What happens when a ray of light hits the boundary of the universe?

[Werg22]

I am not sure if the universe ever expanded at speed inferior to that of light, but if it did, I am curious to know what would have happened (if it didn't happen) if a light ray (or any electromagnetic wave that is) had hit the boundary of the universe?

 

[mathman]

Present theories of the universe all describe a universe without any boundary.

 

[Werg22]

Then what is expansion? Is it merely the increase in distances between galaxies into emptiness, emptiness itself having no boundary? So the term "universe" only means the area of emptiness containing all known matter and energy?

 

[natski]

... Is it just me or does this topic have to be explained every 3 days? Think about the balloon analogy, lots of dots on a balloon represent the galaxies. As they balloon expands, the distance between them increases. A light ray will travel along the surface of the balloon and hence there is no boundary.

 

[Werg22]

Oh, I see, thanks.

 

[Milind_shyani]

Hi
As Hubble explained that when a balloon having dots is blown than the distance between the dots increase and it increases more for those dots which are relatively far from each other.Now the universe has no boundary it is not that you will come to an edge and than you will fall off. Also now i have a weird idea that the universe epands at the speed of light

 

[OSalcido]

the universe expands faster than the speed of light

 

[Milind_shyani]

the universe expands faster than the speed of light

Hi
you are wrong for nothing exceeds the speed of light. You might be saying the above for the only reason that you might have a read an article saying that some experiments of NASA show that universe has expanded on all the sides by a great rate and they also think that the expansion of the universe on such a large scale is only possible if they travel at a speed even greater than light
I have come trough various such articles, but its not proved so you can't boast on it.

 

[yenchin]

Hi
you are wrong for nothing exceeds the speed of light. You might be saying the above for the only reason that you might have a read an article saying that some experiments of NASA show that universe has expanded on all the sides by a great rate and they also think that the expansion of the universe on such a large scale is only possible if they travel at a speed even greater than light
I have come trough various such articles, but its not proved so you can't boast on it.

No. The expansion of the universe is the expansion of spacetime itself. That can expand faster than light (in fact it does, especially in the inflation phase) We are talking about the geometry here. When we say nothing exceeds the speed of light, we are talking about object cannot be accelerated pass the speed of light. These are different notions.

 

[youfourian]

IF the "universe" IS expanding, ... what could it be expanding into?
I gave up on trying to figure this out, as I always come to the conclusion that there must be something beyond in order for something to be expanding. A balloon expands into the air around it ... , but if you blow it up too much, what will happen?
I believe in infinity. I know it is a "cop-out", but it works for me. I spent way too much time thinking about these things. I try to stick to things that pertain to reality, the things around me that I have to deal with every day. My environment is much more interesting.
Even if it is expanding, there is nothing we would be able to do about it, or any consequences thereof.
This is just my personal opinion and is not meant to ridicule science, or anyone who studies it. I am simply not a scientist of that nature
I can't grasp the expansion theory.

 

[Sojourner01]

As Hubble explained that when a balloon having dots is blown than the distance between the dots increase and it increases more for those dots which are relatively far from each other.

However, the balloon analogy implies that the universe folds back on itself. As far as I know, it does not.

 

[cristo]

Hi
you are wrong for nothing exceeds the speed of light. You might be saying the above for the only reason that you might have a read an article saying that some experiments of NASA show that universe has expanded on all the sides by a great rate and they also think that the expansion of the universe on such a large scale is only possible if they travel at a speed even greater than light
I have come trough various such articles, but its not proved so you can't boast on it.

This thread-- superluminal recession speed is common --may help you understand further.

 

[cristo]

IF the "universe" IS expanding, ... what could it be expanding into?

This question probably comes about because you are thinking too much of the "balloon universe." Remember that this is an analogy, and so shouldn't be taken as wholly true.

Note that the word "universe" is taken to mean everything that exists; matter, energy, space and time. Therefore, there can be nothing outside it, and nothing for it to expand into. So we see that, in our three dimensions, the universe is not expanding into anything.

Of course, we could be embedded in a fourth space dimension into which the universe expands (like in the example of the 2D balloon expanding in 3D space). However, this would mean nothing to us, and there would be no way for us to observe this, and so we cannot say whether this is true or not. (cf. the "standard" discussion of small people living on the surface of the balloon. They only know of the two spatial dimensions in which they live, and therefore do not know where they are expanding to. However, to a human living in the 3D embedding space, we see that they are expanding in the third dimension).

 

[Ahmed Abdullah]

What is exactly meant by expansion of universe?
The empty space increases so the distance between galaxies increase; I understand it. The matters do not expand, I hope.

 

[cristo]

The matters do not expand, I hope.

No; spacetime expands, and thus the "distance" between every event in spacetime increases. This is another problem with the balloon model, since if one draws dots on a balloon, then inflates it, the dots will increase in size. However, this does not happen in the actual universe. It is better to represent matter in this thought-model of the balloon as, say, small discs of paper stuck onto the balloon which will not expand when the balloon is inflated.

 

[CraigD]

... Is it just me or does this topic have to be explained every 3 days? Think about the balloon analogy, lots of dots on a balloon represent the galaxies. As they balloon expands, the distance between them increases. A light ray will travel along the surface of the balloon and hence there is no boundary.

Of course this needs to be explained every 3 days. It makes no sense, and has no analogy in life. Even your balloon example fails, because the balloon is expanding into the room you are in. The problem with the question of what is the Universe expanding into is that it is a nonsense question. By definition the universe is everything, so if it is expanding into something, then that something is part of the Universe.

The ESA's new probe Plank hopes to show if the universe is open or closed.

CraigD, AMInstP
www.cymek.com

 

[youfourian]

I still cannot come to terms with the universe having boundaries because if there are boundaries, there has to be something else beyond them. I also cannot grasp the concept of nothingness. There could never have been nothing, and there will never be nothing. There has always been, and will always be something. Something cannot be made out of nothing in a physical environment of any kind and this is a physical environment.
We are just complex organisms who live our lives and co-exist with other organisms on a planet that is capable of supporting this phenomenon. We have an end, and everything that we are exposed to in our environment has an end so we are trained to think that everything, including the universe has an end. We figured out how to measure things and we think that everything must be measurable. Infinity cannot be measured, so we do not believe it, so we try to figure out ways to measure something that simply cannot be measured. We can build all the little gadgets in the world to try and we can search the depths of space forever, but I don't believe that we will ever find an end to time and space. The only end is our own.
I don't believe there was a beginning, nor will there be an end of the "universe". Time and space are infinite. We are simply here for a very short period of time and we take up a very small amount of space. There was something before our solar system and there will always be something after it is gone. We are not the center of the universe, nor will we ever find one, because there simply cannot be one. Nor can there be any outer boundary. We should stop wasting time and energy on this and concentrate our efforts on solving more important problems that we have here in our own environment. A great mind can be wasted on trying to figure out impossible problems when it could be used to better what we do have control over, and that is doing things to make a difference here at home.
After all, IF there is any boundary, what would we do if we did find it?
What would be beyond it?

 

[russ_watters]

I still cannot come to terms with the universe having boundaries because if there are boundaries, there has to be something else beyond them.

The second post in the thread very clearly (and correctly) stated that the current theory holds that there is no boundry.

The surface of a balloon works fine here too because the surface has no 2d boundary. It is impossible, however, to visualize a boundless 3d space, so you're probably better off not trying.

I also cannot grasp the concept of nothingness. There could never have been nothing, and there will never be nothing. There has always been, and will always be something. Something cannot be made out of nothing in a physical environment of any kind and this is a physical environment.

Empty space is not "nothingness", it is empty space - and there isn't any of that "outside" the universe either, since there is no "outside". You can travel as far and as fast as you want, but you'll never get to an area where space looks much different from right where we are now.

 

[BoomBoom]

I don't believe there was a beginning, nor will there be an end of the "universe". Time and space are infinite.

We should stop wasting time and energy on this and concentrate our efforts on solving more important problems that we have here in our own environment.

As for the first part, I too believed that all my life up until a few weeks ago when I came here to try and figure out why cosmologists believe what they do. In the process of trying to educate myself in these matters, I realized how full I was of misconceptions and ignorance about the whole concept (and still am, but working on it).

As far as the 2nd part, one must wonder why you wasted your time writing that post?

 

[BoomBoom]

You can travel as far and as fast as you want, but you'll never get to an area where space looks much different from right where we are now.

What if one could get to the actual edge? What would they see?
Assuming you could separate yourself from the expansion, wouldn't you see nothing to the back of you and a bunch a blueshifted galaxies coming at you? Or if something is coming at you at greater than light speed, would you not be able to see it until it was past you?

 

[BoomBoom]

... Is it just me or does this topic have to be explained every 3 days?

These forums are "physics and math help" are they not?

 

[cristo]

What if one could get to the actual edge? What would they see?

I thought we'd established that there was no boundary to the universe-- i.e. the "edge" does not exist?!

 

[russ_watters]

What if one could get to the actual edge? What would they see?

I'm not sure how many other ways to say it: There is no boundary = There is no edge.

 

[Milind_shyani]

No. The expansion of the universe is the expansion of spacetime itself. That can expand faster than light (in fact it does, especially in the inflation phase) We are talking about the geometry here. When we say nothing exceeds the speed of light, we are talking about object cannot be accelerated pass the speed of light. These are different notions.

When you talk about spacetime expansion, can you describe it more clearly

 

[Mentz114]

Milind says:

When you talk about spacetime expansion, can you describe it more clearly

If the distance between two points in 3D space is expressed in terms of a line element

ds^2 = adx^2 + bdy^2 + cdz^2

then if a,b or c are increasing functions of time, the space is expanding.

 

[Milind_shyani]

Milind says:


If the distance between two points in 3D space is expressed in terms of a line element

ds^2 = adx^2 + bdy^2 + cdz^2

then if a,b or c are increasing functions of time, the space is expanding.

Hi
Please forgive me if i am wrong but i think this equation tells us that greater the displacement of an object in space coordinates the lesser its displacement in time coordinates eg: If an object moves at a velocity close to light than time will move slowly for that object.

 

[Mentz114]

i think this equation tells us that greater the displacement of an object in space coordinates the lesser its displacement in time coordinates

No it does not. There is no time co-ordinate in that line element. I notice that some posters talk about space-time expanding, but in the FLRW metric the only the spatial part expands.

If an object moves at a velocity close to light than time will move slowly for that object

Only when observed from another inertial frame.

 

[youfourian]

As far as the 2nd part, one must wonder why you wasted your time writing that post?

I was just curiously browsing threads, waiting to no avail, to see if anyone would reply to mine and this peaked my interest enough to state my opinion on the subject. I thought when nobody replied to my post that I may be in the wrong place so I decided to browse around to see where I was. My apologies for stumbling into the wrong forum and asking questions before getting to know everyone. I guess I just like to waste time

 

[lionell]

IF the "universe" IS expanding, ... what could it be expanding into?:

Hi
I personly believe that our Universe is expanding in a much much much much bigger and older Universe. That would mean there would be no boundry in a infinit chain of "Universes"

 

[cristo]

Hi
I personly believe that our Universe is expanding in a much much much much bigger and older Universe. That would mean there would be no boundry in a infinit chain of "Universes"

This doesn't make any sense to me. Do you have any links which you can provide us with, have you read this in any books, or is this wild speculation?

 

[havonasun]

The concept of universe being a totality, and the speed of light being the fastest are old. They may be absolute, but I have a hard time believing anything is absolute. 'Always' and 'never' are foolish words. We base our beliefs upon the measurements of instruments, which were created by the minds of men during a certain encapsulatum and agreement of knowledge. We constantly learn new and superceding science, so it is ignorant to believe that what we know today is truth in whole. No matter how much we learn, we will always (hehe) only be able to scratch the surface.

My two cents...
The laws of conservation apply to closed systems. If it can't escape, then it still exists within. If the universe is a closed system, then I do not believe light can escape it; and so light will run temporarily along a 'boundary' of sorts or become parabolic around some gravitic center. If the universe is not a closed system, then light will escape. Time and space are inseparable...one creates the other. The instantaneous movement from one point to another would imply zero-time and zero-space. So it follows that time and space are measurable which objectifies the two. If the universe is an open system, and light escapes, where does the light go? Does it deplete the energy of the universe? Does it add to the energy of something else? For light to escape the universe, time and space must be present to support the existence of that light. If time and space are present, then to what extent. Time/space are either infinite, or they have some boundary outside of the universe which would close the system. It seems that if the universe is an open system and light (mass/energy) escapes, either the laws of conservation fail and the universe dies, or energy is fed into the system from without. If the universe is a closed system and the light (mass/energy) stays within the system, the laws of conservation state that the universe will never die, which implies infinity...an open system. Or, if not infinity, then there are laws of conservation of space and time.

So, I can't answer the question.

 

[russ_watters]

You are making the same mistake others have made in this thread. Please reread it and try to undersand the balloon analogy.

The universe has no boundary. Period.

 

[havonasun]

I don't necessarily subscribe to the boundary idea. It most likely thins out into nothingness and has a very irregular shape. Even so, is there not some point where there is no more universe matter? The alternative is that the universe extends out into infinity and is no longer an object unto itself, but a theologic specimen. Unlikely, but not impossible. Either way, the question of whether the universe is an open system or not is still valid.

 

[russ_watters]

It does not "thin out into nothingness". That's basically a soft boundary anyway. It also does not have to be spatially infinite.

Again, reread the thread.

 

[laurelelizabeth]

... Is it just me or does this topic have to be explained every 3 days? Think about the balloon analogy, lots of dots on a balloon represent the galaxies. As they balloon expands, the distance between them increases. A light ray will travel along the surface of the balloon and hence there is no boundary.

How about some sort of 4 dimensional mobius strip? I'm not sure exactly what i mean by that, I just thought it up because on the balloon the dots are only on one side...

 

[havonasun]

Okay, I have reread the thread. The balloon analogy is a theory. One person said the balloon was a 2d object with no edges. That seems to support the boundless theory. One person says that light traverses between objects along the surface of the balloon. Why not away? Expansion implies boundaries. Tessaracts have expanding and contracting cells which support the boundless theory. Is the balloon theory based on hyperdimensional math and stuffed into a 3d model? According to the theory, one could travel (at whatever speed) and never exit the universe...just go around and around and around... A closed system.

 

[cristo]

Okay, I have reread the thread. The balloon analogy is a theory. One person said the balloon was a 2d object with no edges.

The surface of the balloon is a 2d object with no boundary. That's the point of the model.

One person says that light traverses between objects along the surface of the balloon. Why not away?

Because, to beings living on the surface of the balloon, the third dimension does not exist.

Expansion implies boundaries.

No it doesn't: blowing up the balloon causes it to expand: but it has no boundary.

I really don't see where the problem is here!

 

[russ_watters]

The balloon analogy is a theory.

Yes, it is. And there is very, very good evidence to support it. For example, we can look a very, very long distance and detect none of this "thinning" that you suggest could exist. The universe looks exactly the same in every direction to a very large degree. This implies that there is no boundary. There are other pieces of evidence as well, but until you can understand what the model says, we're getting ahead of ourselves to talk about the evidence for it.

Anyway, you're still not getting it, though cristo did a good job repeating the exact same thing already said a dozen times.

Please read more carefully!

 

[havonasun]

Ok, I'm really trying to grasp what is being said here and I appreciate your comments.

Because, to beings living on the surface of the balloon, the third dimension does not exist.

But, we're not 2d beings. How does this model apply in reality?

 

[russ_watters]

But, we're not 2d beings. How does this model apply in reality?

That's why it is an analogy. The same type of phenomena (finite, but without boundary) that you see on the 2-d surface of a balloon appears to occur in the 3d space of our universe. As the 2d balloon is curved in a 3rd dimenson that isn't necessarily visible from the surface of the balloon (to an amoeba, it may look flat), the universe may also be curved in higher dimensions we cannot see.

 

[havonasun]

Ok, I agree with that.
I found this quote...

Is the universe finite or infinite?

"The observable universe is finite," Sweitzer said, which is to say that it had boundaries -- physical limits. Sort of. "It's a boundary to the events we can see directly, but not a boundary in the sense that New York State has a boundary."

And in an expanding universe, this boundary is constantly moving, as is everything within it. Cosmologists typically invoke a balloon with spots on its surface, representing galaxies, to explain the expanding universe. As the balloon is inflated, the spots grow farther apart. If you stood within one of these spots, you'd see all the others moving away from you, and the most distant spots would move appear to move the fastest.

Sweitzer goes on to say that the observable universe is probably part of a much larger universe, "which could be finite or infinite. Any global statements about the universe, such as overall extent, are speculative because they require extrapolating local mathematical theories and measurements beyond the observable universe."

What I'm understanding is that the balloon analogy was a way to explain the physical movement of the galaxies relative to one another. Ok, so the universe is expanding. Thank you.

 

[russ_watters]

Well, you're missing something there. When they talk about the "observable universe", they are talking about an expanding sphere of what we can see due to the distance the speed of light can travel in a time limited by the age of the universe.

That is different from the entire universe, which need not have any boundary.

 

[marcus]

Havona, you are quoting something over 6 years old that was said for a general public audience by a guy who is not a working cosmologist
but is a "director of education" at something like a natural history museum

http://www.space.com/scienceastronomy/astronomy/universe_overview_010605-1.html

thats OK for starters Havona, but I'm confident you can do better than that (you sound reasonable and serious)

I see your quote comes from page 2 of the article. James Sweitzer, the guy you quote, is being imprecise in that article, or the SPACE.COM reporter was being sloppy. I guess you know not to take everything you read in Space.com for gospel Actually I'd believe either Cristo or Russ Watters over James Sweitzer and popular space media any day! Plus the extra thing about them is you can ask questions back and get clarification.

I'm trying to figure out what you want to understand. What, in this thread, do you find confusing or unsatisfactory? What do you NEED to understand that you might not realize yet that you need to understand it?

 

[marcus]

Well, you're missing something there. When they talk about the "observable universe", they are talking about an expanding sphere of what we can see due to the distance the speed of light can travel in a time limited by the age of the universe.

That is different from the entire universe, which need not have any boundary.

I agree strongly. The observable really is a different kettle of fish. It is merely the part of the whole which we have got light from SO FAR.
The observable universe constantly increases as we get signals coming in from farther and farther

I also think you put your finger on what Havona and a lot of other newcomers are MISSING. they can't picture the 3D analog of the expanding boundaryless 2D surface of the balloon.

Maybe we need a tutorial that everybody gets put thru if they want to discuss cosmology.

(BTW I don't understand why this thread was started in Astrophysics since the topic is cosmology---the shape and dynamics of the universe---not individual stars and star systems.)

As a math student you get told about the 3-sphere early on and a way it may get introduced is as the ONE POINT COMPACTIFICATION OF EUCLIDEAN THREESPACE.

Euclidean R³ is just your familiar x,y,z space, as usual as graph paper. And then you picture adjoining just ONE EXTRA POINT the point at infinity in all directions. And that is the topological space S³,
It is easy to imagine walking around in because it is just like an infinitely big classroom with the extra feature that a straight line heading off in one given direction eventually comes back.

Another way to picture the 3-sphere S³ is as the "skin" on a 4D BALL. And then you have a radius r and you can write down the finite 3D volume of S³ in terms of r.

It is 2 pi² r³
================

Maybe it is too much to expect of newcomers like Havona that they teach themselves to picture living in S³
and pick up a little understanding of it. There is not much, it's so basic.

My hunch is that in Havona case it is not too much to expect at all, but it might be as a general rule.

The good thing is that S³ is FINITE AND BOUNDARYLESS THREE DIMENSIONAL SPACE WHICH CAN EXPAND in an easy-to-picture way: just like the balloon expands which is simply the lower dimensional version of it.

So if a newcomer can picture S³ then there might not be so much reluctance to accept basic cosmology ideas. Just a thought.

 

[M.g.f.s.n.]

Could the universe's boundary simply be defined as the point at which light becomes visible?

A spot 500 Trillion Million light years away obviously hasn't any light for which to define anything, no energy for which to create anything, and no medium for which to house matter. In essence, wouldn't that basically be non-existant?

 

[marcus]

Could the universe's boundary simply be defined as the point at which light becomes visible?

A spot 500 Trillion Million light years away obviously hasn't any light for which to define anything, no energy for which to create anything, and no medium for which to house matter. In essence, wouldn't that basically be non-existant?

just to try to be extra clear, M.g., we are talking about the extent of SPACE.

Professional cosmologists have a small set of models, basically mostly all versions of the same model with different numbers plugged in.

Some models have space be INFINITE in extent. Some have space be FINITE.

In no case do they ever have space be BOUNDED That is a no-no.

the models with infinite space fit the data roughly as well as those with finite space. In the future, to decide which, we need to get more data.
=============

Now your question is about some point that is currently something like a billion trillion lightyears away from us.
that is perfectly OK, it could be a place in space that looks rather much like the Milkyway galaxy or the Andromeda, except it wouldn't be exactly the same. It would differ in details.
The infinite-space model would accommodate that just fine. A billion trillion is no problem.
In that model, expansion began over a broad front----she was already infinite at bigbang time.

But you have a different idea which I never heard a cosmologist say:

A spot 500 Trillion Million light years away obviously hasn't any light for which to define anything, no energy for which to create anything,...

I don't see any reason to suppose this, so I guess you are mistaken.

 

[blighty]

How you see many things has little to do with what "IS" happening.
Eyes are ment to help you find food and evade preditors, not for
scientific purposes, that is what the brain is for.

we can create our own optical illusions and so can the universe.

 

[Gmanme]

To OP:

Mybe the the universe isn't growing at all, and has always been the same size. Mybe the only thing that has changed is TIME in the universe, Making the illusion of a changing distance. You never know.



Naw, just messin with you... :P


Theres no way too, but you have to imagine a finite 3d object with no boundries. The baloon is really only 2d object with no boundry, by simply making a circle. You have to assume anything outside the surface of the baloon is outside the universe, Which is imaginary, like thoughts, outside the universe doesn't exis't physically in any way, Its just a concept.

 

[zankaon]

Within GRT formalism, one can speak of a 4-sphere with 3-faces; or more easily, a 4-cube with 3-faces. So a light ray (null geodesic) would be described in such 4-D. Even more easily, what about a time-like geodesic, like us, in free fall, as for our star. Then such time-like geodesic would never bounce off of anything; rather one could consider it as if it 'rolls over' even if near some conceived near to maximum extent of the manifold (i.e. continuum).

 

[cbd1]

Yes, it is. And there is very, very good evidence to support it. For example, we can look a very, very long distance and detect none of this "thinning" that you suggest could exist. The universe looks exactly the same in every direction to a very large degree. This implies that there is no boundary. There are other pieces of evidence as well, but until you can understand what the model says, we're getting ahead of ourselves to talk about the evidence for it.

Anyway, you're still not getting it, though cristo did a good job repeating the exact same thing already said a dozen times.

Please read more carefully!

There are thin areas in the cosmic microwave background. It is not uniform in all directions. This means space cannot be infinite in all directions, for there would be infinite stars in all directions and light would be shining equally from all directions. right?