What is the issue with the concept of a center in the expanding universe?

[Quinzio]

I'm aware I'm raising a topic which must have been discussed several times, but I couldn't find anything specific to my question.
So...
Let's assume the universe began from a big bang, which seems for sure.
Since then the universe expanded, and it's currently growing.
One question, which leaves me sleepless at night is: inside what is the universe expanding ? Ok, the answer is: into the nothingness.
I doubt there is someone out there who has a faint idea of what a "timeless and spaceless" nothing is. My first objection is that the verb "to expand" requires an outside, otherwise we should use another verb.
The idea of expansion into nothing is senseless to me.

But move on. Something else makes me sleepless. If the universe began from a singularity (the big bang), there must be a boundary, and a center.
I am more keen to the idea of centerless universe, but since nothing travels faster than light, everything must be within a certain distance from the big bang point, wherever it is.
Otherwise, what does it mean that "we cannot look further than 13 billions light years away" ?
I point my telescope to a star, then to a further star, and so on. I cannot see any star further than 13 billion l.y. away. So there is a final star. Where am I wrong ?
If there's no center, every star is the last star.
Something tell me that we are playing with concepts we don't understand at all.
Probably the whole notion of measure loses it's meaning.
I dunno.
I want my sleep back.

 

[Vorde]

The answers to both of your questions are related, and they both stem from a misconception you have about the universe.

I would suggest reading up on the shape/structure of the universe (either by searching PhysicsForums or the web- wikipedia has a good article on the universe), but I will briefly explain the specific answers to your questions.

Regarding your first question, the universe isn't expanding into anything. And I don't mean nothingness, I mean it isn't expanding into anything. Distances between objects within the universe are increasing, which is consistent with expansion but does not require the universe to be expanding into anything. Search 'expansion on the universe' and check out wikipedia's piece on it.

Regarding your second question, the answer is a bit too complicated to briefy explain, but suffice to say the universe does not need to have a boundary and it does not need to have a center. The wikipedia article on 'shape of the universe' does a really good job of explaining this.

 

[DrStupid]

inside what is the universe expanding?

The universe don't need anything to expand inside - not even "nothingness" (whatever that might be).

If the universe began from a singularity (the big bang), there must be a boundary, and a center.

Why?

but since nothing travels faster than light, everything must be within a certain distance from the big bang point, wherever it is

The "big bang point" is everywhere and the expansion of the universe is not limited by the speed of light or any other speed.

Otherwise, what does it mean that "we cannot look further than 13 billions light years away" ?

That means that we cannot see anything that is older than 13 billion years.

Something tell me that we are playing with concepts we don't understand at all.

Majestic plural?

 

[gulfcoastfella]

You may also want to read a brief book called Flatland. It was written some time ago, but it's still as relevant today as it was when it was written. You can extrapolate some things from it's 2D stories that apply equally well in 3D (and 4D).

 

[marcus]

Let's assume the universe began from a big bang, which seems for sure...
... If the universe began from a singularity (the big bang),..

Quinzio, you need to flush that unfortunate misconception out of your brain and make a fresh start.
Professional cosmologists do not imagine that there was a "singularity" from which expansion began.
A "singularity" is what is assumed NOT to have occurred or existed in nature.

It represents a failure of the old-fashioned big bang model.
So researchers are working on new versions, new cosmic models, that do not have a singularity---that do not fail as they go back in time and do not start to predict meaningless infinities. So there are now various "non-singular" cosmic models being studied by experts and to be described and discussed at the major international conferences like MG13 (this July in Stockholm) and GR20 (next year in Warsaw)

 

[sahmgeek]

Quinzio - cosmology is very hard to make sense out of conceptually. That's why scientists use math. Ordinary language doesn't do it justice. The term "Expanding universe" does in fact imply to the layperson that the "universe", in it's entirety (whatever that is), is expanding. If it is the case that "distances between objects within the universe are increasing" as Vorde suggests, then that is an altogether different idea. Confusion always arises when people are not clear with their terminology. From what I have gathered here and there (please someone correct me if I'm misinformed!), when scientists say "universe" they are referring to the "observable universe", the portion of the universe that we can observe and measure and it is within this observable portion that distances between objects appear to be increasing. Calling this an "expanding universe" is, imo, just bad terminology.

p.s. i would recommend some tea with a little valerian root for the insomnia :)

 

[marcus]

What you see in ordinary mainstream cosmology (something like 99% of the published papers) is a model of the universe as something which is spatially without boundary.

That is, you could say, how the concept of universe is defined: spatially speaking it is the thing that has no boundary.

This means that expansion can only be pictured/experienced from the inside.

One experiences and measures expansion as the gradual increase of distances between stationary objects.

Since it has no boundary, space can have no "outside". So expansion cannot be pictured from the outside. This little difficulty normally trips up beginners. They are used to thinking of expansion seen from outside.

You have to learn to think of expansion seen from within, as a pattern of increasing distances---it is currently a percentage growth rate of about 1/140 percent per million years. So naturally the larger distances increase faster.

Sahmgeek,
I haven't mentioned observable universe because that is something different that does not enter the basic discussion. The observable region is the portion of the universe which we can see today. It has a boundary---an horizon, called the "particle horizon". And that horizon pushes constantly farther and farther out: as the age of the U grows there is more time for light to reach us. So as time goes on we get the light from more and more matter.
So the growth of the *observable* universe involves TWO effects, not only the basic Hubble Law expansion which occurs throughout the cosmic model BUT ALSO the increasing age of universe effect that would make the observed region grow even if distances themselves were not expanding. Also the observable portion DOES have a boundary of course, and we are at the center. So let's not complicate matters by adding that secondary concept prematurely. First let's understand the basic idea of the (boundaryless) universe as cosmologists model it, and the pattern of expanding distances.

 

[sahmgeek]

the observable portion DOES have a boundary of course, and we are at the center.

Well, that's like a punch in the gut. I guess I've heard this tossed around before but the implications are just sinking in.

 

[Vorde]

Well, that's like a punch in the gut. I guess I've heard this tossed around before but the implications are just sinking in.

Be careful not to mix up observable universe with universe, we are not at the center of the universe. By definition, we are at the center of the observable universe.

 

[sahmgeek]

Be careful not to mix up observable universe with universe, we are not at the center of the universe. By definition, we are at the center of the observable universe.

right, I got that. so, have we measured that expansion is occurring at the same rate in all directions? does that imply that we are stationary and everything is expanding around us? I may be very confused right now

 

[Mark M]

Well, that's like a punch in the gut. I guess I've heard this tossed around before but the implications are just sinking in.

Why? By definition, we must be at the center of the observable universe. The observable universe is just our light cone. Since light has a finite speed, as an observer looks farther from his position, the older the light he will see. Since the universe is finite in age, there is a portion of the universe that we can see. The 'boundary' is the particle horizon, the point at which we observe the CMB, when the universe started emitting light.

right, I got that. so, have we measured that expansion is occurring at the same rate in all directions? does that imply that we are stationary and everything is expanding around us? I may be very confused right now

No, it is because the universe is expanding everywhere at once at the same rate. Imagine a 2-dimensional universe existing on the surface of a sphere. All that they perceive is the surface. Imagine the sphere growing in size, so that the surface grows. Now, the distance between everything increases. All observers are justified in saying everything is moving away form them.

 

[Vorde]

right, I got that. so, have we measured that expansion is occurring at the same rate in all directions? does that imply that we are stationary and everything is expanding around us? I may be very confused right now

I used to be in the same position as you, I would search (either the web or the forums) for either the raisin analogy or the balloon analogy, those are both good examples of how we can measure the universe expanding all around us and not be in the center of it.

 

[sahmgeek]

I'm on Ned Wright's balloon analogy page. I don't think my brain is working properly :). thanks!

 

[Chronos]

We are at the temporal edge of the universe because nothing in the observable universe is older than us relative to the BB. But, it also appears we are at the center of the observable universe because we can see equally 'distant' in every direction. It is obviously illogical to be both at the center and edge of any geometrical shape aside from a point. The only logical alternative is the universe has no edge or center.

 

[alexg]

I would just like to comment that the balloon analogy seems to have caused more confusion than any other explanation of universal expansion.

Of course, most of the confusion is because of people who don't know what an analogy is.

 

[sahmgeek]

No, it is because the universe is expanding everywhere at once at the same rate. Imagine a 2-dimensional universe existing on the surface of a sphere. All that they perceive is the surface. Imagine the sphere growing in size, so that the surface grows. Now, the distance between everything increases. All observers are justified in saying everything is moving away form them.

yes, exactly. thank you.

I was somehow imagining a 360 degree expansion from a central point (us) and drawing strange conclusions.

 

[Mark M]

I would just like to comment that the balloon analogy seems to have caused more confusion than any other explanation of universal expansion.

Of course, most of the confusion is because of people who don't know what an analogy is.

Yeah, the most common problems with the BA are that people either forget it is only about the surface, or they get the idea the universe must be embedded in a higher dimensional space.

yes, exactly. thank you.

I was somehow imagining a 360 degree expansion from a central point (us) and drawing strange conclusions.

Yes, just remember that the universe is not bounded - there is no mysterious boundary that expands.