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Rolando

How can it be that we can still see the CMBR?

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In standard Big Bang cosmology, the cosmic microwave background radiation (CMBR) originated about 13 billion years ago. Subsequently, it will be visible at a distance of 13 billion light years from its origin. Our own galaxy has only moved a much shorter distance from a place close to this origin since then. My question is how it can be that we, nevertheless, still can see this radiation.

Text books assure us that the CMBR is a blackbody radiation that expands with the universe and so becomes more long-waved. This would require either that the universe was (1) infinite or (2) surrounded by a reflecting wall or (3) expanding like the surface of an inflating balloon. Alt. (3) was tenable until it began to be claimed that the geometry of the universe is flat, alt. (2) was always denied and in alt. (1) there is no Big Bang. So, on which additional alternative rests the present doctrine?

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10 minutes ago, Rolando said:

In standard Big Bang cosmology, the cosmic microwave background radiation (CMBR) originated about 13 billion years ago. Subsequently, it will be visible at a distance of 13 billion light years from its origin. Our own galaxy has only moved a much shorter distance from a place close to this origin since then. My question is how it can be that we, nevertheless, still can see this radiation.

Because the source of the radiation was everywhere in the universe. So, after 1 billion years we would have seen background radiation from 1 billion light years away; after 2 billion years we would have seen background radiation from 2 billion light years away; after 5 billion years we would have seen background radiation from 5 billion light years away; now we see background radiation from 13.8 billion light years away.

(Note, when I say "x billion light years away" I really mean "x billion years light travel time"; that was less than x billion light years when the light was emitted and much more than x billion light years now.)

13 minutes ago, Rolando said:

This would require either that the universe was (1) infinite

The universe could be finite or infinite. It makes no difference.

14 minutes ago, Rolando said:

Alt. (3) was tenable until it began to be claimed that the geometry of the universe is flat

It could be a very large "surface" (also this is just an analogy, so there is a limit to how far you ca stretch it, if you will exclude the pun).

The universe could finite and bounded (like the surface of the balloon) and still be flat. It depends on the topology. For example, a torus is geometrically flat but is finite.

Or it could be infinite.

15 minutes ago, Rolando said:

(1) there is no Big Bang.

Not true. The Big Bang model works for an infinite universe.

Quote

So, on which additional alternative rests the present doctrine?

We don't know if the universe is finite or infinite. We don't know the topology. So that can't really be answered.

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2 hours ago, Strange said:

Because the source of the radiation was everywhere in the universe. So, after 1 billion years we would have seen background radiation from 1 billion light years away; after 2 billion years we would have seen background radiation from 2 billion light years away; after 5 billion years we would have seen background radiation from 5 billion light years away; now we see background radiation from 13.8 billion light years away.

This everywhere was enclosed within a space in which everything was clother than 1 billion light years to everything else – and our galaxy has not moved outside this space.

Under these circumstances, in a flat geometry, a ray of light between the source and us can have a length of 13.8 billion light years only if it is reflected on its way. In various non-flat geometries, such a length can be obtained without reflection.  

I have yet to look at the link you provided in your second response. Thank you.

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13 minutes ago, Rolando said:

This everywhere was enclosed within a space in which everything was clother than 1 billion light years to everything else – and our galaxy has not moved outside this space.

What? Why would it be closer than 1 billion light years? What do you think was outside that?

The universe was uniformly full of matter - it extended equally in all directions. For 380,000 years that matter was too hot and dense (and ionised) for light to go through it. Then it cooled enough for atoms to form and became transparent. At that point light began travelling vast distances. The microwaves we see now started something like 4 billion light years away and has taken 13.8 billion light years to get here. We will continue to see the CMB as it arrives from further and further away.

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21 minutes ago, Strange said:

atoms to form and became transparent. At that point light began travelling vast distances. The microwaves we see now started something like 4 billion light years away and has taken 13.8 billion light years to get here. We will continue to see the CMB as it arrives from further and further away.

This sounds reasonable. If the photons we see now started approximately 4 billion years ago (or away) they must have been reflected there if they were originally emitted at the surface of last scattering and have been on their way for 13.8 billion years, but this is not usually told. The text in https://ned.ipac.caltech.edu/level5/March03/Lineweaver/Lineweaver7_2.html lacks this information.

Edited by Rolando

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1 hour ago, Rolando said:

This sounds reasonable. If the photons we see now started approximately 4 billion years ago (or away) they must have been reflected there if they were originally emitted at the surface of last scattering and have been on their way for 13.8 billion years, but this is not usually told.

I can't quite work out what your mental image is, so it is hard to explain where you are going wrong... :)

The surface of last scattering is a sphere, centred on us and expanding at the speed of light. The photons we see now started from the surface of last scattering 4 billion light years away and 13.8 billion years ago.

It took them 13.8 billion years to travel that 4 billion light years because the expansion of the universe means they had an ever increasing distance to travel - a bit like running up the down escalator. There is no reflection involved. I'm not sure why you think there would be.

Edited by Strange

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If there was no reflection, why had the light to travel 4 billion light years? Our galaxy has only moved a negligible distance during the past 13.8 billion years.

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I'm trying to figure out the image you have in mind Rolando, to allow me to contribute: 

3 minutes ago, Rolando said:

Our galaxy has only moved a negligible distance during the past 13.8 billion years

Moved a negligible distance relative to what? As strange said, the sphere where the light originated is centered on us.

 

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30 minutes ago, Strange said:

The surface of last scattering is a sphere, centred on us and expanding at the speed of light. The photons we see now started from the surface of last scattering 4 billion light years away and 13.8 billion years ago.

If this is the definition of "surface of last scattering", it is a misnomer. The literal meaning of  "surface of last scattering" is a surface at which which the photons are no longer scattered by particles, which happens when the temperature of the surface due to its expansion goes below very roughly 3000 K. This surface does not expand.

9 minutes ago, Ghideon said:

Moved a negligible distance relative to what? As strange said, the sphere where the light originated is centered on us.

I said it a little less categorically: we have not moved much relative to the sphere where the light originated. 

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29 minutes ago, Rolando said:

If there was no reflection, why had the light to travel 4 billion light years? Our galaxy has only moved a negligible distance during the past 13.8 billion years.

Because it is "swimming upstream." It is nothing to do with the movement of our galaxy but the expansion of the space between us and the source.

Let's try this. (I haven't got time to work out the real numbers so this is just an example.)

When the radiation starts out it is 4 billion light years away.

After 2 billion years, it has travelled 2 billion light years. But in the meantime, the distance has increased to 6 billion light years so it still has 4 billion light years to go.

After another 2 billion light years, it has travelled another 2 billion light years. But expansion has increased the distance remaining again so it has 5 billion light years to go

After another 2 billion light years, it has travelled another 2 billion light years. But expansion has increased the distance remaining again so it has 4 billion light years to go

And so on. It ends up taking about 14 billion light years to travel the distance (which, by now is about 45 billion light years).

8 minutes ago, Rolando said:

If this is the definition of "surface of last scattering", it is a misnomer. The literal meaning of  "surface of last scattering" is a surface at which which the photons are no longer scattered by particles, which happens when the temperature of the surface due to its expansion goes below very roughly 3000 K. This surface does not expand.

The place at which photons were no longer scattered is not a surface, it is the entire volume of the universe.

The surface of last scattering is the sphere from which we are receiving the first photons that are no longer being scattered.

Edited by Strange

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3 minutes ago, Strange said:

And so on. It ends up taking about 14 billion light years to travel the distance (which, by now is about 45 billion light years).

To my understanding, the distance between us and the source of the radiation is now still close to 0 light years, in any case < 1 billion.

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2 minutes ago, Rolando said:

To my understanding, the distance between us and the source of the radiation is now still close to 0 light years, in any case < 1 billion.

I don't know where you get that from. The source of the radiation we are seeing is now about 45 billion light years away.

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46 minutes ago, Strange said:

I don't know where you get that from. The source of the radiation we are seeing is now about 45 billion light years away.

We have only moved away a negligible distance from the source of the radiation. It is the radiation front that has moved away very far from us - not its source.

Edited by Rolando
precision

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I'll try an analogy as a complement to "swimming upstream": I think one could imagine it as an ant that walks along a rubber band while someone slowly stretches the rubber band. Now lets start with several ants equally spaced along the rubber band. The ants represent photons leaving the surface of last scattering and one end of the rubber band is us on earth. All ants start walking, so there will be that ants arriving one at a time. They started from a distance further away as time passes and they have to walk a longer and longer distance to get to the end. The analogy I'm trying to make is that the ants/photons don't start at the same point on the rubber band but they start at the same time. The sphere @Strange describes grows because the photons that started inside the sphere has already arrived here, and photons arriving in the future started from a greater distance. 

(Getting late here, not sure if I make any sense...)

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31 minutes ago, Rolando said:

We have only moved away a negligible distance from the source of the radiation. It is the radiation front that has moved away from us - not its source.

It is nothing to do with our movement. The source of the photons was about 380,000 years after the Big Bang, or about 13.8 billion years ago. Therefore about 4 billion light years away.

The photons that were released here, are now 45 billion light years away.

You can deny this, but all that demonstrates is your lack of understanding. You need to read some introductory texts on the Big Bang model.

https://simple.wikipedia.org/wiki/Big_Bang

https://www.space.com/20330-cosmic-microwave-background-explained-infographic.html

https://www.space.com/33892-cosmic-microwave-background.html

https://en.wikipedia.org/wiki/Cosmic_microwave_background

6 hours ago, Rolando said:

In standard Big Bang cosmology, the cosmic microwave background radiation (CMBR) originated about 13 billion years ago. Subsequently, it will be visible at a distance of 13 billion light years from its origin.

In standard Big Bang cosmology, the cosmic microwave background radiation (CMBR) originated about 13 billion years ago. Subsequently, it will be visible at a distance of 45 billion light years from its origin.

Edited by Strange

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3 hours ago, Rolando said:

This text describes in its first passage how the reasoning goes within Big Bang cosmology:
https://ned.ipac.caltech.edu/level5/Glossary/Essay_lss.html

The description is intelligible to me. It answers my question and confirms my concern.

 

That is the same description I linked to earlier. If you think it has confirmed your misunderstanding then clearly you haven’t seen nderstood it. 

It says:

Quote

This radiation appears to come from a spherical surface around the observer such that the radius of the shell is the distance each photon has travelled since it was last scattered at the epoch of recombination. This surface is what is called the last scattering surface.

So the radius of the sphere that we are currently receiving photons from is 13.8 billion light years.

Not “less than 1.” Obviously. 

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Let me reformulate my question:

What happens to light when it moves farther than the material universe has expanded?

After the standard Big Bang universe had become transparent - the universe expanded at less than 100 km/s then - what happened to light when it reached the boundary of the universe? Friedman models do not tell this. Instead of matter and radiation, these contain only an abstract fluid, and this is taken to represent the motion of matter. Light moves faster and further in free space.

It has sometimes been claimed that the Big Bang universe has no boundary,  but this is true only for omnidirectionally closed universes. Our universe is nowadays claimed to be flat and open.

Yet I have never seen it explicitly claimed that the universe has a boundary at which light is reflected back. If there is no such boundary, then the material universe is surrounded by a much larger universe that contains only radiation. In order to see the CMBR, one would need to be at the boundary of this larger universe.

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4 minutes ago, Rolando said:

What happens to light when it moves farther than the material universe has expanded?

After the standard Big Bang universe had become transparent - the universe expanded at less than 100 km/s then - what happened to light when it reached the boundary of the universe?

There is no boundary.

5 minutes ago, Rolando said:

Yet I have never seen it explicitly claimed that the universe has a boundary at which light is reflected back. If there is no such boundary, then the material universe is surrounded by a much larger universe that contains only radiation.

Why are you making up nonsense like this?

5 minutes ago, Rolando said:

In order to see the CMBR, one would need to be at the boundary of this larger universe.

We can see it where we are. And so could an observer at every other location in the universe.

I have suggested this be move to Speculations as you are no longer asking questions but promoting your own crackpot physics.

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50 minutes ago, Strange said:

There is no boundary.

The omnidirectionally closed universe (analogous to the surface of an inflating balloon) appears to be incompatible with the observations that suggest the universe to be flat.  In a flat and open universe, one can talk of two boundaries, both somewhat indistinct. The first one tells how far the matter (the web of galaxies) has expanded. The second one tells how far light and other radiation has propagated.

I am not promoting any of these or any other alternative. I just wish to know the reasoning within the frame of standard Big Bang cosmology.

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Just now, Rolando said:

The omnidirectionally closed universe (analogous to the surface of an inflating balloon) appears to be incompatible with the observations that suggest the universe to be flat. 

I have already explained why that is (a) wrong and (b) irrelevant. 

You would be better off learning the basics of the Big Bang model rather than wasting your time inventing nonsense like this. 

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16 hours ago, Rolando said:

Let me reformulate my question:

What happens to light when it moves farther than the material universe has expanded?

After the standard Big Bang universe had become transparent - the universe expanded at less than 100 km/s then - what happened to light when it reached the boundary of the universe? Friedman models do not tell this. Instead of matter and radiation, these contain only an abstract fluid, and this is taken to represent the motion of matter. Light moves faster and further in free space.

It has sometimes been claimed that the Big Bang universe has no boundary,  but this is true only for omnidirectionally closed universes. Our universe is nowadays claimed to be flat and open.

Yet I have never seen it explicitly claimed that the universe has a boundary at which light is reflected back. If there is no such boundary, then the material universe is surrounded by a much larger universe that contains only radiation. In order to see the CMBR, one would need to be at the boundary of this larger universe.

!

Moderator Note

This strays significantly from mainstream physics, so I'm moving it to Speculations. Please provide some evidence to support your explanations and arguments. This will help raise them above mere guesswork. 

 

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