Could We See the Edge of the Universe?

[Wings of Pegasus]

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]

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]

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]

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]

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]

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]

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]

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 traveling in converge? And then wouldn't this be approximately the point of the big bang?

 

[Leonardo Sidis]

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]

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]

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]

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]

"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]

Once again I must suggest reading this excellent little paper [/PLAIN]
on misconceptions regarding expansion.

 

[matt.o]

Yes indeed, can't recommend that paper enough.

 

[|Orion's Thought|]

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, isn't it possible that there is one other galaxy going the same direction as we are?

If Andromeda is supposed to collide with us, doesn't that defeat the thing about everything moving away from everything else? Am i missing something?

 

[chroot]

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|]

But once you "take the air out of the balloon", wouldn't 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?

 

[DaveC426913]

But once you "take the air out of the balloon", wouldn't 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?

There's an ant living on every dot. Each ant looks around his universe and sees the other nearby ants. When the balloon is expanding he sees every other ant on every other dot receding from him. Nearby ants-on-dots are receding slowly, ants on more distant dots are receding faster. All ants are (on average) receding.

Now reverse it. When the balloon is deflating, each ant sees every other ant-on-a-dot rushing towards it.

Each ant sees himself at the centre of the universe. Which one is right?


You, in your 3D view of the balloon may be able to point to the centre of the balloon, but the ants, living only in two dimensional surface, can only point to a spot on the surface of the balloon. Can any ant point to the center of expansion/contraction? Well, yes, sort of, they all point at the dot under their own feet!

 

[|Orion's Thought|]

Suppose that one ant, smarter than the rest, takes into calculation his position and movement relative to the other ants and their positions and movements and creates an equation for a line and figures that the center of the balloon is somewhere on that line, and he is moving away from it, so now he knows what direction in which to look. Then, this smart little ant figures out an equation for another ant on another dot. He finds where the two lines intersect and figures that must be the center of the balloon, and he then checks this with another line from a third dot... etc.

Can any ant point to the center of expansion/contraction? Well, yes, sort of, they all point at the dot under their own feet!

Ants are next in line for big brains... Just kidding.

 

[DaveC426913]

You're missing the point. The ants are not in error. Every measurement they make (and they make corerct measurements) says that all ants nearby are moving away from them symmetrically (this is what actually happens).

Pretend the balloon is painted up like a soccer ball - with an ant at the center of each hex. At 3" in diameter, every hex shape is (I dunno) 1/2" across. The balloon expands to 10x its size, now the balloon is 30" in diameter. Every hex is now 5" across.

You see, each and every hex, and thus each and every ant's viewpoint is equally valid. It's not a perception thing, it really is true.

 

[|Orion's Thought|]

I'm sorry, I just don't understand what you mean. If every ant thinks that the center of the balloon is below their own feet, then each and every one of them is right, but couldn't one measure where another ant thinks is below its feet, and compare, and therefore tell exactly where the center of the balloon is? And because they all move relative to one another, where they think the center should be shouldn't change.

 

[SpaceTiger]

I'm sorry, I just don't understand what you mean. If every ant thinks that the center of the balloon is below their own feet, then each and every one of them is right, but couldn't one measure where another ant thinks is below its feet, and compare, and therefore tell exactly where the center of the balloon is? And because they all move relative to one another, where they think the center should be shouldn't change.

Perhaps you're being confused by the fact that the balloon has a center in three dimensions. Dave is saying that there's no center on the two-dimensional "surface" of the balloon. One can imagine stretching an infinite rubber sheet and a similar effect would occur. This may be more analogous to the real universe because a balloon has curvature. We measure the universe to be flat (which is discussed in this thread).

 

[chroot]

Yes, sorry I didn't make it clearer in my post that the ants living on this balloon only experience a (curved) two-dimensional space.

What may be surprising is that the mathematics of such curved 2D spaces is well-defined even without any reference to a higher-dimensional space. In other words, my analogy dealt with a 2D surface (a balloon) embedded in normal 3D space. Mathematically, however, there's no need for any such external space to exist. In such a case, there simply is no direction called "down" for the ants. The totality of their world is two-dimensional, in exactly the same way that the totality of ours is three-dimensional.

- Warren

 

[Chronos]

. . . If Andromeda is supposed to collide with us, doesn't that defeat the thing about everything moving away from everything else? Am i missing something?

No. The cosmological constant [dark energy] is an extremely feeble force. Gravitation, in the local group of galaxies, is much stronger. Some theories predict that may change someday [e.g., the big rip], but not in the foreseeable future.

 

[|Orion's Thought|]

In such a case, there simply is no direction called "down" for the ants.

Is there a direction "away" and "towards"?

 

[chroot]

Is there a direction "away" and "towards"?

No. In a two-dimensional world, even one which is curved, there are only two "axes": left and right, and forward and back, for example.

- Warren

 

[|Orion's Thought|]

Oh, ok, I see what you mean. Thanks for being patient with me.

 

[DaveC426913]

Keep in mind that, in this 2D world, light rays are also constrained to 2D. Just as in our 3D world light is constrained to 3D.

As surely as we cannot point in the direction of this higher dimension, so are we unable to shine a light beam that way.

This is just one example of how internally consistent these N-dimensional worlds are.

 

[|Orion's Thought|]

Ok, that also explains why we can't find a line going "downwards" or "away". Thanks everyone!

 

[OSalcido]

if everything is receding from everything... how is it that our galaxy will eventually collide with the andromeda galaxy?

 

[Chronos]

Gravity is much stronger than the Hubble flow [expansion] at 'short' [less than megaparsec] distances. The proper motion of the Andromeda galaxy puts it on a collision course with our galaxy in about 5 billion years.