When light arrives at the end of space, what happens?

[Jinsuk Kim]

In the big bang theory, inevitably we have the end of space. Expansion ratio is same everywhere in space. The end of the universe moves from us at the very high speed, but there space also hardly expands(Ho << 1). Then what happens to the light that arrives at the end of space? Will it stop, disappear or reflect?

 

[russ_watters]

In the big bang theory, inevitably we have the end of space. Expansion ratio is same everywhere in space. The end of the universe moves from us at the very high speed, but there space also hardly expands(Ho << 1). Then what happens to the light that arrives at the end of space? Will it stop, disappear or reflect?

No, you misunderstand the Big Bang theory. There is no end of space, no center, etc.

That space is undergoing metric expansion is shown by direct observational evidence of the Cosmological principle and the Copernican principle, which together with Hubble's law have no other explanation. Astronomical redshifts are extremely isotropic and homogeneous,[20] supporting the Cosmological principle that the universe looks the same in all directions, along with much other evidence. If the redshifts were the result of an explosion from a center distant from us, they would not be so similar in different directions.

Measurements of the effects of the cosmic microwave background radiation on the dynamics of distant astrophysical systems in 2000 proved the Copernican principle, that, on a cosmological scale, the Earth is not in a central position.[81] Radiation from the Big Bang was demonstrably warmer at earlier times throughout the universe. Uniform cooling of the CMB over billions of years is explainable only if the universe is experiencing a metric expansion, and excludes the possibility that we are near the unique center of an explosion.

https://en.wikipedia.org/wiki/Big_Bang#Hubble's_law_and_the_expansion_of_space

 

[Drakkith]

In the big bang theory, inevitably we have the end of space.

Not true. The universe is believed by most scientists to be infinite in size. An alternative could be that it is finite, but wraps back around on itself. In either case there is no boundary for light to arrive to.

 

[Jinsuk Kim]

What do you think about the size of the universe, especially related to this problem?

 

[russ_watters]

What do you think about the size of the universe, especially related to this problem?

As @Drakkith said, the universe is believed to be infinite, but whether infinite or finite it does not have an edge.

 

[Jinsuk Kim]

Some physicists think that the universe is infinite. But the others maybe don't think so. Am I right?
We can see from the several materials including Wikepedia and textbook that the the size of the universe is ~.
If the universe is fininite and has a size, how about the end?
The word "size" without "end" is possible?

 

[russ_watters]

Some physicists think that the universe is infinite. But the others maybe don't think so. Am I right?

Yes: it is an open question.

The word "size" without "end" is possible?

Yes. Does a mobius strip have an "end"? Does it have a size (length? surface area?)

 

[Jinsuk Kim]

mobius strip is a strip. The universe is a volume. It can have a radius, size, end, edge. Though the issue submitted seems to be simple, i think, it is important and has a meaning to talk together. I have no mind to oppose you. Let's considerr more.

 

[russ_watters]

mobius strip is a strip. The universe is a volume. It can have a radius, size, end, edge. Though the issue submitted seems to be simple, i think, it is important and has a meaning to talk together. I have no mind to oppose you. Let's considerr more.

That isn't how Physics Forums works. Physics Forums is a place for learning accepted science, not for speculation about things that go against accepted science. Please review the physicsforums guidelines on speculative discussions:

https://www.physicsforums.com/threads/physics-forums-global-guidelines.414380/
https://www.physicsforums.com/help/speculation/

If you want to discuss why you are misunderstanding, we can continue. If you want to pursue the idea that the universe has an edge, we cannot.

 

[Simon Peach]

Light is red shifted further into the red as the object recedes from our viewpoint. So when the object gets so far away that the red shift goes into the infer-red it then disappears from our view at the same time it will be going at a very large percentage of light speed (99.999...%). So light "when light arrives at the end of space" the effect will be that it slows down and down and down. It won't disappear, as it can't, but it'll get slower and slower. But from it's point of view it will be traveling at it's normal speed it will be use that are slowing down.

 

[Jinsuk Kim]

Light is red shifted further into the red as the object recedes from our viewpoint. So when the object gets so far away that the red shift goes into the infer-red it then disappears from our view at the same time it will be going at a very large percentage of light speed (99.999...%). So light "when light arrives at the end of space" the effect will be that it slows down and down and down. It won't disappear, as it can't, but it'll get slower and slower. But from it's point of view it will be traveling at it's normal speed it will be use that are slowing down.

Thank you Mr. Simon Peach,
I understand what you mean.
But if my question is strange to you, maybe I don't understand you.
I will think again your comment.
My question is - The universe is homogeneous, so here and end of space is in same condition.
Speed of light is same. But there, end of space, light meets end of space. It cannot go anymore, or it can open or expand space by itself, can it disappear?, maybe it can't reflect.
What happens to the light? - I am glad to discuss with you. Thank you.

 

[weirdoguy]

But there, end of space

As been already pointed out to you, there is no end of space. If you don't understand it just ask questions about it, but don't ignore it just because you don't like the answer.

 

[Bandersnatch]

mobius strip is a strip. The universe is a volume. It can have a radius, size, end, edge.

The surface of a sphere is a finite space with no end. The 3D analogue of the sphere is the hypersphere, which has finite volume and no edge. Whenever you read about 'some physicists' considering a spatially finite universe, they're talking about something like the hypersphere, or another kind of three-dimensional shape with similar properties.

We can see from the several materials including Wikepedia and textbook that the the size of the universe is ~.

Whenever there is the size of the universe mentioned, it usually just means the size of the observable universe. Very occasionally, the size of the hypersphere (or similar) might be discussed. These distinctions should always be made clear in the article, providing the reader pays attention and knows what to look for.
The observable universe is just a small patch in the larger universe, from which light had a chance to reach us in its finite age. There is quite clearly more universe beyond the limit of what we can see today, since the microwave background radiation keeps arriving from ever further away.

That there be no edge to the universe is an essential feature of the big bang theory, because otherwise the equations that describe the expansion so accurately, could not work.

 

[Jinsuk Kim]

Mr. Bandersnatch,
Thank you for your answer.
I know what you mean, and your comments are helpful to me.
Now I ask a question related to the above my comments. But this is not for the purpose to oppose you.
If there wasn't the big bang theory, I think that there is no end of space.
- In the beginning, the universe was a point, or nearly a point.
- If anybody ask me the size, volume, end of the universe, I will answer size 0 or near 0, volume also 0, about end, I will answer point.
- Thousand years after big bang, I will answer size R(t=thousand years), volume(thousand years), for end, I answer the surface of sphere of radius R.
This is why I don' leave the end of the universe.
Then for the present time, Idon't know, but the above flow can't be abandoned.

 

[Bandersnatch]

Then for the present time, Idon't know, but the above flow can't be abandoned.

It certainly can, considering the last hundred years of advances in cosmology.

 

[DrClaude]

- In the beginning, the universe was a point, or nearly a point.

Your starting point is incorrect. The universe is believed to have been infinite at the Big Bang. The visible universe started out point-like.

 

[jbriggs444]

Your starting point is incorrect. The universe is believed to have been infinite at the Big Bang. The visible universe started out point-like.

That phraseology startled me. In an idealized expanding universe featuring an initial singularity, the universe does not even have a starting point at which to assess a size.

What we could possibly do would be to take the limit of the size of the universe as the singularity is approached (always infinite, so an infinite limit) and the limit of the size of the observable universe as the singularity is approached (decreasing toward zero, so a zero limit).

 

[stefan r]

Some physicists think that the universe is infinite. But the others maybe don't think so. Am I right?
We can see from the several materials including Wikepedia and textbook that the the size of the universe is ~.
If the universe is fininite and has a size, how about the end?
The word "size" without "end" is possible?

When people talk/write "size of the universe" it is a short form of "size of the observable universe". Light traveled 13.8 billion years and is now getting here. In 1.2 billion years we can start seeing light that traveled 15 billion light years. 10 billion years ago the observable universe had 3.8 billion light year radius.

As other people pointed out there is no reason to believe there is an edge at 13.8 billion light years. A photon that originated from further away is still on its way toward us and has not arrived yet.

 

[stefan r]

That phraseology startled me. In an idealized expanding universe featuring an initial singularity, the universe does not even have a starting point at which to assess a size.

What we could possibly do would be to take the limit of the size of the universe as the singularity is approached (always infinite, so an infinite limit) and the limit of the size of the observable universe as the singularity is approached (decreasing toward zero, so a zero limit).

This paper claims it is not infinite. If you have enough observable universes you find an exact copy: e^e180 or 10¹⁰⁷⁷ . That is not "curving back" it is just an observer sees an identical universe and is therefore in the same place.

 

[weirdoguy]

This paper claims it is not infinite.

This paper is about multiverse hypothesis so it's not really relevant to the OP issue.

 

[Jinsuk Kim]

Can I fix my question as follows?
At million years after big bang, the universe is a sphere? If yes,
what happens to the light that arrives at the inner surface of the sphere(universe)?
Would it stop, disappear or reflect at that time?

 

[stefan r]

Can I fix my question as follows?
At million years after big bang, the universe is a sphere? ...

Yes, asking the question is fine.

No, the universe is not a sphere. A million years after the big bang and observer can only see light that has traveled 1 million light years. So you would see light from objects inside spherical area. The structures producing the light are not effected by any type of boundary. An observer at any location sees background radiation coming from the same distance in all directions.

Also the distance changes. Light could travel 1 billion years and then reflect off of a mirror. It takes longer than 1 billion years to get back to the source. If we reflect light from 13 billion years ago the reflection will never reach the source because the source is moving away at more than the speed of light.

 

[unusually_wrong]

Can I fix my question as follows?
At million years after big bang, the universe is a sphere?

No, the universe is not a sphere. It existed before the big bang and it is infinite.

what happens to the light that arrives at the inner surface of the sphere(universe)?
Would it stop, disappear or reflect at that time?

There is no surface, wall, boundary, et cetera to the universe. That is like asking where infinity starts and stops.

 

[russ_watters]

No, the universe is not a sphere. It existed before the big bang and it is infinite.

It is not necessarily true that the universe is infinite, but it doesn't have to be infinite to be boundless. Either way, the first part is right: not a sphere.

 

[phinds]

No, the universe is not a sphere.

correct

It existed before the big bang

We don't know that and the Big Bang Theory says nothing about any "before"

and it is infinite.

Also not known for sure. Seems to be the most likely but definitely not know to be a fact.

 

[Jinsuk Kim]

Maybe my question is meaningful.
1 minute after big bang, its size is finite or infinite?
1 minute after big bang, if its size is finite, how about its shape?
1 minute after big bang, is the universe is a sphere or not?
If not sphere, can it be unknown?
Sphere means that all direction expansion, am I right?

 

[phinds]

Maybe my question is meaningful.

If you mean your question about what happens when light "reaches the end of the universe", then no, that is not a meaningful question since there IS no "end of the universe".

1 minute after big bang, is the universe is a sphere or not?

No, it was not

Its size is finite or infinite?

We don't know. If it was infinite then, it is infinite now and if it was finite then it is finite now.

If its size is finite, how about its shape, not sphere or unknown?

unknown.

Sphere means that all direction expansion, am I right?

No, expansion is in all directions but that has nothing to do with a sphere.

 

[Drakkith]

Sphere means that all direction expansion, am I right?

No. A cylindrical-like surface/space is indistinguishable from a spherical one without being able to view it from a higher dimension. I'm talking about mathematical dimensions.

I highly recommend looking into some basics about topology if you want to understand this material. Thing will make MUCH more sense then.

 

[stefan r]

Maybe my question is meaningful.
1 minute after big bang, its size is finite or infinite?
1 minute after big bang, if its size is finite, how about its shape?
1 minute after big bang, is the universe is a sphere or not?
If not sphere, can it be unknown?
Sphere means that all direction expansion, am I right?

Suppose an observer is confined to an office chair in a field in Kansas. He can see the horizon 360 degrees by spinning around. His view of Earth is limited to a within circular ring. Iowa is out there somewhere but he cannot see if from the chair because Iowa is over the horizon. There is no boundary where he stops seeing and things become invisible. He could for example stand up in the chair and change his view slightly.

Earth orbits the Sun so we get a 360 by 180 view except in places where gas and dust are blocking. Over time the distance to the limit of what we can see changes. We see galaxies all the way out. At the extreme it is galaxies forming because the light has been traveling a long time. It is like the guy in the chair seeing harvested cornfield. There is no reason to believe that over the horizon looks different from within the horizon. Every stalk of corn in Kansas is unique and every galaxy is unique.

The guy in Kansas only knows that the fields extend over the horizon. There is a Canadian arctic and a Gulf of Mexico. He cannot make that observation while remaining on the office chair. From the solar system observations indicate more galaxies forming by the same mechanics in all directions. The existence (or non-existence) of the Gulf of Mexico would be speculation. Guessing how far over the horizon is just guessing.

It looks like corn was planted sooner close to the chair but only because it takes most of a full season for the light to reach the chair.

 

[Jinsuk Kim]

I read all the replies in this post.
I may not understand them.
Thank you for the above three replies.
Sorry, but I am not satisfied about No.26 questions.
I agree with "recommend looking into some basics about topology". It is good for me.
But I ask about "1 minute after big bang"
Maybe it will not be complex.
I changed "million years or present" to "1 minute" to simplify the situation and get better understanding about size and shape of the universe.