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Andrew D
2009-Nov-30, 07:48 PM
Hello everyone, first post here.

Here's my question:

Since the Universe is constantly expanding, is it possible to observe the direction of expansion and define a point which matter is expanding from, i.e., the center of the Universe?

Swift
2009-Nov-30, 07:51 PM
Hi Roobydo, welcome to BAUT.

As to your question - no. One analogy that is often used is to compare the universe to the surface of a balloon. The balloon expands, the points on its surface continue to become further and further apart, but to an ant walking around on the surface of that balloon, there is no center.

Andrew D
2009-Nov-30, 08:19 PM
But isn't most expansion be contributed to inertia? If so, matter has to be moving from somewhere, to somewhere, that is, from the location of the big bag into "space", and not just away from other matter.

As the surface of the balloon expands into three dimensional space, the points on the surface don't just move further away from each other, they all also move away from the center of the three dimensional shape that the balloon forms.

01101001
2009-Nov-30, 08:35 PM
Welcome to BAUT Forum.

Sean Carroll's Cosmology FAQ (http://preposterousuniverse.com/writings/cosmologyprimer/faq.html):
Question: Does the Universe have a center? (http://preposterousuniverse.com/writings/cosmologyprimer/faq.html#center)


No. Our observable universe looks basically the same from the point of view of any observer. [...]

BAUT Forum topic ** FAQs ** Resources On The Web (http://www.bautforum.com/space-astronomy-questions-answers/22865-faqs-resources-web.html) is an excellent resource.

cosmocrazy
2009-Nov-30, 09:06 PM
But isn't most expansion be contributed to inertia? If so, matter has to be moving from somewhere, to somewhere, that is, from the location of the big bag into "space", and not just away from other matter.

As the surface of the balloon expands into three dimensional space, the points on the surface don't just move further away from each other, they all also move away from the centre of the three dimensional shape that the balloon forms.

Its not as simple as the analogy infers. The geometry of space/time is more complicated than the surface of the balloon. In the analogy the 2D surface of the balloon is crudely representing the whole of space/time (4D). The centre of the balloon and the space between and around the surface that you imagine in this analogy is actually what we would consider outside of space/time (if any such thing actually exists).

Based on the big-bang model and how the expansion of space appears to propagate then as Swift pointed out there appears to be no defined centre, but the whole of the universe or any point you choose is the centre expanding very rapidly.

Andrew D
2009-Nov-30, 09:09 PM
I understand; it's not that the big bang occured at any one point within the universe, but at the time of the big bang that point was the entire universe.

However, in order to expand, the universe would have to be occupying more 'space' at any given moment than it did the moment before, but that's not the case. It doesn't occupy more space, it contains more space. To say the universe is expanding, you have to define the 'space' that the universe is expanding into, and by its very nature, anything outside of the universe is undefined. It is much easier to visualize a universe with a finite border that contains matter which is constantly shrinking than one that is expanding into the absence of itself.

Swift
2009-Nov-30, 10:04 PM
To say the universe is expanding, you have to define the 'space' that the universe is expanding into,
I don't believe that is correct. I don't think you have to define what the universe is expanding into, it is not expanding into anything. It is just expanding.

antoniseb
2009-Dec-01, 12:17 AM
... To say the universe is expanding, you have to define the 'space' that the universe is expanding into, and by its very nature, anything outside of the universe is undefined. ...

Are you, by any chance, letting the English language, and the nuances it contains when describing small things interfere with your understanding of things at the largest scale?

I can't tell from your writing if you are letting the limits of language rule you, or if you are fighting them. It appears you probably understand what the mainstream cosmology really says... well, except for that idea about inertia...

01101001
2009-Dec-01, 01:23 AM
To say the universe is expanding, you have to define the 'space' that the universe is expanding into, and by its very nature, anything outside of the universe is undefined. It is much easier to visualize a universe with a finite border that contains matter which is constantly shrinking than one that is expanding into the absence of itself.

Yeah, the Universe's nature and your ease of visualization may not be compatible.

Sean Carroll's Cosmology FAQ (http://preposterousuniverse.com/writings/cosmologyprimer/faq.html):
Question: What is the universe expanding into? (http://preposterousuniverse.com/writings/cosmologyprimer/faq.html#into)


As far as we know, the universe isn't expanding "into" anything. When we say the universe is expanding, we have a very precise operational concept in mind: the amount of space in between distant galaxies is growing. [...]

forrest noble
2009-Dec-01, 04:32 AM
Roobydo,


Since the Universe is constantly expanding, is it possible to observe the direction of expansion and define a point which matter is expanding from, i.e., the center of the Universe?


One way to explain the expansion of the universe according to the Standard Model, involves the proposed expansion of space. An original "Big Bang" accordingly involved the expansion of space from a single point or relatively small size. From this perspective the space surrounding us, and everywhere else in the universe, occupies that original space -- which has expanded. You might say, accordingly, that the universe is expanding as it creates new space.

According to the prevailing explanation no one part of space is any more significant than any other part of space. For this reason the Big Bang was not, accordingly, a normal explosion stretching out spherically where one could identify its center.

If space is completely flat then the universe would accordingly have a three dimensional volume to it (or flat space-time) that would have relative positions whereby a general central area might be recognized. This is not the standard model of the universe however.

Einstein proposed a fourth physical dimension to space, that the warping of the entire mass of the universe closes space. From this perspective if one would travel in a straight line one would end up in the same location that he started. This is analogous to the explanation concerning no center to the universe.

quotes from this link (at bottom): http://math.ucr.edu/home/baez/physics/Relativity/GR/centre.html


Of course there are many other even less uniform shapes the universe could have, with or without an identifiable centre. If it turned out to have a centre on some scale beyond the observable universe, such a centre might turn out to be just one of many "centres" on much larger scales, just as the centre of our galaxy did before.


In other words, although the standard Big Bang models describe an expanding universe with no centre, and this is consistent with all observations, there is still a possibility that these models are not accurate on scales larger than we can observe. We still have no real answer to the question "Where is the centre of the universe?".

forrest noble
2009-Dec-01, 04:50 AM
Roobydo,


It is much easier to visualize a universe with a finite border that contains matter which is constantly shrinking than one that is expanding into the absence of itself.

Although you are completely correct concerning "visualization," this is not the Standard Model nor one of the accepted Standard Model hypothetical possibilities. Instead, accordingly, one might say that the universe defies Newtonian visualization.

Andrew D
2009-Dec-01, 04:07 PM
"more like an expanding infinite sheet than a finite spherical surface"

-Andrei Dmitriyevich Linde


got it.