obsolete based on this concept.

I always had a "feeling" or "sense" that time exists only because there is an expansion of the universe and time was really a space dimension (4th dimension) and our 3 dimensions of our reality/universe are expanding coplanar with a 4th space dimension (not sure if this made any sense to anyone but perhaps that this is a good reason -- it may not be of any sense). But, based on the article and what you are describing then, time would still exist if the universe was static and not expanding. Or, if the universe was contracting. Is this correct or my question makes any sort of sense?

mugaliens
2010-Apr-13, 03:11 AM
It is everywhere, yet it is nowhere.

At least so far as we're able to discern, as it appears to extend right up to the edge of the observable universe, and quite probably beyond it.

manxman
2010-Jun-30, 03:00 AM
Welcome fellow manxie.

In reply to your OP, Washington.