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-   -   Could We See the Edge of the Universe? (http://www.physicsforums.com/showthread.php?t=108385)

 Wings of Pegasus Jan27-06 03:00 PM

Could We See the Edge of the Universe?

Everywhere I've looked, it says that the universe is at, getting to, or already past the speed of light and quickly accelerating. Can we ever get to see the edge of the universe? Could we see it as it was in the past, because it seems we will never see it in the present?

 chroot Jan27-06 03:07 PM

Light travels at a finite velocity. Since the universe is not infinitely old, light from one place in the universe can only have gone some finite distance away from its origin. This means that there are (or at least, could be) things in the universe that are so distant that light from them has not yet had time to reach us.

The furthest distance an object may be from us and still have had time for its light to reach us is called the 'particle horizon.' Objects which are visible to us are said to be within our 'past light cone.'

As you mentioned, because the expansion of the universe appears to be accelerating, the number of objects visible in our past light cone is not rising as quickly as the radius of our past light cone. This means the number of objects visible to us is actually going down over time.

- Warren

 Wings of Pegasus Jan27-06 06:49 PM

So, by studying the direction from which everything is moving away, we could theoretically find the point from where the big bang theory began?

 Leonardo Sidis Jan28-06 10:52 AM

Quote:
 Quote by Wings of Pegasus So, by studying the direction from which everything is moving away, we could theoretically find the point from where the big bang theory began?
Yes, but I think there is a cluster of galaxies near us that are moving as a group in the same direction, so if the universe continues expanding forever (if the average density of the universe isn't enough to reverse the expansion from the big bang; called the big freeze theory), eventually we will only be able to see the cluster of galaxies moving in the same direction as us.

 DaveC426913 Jan28-06 11:29 AM

Quote:
 Quote by Wings of Pegasus So, by studying the direction from which everything is moving away, we could theoretically find the point from where the big bang theory began?
No. There is no centre/the centre is everywhere.

 matt.o Jan28-06 11:30 AM

if we studied the direction with which everything is receding from us, disregarding local galaxies, we would find that we are at the point where the big bang began...

 chroot Jan28-06 02:36 PM

As has been said, there is no "center" of the expansion. Everything is moving away from everything else.

Some people have likened the expansion of the universe to the rising of a loaf of raisin bread. As the bread rises, every raisin gets further away from every other raisin.

No matter where you are in the universe, the expansion will look the same -- as though everything is moving away from you.

- Warren

 Leonardo Sidis Jan28-06 02:36 PM

Quote:
 Quote by matt.o if we studied the direction with which everything is receding from us, disregarding local galaxies, we would find that we are at the point where the big bang began...
I don't understand. Certain galaxies are moving faster away from us than others are, and this can determined by redshift, right? The farther away a galaxy is, the faster it is moving away. So if you measure the redshift of many different galaxies, shouldn't you be able to develop an idea as to where the lines which they are travelling in converge? And then wouldn't this be approximately the point of the big bang?

 Leonardo Sidis Jan28-06 02:39 PM

Quote:
 Quote by chroot As has been said, there is no "center" of the expansion. Everything is moving away from everything else. Some people have likened the expansion of the universe to the rising of a loaf of raisin bread. As the bread rises, every raisin gets further away from every other raisin. No matter where you are in the universe, the expansion will look the same -- as though everything is moving away from you. - Warren
Ah, that helps a little. But if this is true, then how could there have been a big bang? What would have had to have happened during it to have resulted in the current state of the universe?

 chroot Jan28-06 02:43 PM

That's a relatively large question, Sidis, one which I cannot answer appropriately with a forum response. I suggest you pick up a book which includes some cosmology (like Carroll and Ostlie's Introduction to Astrophysics). If you have some specific questions about big-bang cosmology, I can try to answer them, however.

- Warren

 Leonardo Sidis Jan28-06 02:58 PM

Quote:
 Quote by chroot That's a relatively large question, Sidis, one which I cannot answer appropriately with a forum response. I suggest you pick up a book which includes some cosmology (like Carroll and Ostlie's Introduction to Astrophysics). If you have some specific questions about big-bang cosmology, I can try to answer them, however. - Warren
Thanks! I was just thinking, the empty space between galaxies is expanding at an increasing speed and faster than the speed of light, right? This doesn't violate Einstein's rule that the speed of light is the maximum speed in the universe because it's empty space that's expanding, correct? So is that basically the main idea of the big-bang theory, that the empty space between the components of an incomprehensibly dense particle expanded in a "bang"?

 Leonardo Sidis Jan28-06 03:04 PM

Also, is it true that a galaxy facing North Pole is receding from Earth just as fast as a galaxy facing the south side of Earth if the two galaxies are the same distance from Earth? Or would it depend on how much matter is between Earth and the galaxies?

 matt.o Jan28-06 03:06 PM

"big bang" was a phrase coined by Fred Hoyle in an effort to discredit the theory (he had his own pet steady state theory). It has nothing to do with an explosion, since when people think of an explosion they think of something expanding into something else. The Universe does not expand into something else.

Superluminal recession "velocities" do not violate special relativity since nothing is moving through space, rather the space between the observer and the observed is "stretching".

 mijoon Jan28-06 03:36 PM

Once again I must suggest reading this excellent little paper
on misconceptions regarding expansion.

 matt.o Jan28-06 03:40 PM

Yes indeed, can't recommend that paper enough.

 |Orion's Thought| Jan29-06 05:11 PM

Quote:
 Everything is moving away from everything else.
Isnt there one possible scenario? That the big bang exploded in a sphere/spherical shape, so everything is moving away from everything else, but there is a common point of origin? And because there are galaxies in front of and behind us, isnt it possible that there is one other galaxy going the same direction as we are?

If Andromeda is supposed to collide with us, doesnt that defeat the thing about everything moving away from everything else? Am i missing something?

 chroot Jan29-06 05:32 PM

Quote:
 Quote by |Orion's Thought| Am i missing something?
Yes, you are missing something -- an understanding of non-Euclidean three-dimensional geometry.

As another analogy, consider the surface of a balloon. This surface is only two-dimensional, rather than three, but illustrates the expansion of the universe quite well. This two-dimensional space is finite, yet has no boundaries.

Imagine that the balloon was originally very small, and you drew a few dots on its surface, representing galaxies. As you add air to the balloon, all of the dots become further apart. As you blow the balloon up, the two-dimensional space "expands." However, there is no "center" to the expansion -- everything is moving away from everything else. The further two dots are from each other to begin with, the faster they will appear to be moving apart as you blow the balloon up. No matter which dot you consider to be "home base," it looks as it all other dots are moving away from home base, with the same speed/distance depedence.

Our universe appears to be the same sort of space, only with three dimensions instead of two. The inclusion of another dimension does not, however, affect any of the features I've mentioned.

- Warren

 |Orion's Thought| Jan29-06 06:10 PM

But once you "take the air out of the balloon", wouldnt all those dots come back to a certain area? Or are you saying that the dots were never at the same point in the first place?

Andromeda?

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