B 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.

 

[Drakkith]

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.

There's nothing different between 1 minute and 1 million years except for scale. If the universe is currently infinite in size, then it was also infinite 1 minute after the big bang, 1 year after the big bang, or a million years after the big bang. If the universe is not infinite, but is unbounded (meaning there's no edge), then the shape stays the same but the size increases.

The exact shape of the universe is unknown. It could be a hypersphere, or it could be one of several other shapes.

If these do not answer your questions then you're going to need to elaborate on what you don't understand.

 

[Jinsuk Kim]

You said that "There's nothing different between 1 minute and 1 million years except for scale. If the universe is currently infinite in size, then it was also infinite 1 minute after the big bang, 1 year after the big bang, or a million years after the big bang."
I agree with you and that is the reason why I say "1 minute".
Then can you answer me about the question "1 minute after big bang, its size is finite or infinite?"

 

[weirdoguy]

It's been answered already:

The exact shape of the universe is unknown. It could be a hypersphere, or it could be one of several other shapes.

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

 

[Drakkith]

Then can you answer me about the question "1 minute after big bang, its size is finite or infinite?"

We don't know. It is an open problem in cosmology.

 

[Jinsuk Kim]

I keep "The exact shape of the universe is unknown. It could be a hypersphere, or it could be one of several other shapes." as a good answer. But I don't agree with this answer because we can have the better answer. Of course, this doesn't mean at all that answer is wrong. It's good.

 

[Jinsuk Kim]

Mr, Drakkith,
You said that "We don't know. It is an open problem in cosmology."
I respect you. This word is not a joke. Really.
I would like to express my respect to all participants in this post. Especially, if he seems to be good.
But the others who answered me also think like you?
I remember that many persons answered me the (present) universe is infinite.
First, I can't agree with you.
Let's wait the others' replies about question No.26.

 

[stefan r]

You said that "There's nothing different between 1 minute and 1 million years except for scale. If the universe is currently infinite in size, then it was also infinite 1 minute after the big bang, 1 year after the big bang, or a million years after the big bang."
I agree with you and that is the reason why I say "1 minute".
Then can you answer me about the question "1 minute after big bang, its size is finite or infinite?"

Right now, without using anyone else's data prove that the surface of Earth is spherical and finite. It is not as simple as some people make it out to be. Eratosthenes was the first person we know of who measured Earth's circumference. He needed measurements at 2 locations that were separated by 900 km. Of course you could get in an airplane and fly around the world but that only works because we have the ability to fly that distance.

I can measure my back yard. Based on those measurements I can easily prove that Earth's circumference must be much larger than my back yard. The overall shape of Earth is not clear. I can use a bubble level to show that it is fairly flat. I can use plum lines to show gravity is basically going in one direction. Those measurements would not indicate much useful information about Earth's overall shape. The sphere is large enough that the plumb lines look nearly parallel. The deviation from parallel is much less than the width of the string.

Receiving data from an alien civilization located a few billion light years away could clarify some cosmology issues. If that data stream exists we have not found it yet. We might get some constraints by watching distant galaxies drop over the event horizon over the next few million years. Astronomy only has measurements taken from one solar system and distant galaxies have only been observed for a few decades.

 

[davenn]

First, I can't agree with you.

Well that is your problem, not ours ... you have been given very good answers based on today's knowledge of cosmology

Let's wait the others' replies about question No.26.

I seriously doubt that you will get anything different to what you have already been told

If you cannot accept it, then go and do some serious study and you will see that what you have been told here aligns with current theories of cosmology

 

[phinds]

First, I can't agree with you.
Let's wait the others' replies about question No.26.

@Jinsuk Kim I find this very odd. You have joined a forum on science where people know what they are talking about, for the purpose of finding the answers to some questions. You have been given answers which are correct to the extent that science currently knows the answers. THEN you decide that the people on the forum do NOT know what they are talking about because you don't like the answers and you decide to ignore the repeated and consistent answers that you have been given. Very odd.

 

[Klystron]

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

I also recommend Steven Weinberg's excellent book "The First Three Minutes". Prof. Weinberg presents a detailed perspective on current theory -- last time I read this book was revision 6 including data and observations since original publication.

 

[phinds]

@Jinsuk Kim I also recommend the link in my signature

 

[diogenesNY]

@Jinsuk Kim , why not have a look at the following paper:

http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf

This is a very readable and lucid paper by Charles Lineweaver and Tamara Davis that was published as an article in Scientific American in 2005. Your question(s) are fairly answerable in terms of what we understand about the universe (more or less) but sometimes the explaining gets a bit confusing.

This article should shed a bit of light on your inquiries, and answer a bunch more questions that I suspect you may have (or soon will).

Highly recommended, and highly credible.

diogenesNY

 

[Jinsuk Kim]

It has not been discussed at all about “What happens to the light that arrives at the end of space?” I raised this question to feel and understand about the concept and meaning of space. This cannot be discussed because most people answered “infinite.” So I will not touch on this question anymore.

I am not an expert in astrophysics but I am not unfamiliar in the big bang theory. In this post, it seems that “the size of the univere is infinite” is the mainstream. “finite” seems to be the non-mainstream. But at No.24 Mr. russ_watters said “It is not necessarily true that the universe is infinite”. I suppose he is an expert. I know that “finite” is not non-mainstream. Then why many people insist “infinite”?

My final question in this post is why the theories that insist “the size of the universe is infinite” has been established?

1. Because the newly observed results suggest it.

2. There is no observed result suggesting it. But because theoretically it is correct.

Will you answer me? If you don’t mind, please explain it easily and simply, not in detail.

 

[Andrea Panza]

Hi Jinsuk,
I am afraid that we cannot really answer to your question "What happens to the light that arrives at the end of space", because it is made with the incorrect assumption that the universe has a boundary (an edge of some sort), but our current understanding suggests that the universe has no edges.
If we compare your light beam with an ant walking on the surface of a sphere the ant can go on and move forever, even if the sphere is not infinite.
And the universe does not need to be a sphere or a an hypersphere, it is enough that the universe has some kind of closed geometry so that it loops back on itself (with looping back I mean that if you proceed in a straight line for long enough you will find yourself at the starting point).
If the universe does not loop back it is probably infinite so, again the light will not reach the edge because there is no edge, as there is no upper bound the the series of integers, your light beam will go on and on across the space and the space will be always filled with stars and galaxies as all the universe as we know it started with the big bang and the big bang happened everywhere at the same time.
Since the light speed has a finite value and the universe is expanding at a certain point the light will reach the limit of the observable universe with respect of the initial starting position: this means that we cannot see what is beyond a given distance from us because the space is expanding and the distance every second increases more than the distance the light can travel in a given second (it is like trying to run up on an escalator that is going in the opposite direction with a speed faster than your running speed). Please note that this horizon, this limit of the observable universe (the point where the 'escalator'-space expansion becomes faster than light) is not an hard edge, but is just an observer dependent phenomenon, every point in the universe has an horizon and two places even just few millimeters apart on the surface of Earth will have very similar but slightly different horizons.
So as per our understanding there is no real edge in the universe.
I hope it helps,
regards.

 

[phinds]

It has not been discussed at all about “What happens to the light that arrives at the end of space?” I raised this question to feel and understand about the concept and meaning of space. This cannot be discussed because most people answered “infinite.” So I will not touch on this question anymore.

You continue to refuse to listen to what you are being told. It is NOT true that your question is meaningless because space may be infinite or not. It is meaningless because there is no "end" to space whether or it is infinite or not, as you have been told repeatedly

 

[phinds]

My final question in this post is why the theories that insist “the size of the universe is infinite” has been established?

AGAIN, you continue to not listen to what you are being told. It is NOT established that the universe is infinite. That seems to be most likely with our current understanding but it is NOT established fact, and you have been told that repeatedly.

 

[Andrea Panza]

My final question in this post is why the theories that insist “the size of the universe is infinite” has been established

As others already pointed out, is not that easy to determine if something is infinite for real or just way bigger than the detection limit of your tools:

If I draw some circles on a piece of paper and I ask you to calculate the curvature (or at least find the approximate center) the task is not difficult if the size of the circles is smaller than the dimensions of the piece of paper. It is still doable even if the circle is bigger than the piece of paper (so you can only see a portion of the circle), if the curvature is marked enough, but will you be able to find the center of the circle if what I give you looks exactly as a straight line?

The current scientific observations suggest that the universe is either finite but way bigger than what we can observe or really infinite.

I might add that the universe is not only defined by the 3 spatial dimensions but there is also a time dimension and at least for what I understand the flow of time is already considered to be infinite in the direction of the future (in the sense that the universe will slowly cool and things will spread and decay, but it does not look like that there will be a 'final second' before everything shuts down).

I hope it helps

 

[CWatters]

Perhaps the OP should google "what is the universe expanding into".

If the universe was one dimensional everything would be "on one line", the concept of something being "off the line" would be meaningless.

If the universe was two dimensional everything would be "on one plane", the concept is something being "off the plane" would be meaningless.

If the universe was three dimensional everything would be "in a volume". The concept of something being " outside that volume" would be meaningless.

 

[Jinsuk Kim]

As others already pointed out, is not that easy to determine if something is infinite for real or just way bigger than the detection limit of your tools:

If I draw some circles on a piece of paper and I ask you to calculate the curvature (or at least find the approximate center) the task is not difficult if the size of the circles is smaller than the dimensions of the piece of paper. It is still doable even if the circle is bigger than the piece of paper (so you can only see a portion of the circle), if the curvature is marked enough, but will you be able to find the center of the circle if what I give you looks exactly as a straight line?

The current scientific observations suggest that the universe is either finite but way bigger than what we can observe or really infinite.

I might add that the universe is not only defined by the 3 spatial dimensions but there is also a time dimension and at least for what I understand the flow of time is already considered to be infinite in the direction of the future (in the sense that the universe will slowly cool and things will spread and decay, but it does not look like that there will be a 'final second' before everything shuts down).

I hope it helps

Thank you for your kindness. You are very gentle. I will read your comments once again.

 

[PeterDonis]

There is no "end of space", so the question being asked in this thread is based on a misconception. Either the universe is spatially infinite, or it is spatially finite without a boundary (a hypersphere). Either way there is no "end of space".

Thread closed.