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

1. May 23, 2007

### 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?

2. May 23, 2007

### mathman

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

3. May 23, 2007

### 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?

4. May 23, 2007

### 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 ballon expands, the distance between them increases. A light ray will travel along the surface of the balloon and hence there is no boundary.

5. May 23, 2007

### Werg22

Oh, I see, thanks.

6. May 24, 2007

### Milind_shyani

Hi
As hubble explained that when a ballon 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

7. May 24, 2007

### OSalcido

the universe expands faster than the speed of light

8. May 24, 2007

### Milind_shyani

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 cant boast on it.

9. May 24, 2007

### yenchin

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.

10. May 24, 2007

### 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.

11. May 25, 2007

### Sojourner01

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

12. May 25, 2007

### cristo

Staff Emeritus

13. May 25, 2007

### cristo

Staff Emeritus
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).

Last edited: May 25, 2007
14. May 25, 2007

### 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.

15. May 25, 2007

### cristo

Staff Emeritus
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 ballon is inflated.

Last edited: May 25, 2007
16. May 25, 2007

### CraigD

Of course this needs to be explained every 3 days. It makes no sense, and has no analogy in life. Even your ballon 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

17. May 25, 2007

### 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?

18. May 25, 2007

### Staff: Mentor

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.
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.

19. May 25, 2007

### BoomBoom

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?

20. May 25, 2007

### BoomBoom

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

21. May 25, 2007

### BoomBoom

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

22. May 25, 2007

### cristo

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

23. May 25, 2007

### Staff: Mentor

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

24. May 26, 2007

### Milind_shyani

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

25. May 26, 2007

### Mentz114

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.