What happens to light when it reaches the edge of the universe?

[George Jones]

Is our universe closed (like the surface of as sphere)? If it is closed, it is just barely closed. If our universe is closed, can light, in principle (neglecting the spreading of light that Dorothy mentioned), circumnavigate our universe? My calculations indicate that even if our universe is closed, light traveling forever only makes it a small fraction of "all the way 'round". Why? Because: 1) if our universe is closed, it is only just barely closed; 2) the expansion of the universe is accelerating. A long-winded expansion (pun intended) on this follows.

Standard models of the universe are either open or closed. The average density of matter and energy determines if a universe is open or closed. If this density is at or less than a critical density, the universe is open; if this density is greater than the critical density, the universe is closed. Here, "open", "closed", and "flat" all refer to 3-dimensional space, not to 4-diemsional spacetime. A flat universe is an open universe that is on the border of being closed, i.e., right at critical density.

Standard models of the universe either expand forever, or reach maximum expansion, contract, and end in a Big Crunch.

Without dark energy, the preceding two paragraphs are linked. A universe expands forever if and only if it is open; a universe ends in a Big Crunch if and only if it is closed. Without dark energy, a photon circumnavigates a closed universe exactly once during the time from Big Bang to Big Crunch.

Dark energy, repulsive gravity, changes the picture. With dark energy, closed universes can still end in a Big Crunch if the density of dark energy is small enough, but, because of repulsion, the time taken to get to Big Crunch will be longer. Consequently, a photon in this type of universe will circumnavigate the universe more than once. With just the right amount of dark energy, a photon can circumnavigate the universe an arbitrarily large number of time before the Big Crunch.

If, however, the amount of dark energy is large enough, a closed universe will expand forever, and, at late times, expansion will proceed exponentially quickly. Because of this exponential expansion, the position of the photon in space approaches a limiting value as time gets large. Depending on the amount of dark energy, this limiting position can be made arbitrarily small (lots of dark energy, early onset of exponential expansion) or arbitrarily large.

Observations indicate that our universe is near the border between open and closed, and that even if our universe is closed, it will expand forever.

My calculations seem to verify all this. As I said above, in a closed universe that fits our observations, a the limiting value is a small fraction of the circumference of the universe.

 

[Johninch]

With just the right amount of dark energy, a photon can circumnavigate the universe an arbitrarily large number of time before the Big Crunch...(but)...Observations indicate that our universe is near the border between open and closed, and that even if our universe is closed, it will expand forever.

Thank you George. Putting your explanation together with those of Dorothy and Russ, I conclude that it's not possible to have another BB as a result of the curvature of the universe, even if the curvature is complete.

If as you write, it's possible that photons "can circumnavigate the universe an arbitrarily large number of times" I am left wondering what the universe would look like in the scenario where all photons do this. Would the universe be a sea of photons? I suppose this is what is meant by the big freeze.

Further I conclude that, as the galaxies (or their diffused remains) cannot come together due to curvature, then they could only come together due to gravity, if at all. So the shape of the universe does not seem to make any difference.

Have I got it right yet?

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[George Jones]

Got caught up in this discussion now! I think the answer is no, because of the 1/r squared rule. (I hope this comes out OK!)

Hi Dorothy.

In terms of reduced intensity, things are even worse than this, because of the expansion of the universe.

The expansion of the universe in two ways diminishes the energy flux that is received. The energy of light is inversely proportional to its wavelength. As light travels, the expansion of the universe expands the wavelength of the light by a factor of $a_o / a_e$, where $a_e$ is the scale factor of the universe when the light was emitted, and $a_o$ is the scale factor of the universe when the light is observed. Also, the expansion of the universe decreases the rate at which we receive photons, as compared to the rate at which photons left the light source, by another factor of $a_o / a_e$.

 

[Johninch]

1. That doesn't logically follow. Any arbitrary area or volume can be tracked and observed to expand (or not) regardless of its size, curvature or arbitrary or lack of boundary.

2. False. I have a ruler sitting on my desk to compare the size of the universe to.

1. The universe is not an arbitrary area or volume. We cannot observe the universe to be expanding, because it is everything and we are in it. The observable universe is an area of volume which is not increasing in size.* Just because we observe the most distant galaxies to accelerate out of view, we cannot assume that it gets bigger on the other side.

2. We cannot measure the universe from the inside, therefore not at all. In order to measure something, we have to select limiting parameters, normally the boundaries. If we cannot identify the limiting parameters, we cannot make measurements to determine that the universe is growing.*Edit: I'm not sure about this, please tell me if I'm wrong on this point, but it doesn't affect the rest of my argument.

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[Bandersnatch]

1. The universe is not an arbitrary area or volume. We cannot observe the universe to be expanding, because it is everything and we are in it. The observable universe is an area of volume which is not increasing in size.* Just because we observe the most distant galaxies to accelerate out of view, we cannot assume that it gets bigger on the other side.

2. We cannot measure the universe from the inside, therefore not at all. In order to measure something, we have to select limiting parameters, normally the boundaries. If we cannot identify the limiting parameters, we cannot make measurements to determine that the universe is growing.


*Edit: I'm not sure about this, please tell me if I'm wrong on this point, but it doesn't affect the rest of my argument.

Arbitrary means "any sort you want". Of course the volume of the universe falls into that category.

The point was that all you need to do to observe expansion of space is to measure some distances at two different times. You do not need to look at the space from "outside".
In the workhorse analogy of the expanding balloon, the 2d inhabitants on its surface do not need to move into the third dimension to notice expansion. They just measure distances on the surface, see them increase with time, and conclude that there's expansion.

The observable universe is most definitely increasing in size. Before the primordial plasma cooled enough to recombine with electrons, it was merely as big as the average length of photon scattering. Now it's 80-something billion ly across.

 

[Johninch]

The point was that all you need to do to observe expansion of space is to measure some distances at two different times. You do not need to look at the space from "outside".
In the workhorse analogy of the expanding balloon, the 2d inhabitants on its surface do not need to move into the third dimension to notice expansion. They just measure distances on the surface, see them increase with time, and conclude that there's expansion.

The observable universe is most definitely increasing in size. Before the primordial plasma cooled enough to recombine with electrons, it was merely as big as the average length of photon scattering. Now it's 80-something billion ly across.

Your first point about “observing the expansion of space by measuring some distances” is a strawman argument. Of course you are right. The observed distance between here and a galaxy is finite and measureable. But you can’t do that for the universe.

I know it is a workhorse analogy, but it doesn’t help me to compare an expanding balloon or fruitcake to the universe, in the first place because balloons and fruitcakes have clearly defined boundaries. Feynman told us to stop making such potentially misleading analogies.

I see now that I was wrong to suggest that the observable universe is not expanding. It is expanding each day in the sense that it takes light longer to travel between two opposite and receding boundaries. The boundaries of the observable universe are represented by the limits of our potential observation through spacetime.

On the other hand, the universe has no boundaries. Apparently it is meant that the universe has no spatial boundaries, only a time boundary. So I am trying to relate the time boundary of 13.7 bn years to a spatial origin of the universe which is not based on point volume. The suggestion seems to be that when density is too great, time cannot tick. Perhaps someone can help me on this one too.

I have noticed diverging opinions about whether the spatial origin of the universe was point volume or not and I have come down on the side that it was not.

I have noted the following postings in this thread:

An infinite unievres must of always been infinite, at the big bang in an infite unievres, you basically have an inifte, but infinitely dense space, which expands (in other words becomes less dense).

But because "singularity" sounds like the word "single"
it suggests to many people that there is a single point
where the singularity happens!
so they imagine a single isolated point
this is a wrong image and leads to much confusion

the BB singularity may have been confined in a small point-like region, but this is not the prevailing view. It could also have occurred at every point of an infinite 3D hypersurface---this is nowadays a very common view

We know nothing about what the big bang was. We only know that as we look backwards in time it appears that the universe gets denser and denser. We extrapolate this out using known laws and it leads to a point where our math simply breaks down and we get a singularity. It is very likely we simply don't know how the universe works at the densities and temperature of the extremely early universe, leading to this breakdown.

The talk of the expansion is simply because we observe that all objects not bound to us through gravity are receding from us. Moving away.

 

[Drakkith]

2. We cannot measure the universe from the inside, therefore not at all. In order to measure something, we have to select limiting parameters, normally the boundaries. If we cannot identify the limiting parameters, we cannot make measurements to determine that the universe is growing.

Nonsense. We can easily measure the rate of expansion. Just because we can't measure the total size of the universe doesn't mean we can't say that it's getting larger. Instead of saying the universe has increased in size by X cubic meters, we simply say that it has increased in size by X percent.

The suggestion seems to be that when density is too great, time cannot tick. Perhaps someone can help me on this one too.

That would not be correct. An observer ALWAYS measures time passing at a normal rate in their own reference frame.

 

[Johninch]

We can easily measure the rate of expansion. Just because we can't measure the total size of the universe doesn't mean we can't say that it's getting larger. Instead of saying the universe has increased in size by X cubic meters, we simply say that it has increased in size by X percent.

We can easily measure the rate of expansion of what – the observable universe? How do you get from that to the expansion of the universe, which is assumed to be much larger?

If you are of the opinion that the universe is getting larger, I suppose you are also saying that the universe is finite in size?

That would not be correct. An observer ALWAYS measures time passing at a normal rate in their own reference frame.

I think that I did not express my question very well. I was referring to the initial stages of inflation, which some say started with point volume and others say was much larger – everywhere, but with extremely high density. If the latter, we have the phenomenon that space existed in large amounts, before time. So suddenly, time started ticking, although space already existed. That's why I offered the explanation that time cannot or did not exist because of the high density. I know this is all hypothetical, but we still have to know what we mean. I don’t think this has anything to do with an observer.

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[Bandersnatch]

I know it is a workhorse analogy, but it doesn’t help me to compare an expanding balloon or fruitcake to the universe, in the first place because balloons and fruitcakes have clearly defined boundaries. Feynman told us to stop making such potentially misleading analogies.

You are taking the analogy too far - it requires you to think of the 3d universe as a 2d surface of a balloon. The 2d surface of a balloon has no boundaries in 2d space. The third dimension is completely irrelevant, and mathematically unnecessary for expansion.
Yes, it's misleading in the sense that its hard to stop imagining the surface as being embedded in higher-dimensional space, but it does the job - as long as you follow the instructions on the tin.

Your first point about “observing the expansion of space by measuring some distances” is a strawman argument. Of course you are right. The observed distance between here and a galaxy is finite and measureable. But you can’t do that for the universe.

I think I understand what you're getting at - you're simply challenging the validity of the cosmological principle outside the observable universe. I can sort of get behind that. After all, we do have the idea of eternal inflation, where parts of the universe are still inflating, while others are not.
Still, since we cannot, in principle, know what's outside the observable universe, it seems sensible to keep the assumption that has worked so splendidly so far in what we do observe.

But your original argument was:

The distances between galaxies is increasing, but the universe cannot be expanding, because it is everything.

Which is what was addressed. In the balloon analogy, whether you compare two close-by distances on the surface, or measure the complete circumference, or even the area(so, "everything"), you get the same result of expanding space, even though the surface is all there is, and there's no more surface to expand into.

 

[Drakkith]

We can easily measure the rate of expansion of what – the observable universe? How do you get from that to the expansion of the universe, which is assumed to be much larger?

From the belief that the laws of physics work the same everywhere. There's no reason to think that only our little corner of the universe is expanding.

If you are of the opinion that the universe is getting larger, I suppose you are also saying that the universe is finite in size?

Nope. The universe could be either finite or infinite and still expand.

I think that I did not express my question very well. I was referring to the initial stages of inflation, which some say started with point volume and others say was much larger – everywhere, but with extremely high density. If the latter, we have the phenomenon that space existed in large amounts, before time. So suddenly, time started ticking, although space already existed. That's why I offered the explanation that time cannot or did not exist because of the high density. I know this is all hypothetical, but we still have to know what we mean. I don’t think this has anything to do with an observer.

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Where did you get the idea that time didn't exist at/prior to inflation? Space and time are inexorably linked together. You cannot separate the two.

 

[Johninch]

In the balloon analogy, whether you compare two close-by distances on the surface, or measure the complete circumference, or even the area(so, "everything"), you get the same result of expanding space, even though the surface is all there is, and there's no more surface to expand into.

“the surface is all there is, and there's no more surface to expand into.” Wrong. The balloon inflates, which causes the surface to get bigger. We don’t need to discuss a third dimension because the analogy already doesn’t work with two.

Irrespective of analogies, you are saying that the total universe, which is everything, can expand. Are you really sure that you don’t just mean that the observed distances between the observed galaxies are observed to be increasing? How does it look with a finite number of galaxies? What do they collectively expand into? There is no boundary which can be pushed out, unless there is available space beyond all the galaxies. This would be contrary to the Cosmological Principle which requires an isotropic and homogeneous universe.

I think I understand what you're getting at - you're simply challenging the validity of the cosmological principle outside the observable universe. I can sort of get behind that. After all, we do have the idea of eternal inflation, where parts of the universe are still inflating, while others are not.
Still, since we cannot, in principle, know what's outside the observable universe, it seems sensible to keep the assumption that has worked so splendidly so far in what we do observe.

It is not my place to question established principles, although I have noted the discovery of the Hercules-Corona Borealis Great Wall and other problematic structures.

I only see a redistribution of matter and energy. There is the question of what space contains, but I don’t want to address this here, because it is getting too far off topic.

Could you just tell me the reasons for why the universe must be expanding in size and not redistributing itself.

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[Drakkith]

Irrespective of analogies, you are saying that the total universe, which is everything, can expand. Are you really sure that you don’t just mean that the observed distances between the observed galaxies are observed to be increasing?

Which would require that the universe expand.

How does it look with a finite number of galaxies? What do they collectively expand into?

Nothing. Distances grow larger over time, that's it. If the universe is finite and 100 billion light years across, then after a period of time it will be larger than 100 billion light years.

There is no boundary which can be pushed out, unless there is available space beyond all the galaxies. This would be contrary to the Cosmological Principle which requires an isotropic and homogeneous universe.

You are correct in that there is no barrier. Nothing is pushed anywhere. Distances grow thanks to the geometry of spacetime. Note that no galaxies actually travel anywhere. Distances between them grow, and nothing gets any closer to anything else.

Could you just tell me the reasons for why the universe must be expanding in size and not redistributing itself.

Redistributing itself according to what known laws? Nothing in science at this time can explain the fact that galaxies appear to be receding from each other better than current theories.

 

[Johninch]

Which would require that the universe expand.

Not necessarily. We do not observe the universe as a whole to be getting bigger.

Nothing. Distances grow larger over time, that's it. If the universe is finite and 100 billion light years across, then after a period of time it will be larger than 100 billion light years.

But we don’t know the diameter of the universe. We can’t define the universe as an object with a diameter because that would give it a boundary. The universe is simply everything. The only way to talk about diameter is to talk about a finite number of galaxies occupying a portion of the universe.

You are correct in that there is no barrier. Nothing is pushed anywhere. Distances grow thanks to the geometry of spacetime. Note that no galaxies actually travel anywhere. Distances between them grow, and nothing gets any closer to anything else.

Agreed, with the exception of gravitationally-bound galaxies.

Redistributing itself according to what known laws? Nothing in science at this time can explain the fact that galaxies appear to be receding from each other better than current theories.

I didn’t know that the hypothesis of dark energy is good enough yet to be called a theory. I thought it was more like a stopgap. It is hypothesized that the vacuum strength of dark energy has been increasing in relation to the gravitational strength of dark and baryonic matter. The turning point came about 5 bya when dark energy got the upper hand and started to accelerate the expansion of space. This is an example of what I call redistribution. I assume that the total energy resource of the universe has not increased since the BB.

Wiki (Metric Expansion of Space): “the way we define space in our universe in no way requires additional exterior space into which it can expand since an expansion of an infinite expanse can happen without changing the infinite extent of the expanse. All that is certain is that the manifold of space in which we live simply has the property that the distances between objects are getting larger as time goes on.”

Wiki is saying that for practical purposes we can regard the universe as infinite. I prefer to say that the universe is everything, but I don’t want to quibble. Using Wiki’s formulation, I conclude that we are both right: you are right that the universe appears to be expanding and I am right that it is not getting bigger.

>
> I will respond to your posting #80 on the question of space and time separately.

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[russ_watters]

I didn’t know that the hypothesis of dark energy is good enough yet to be called a theory. I thought it was more like a stopgap. It is hypothesized that the vacuum strength of dark energy has been increasing in relation to the gravitational strength of dark and baryonic matter. The turning point came about 5 bya when dark energy got the upper hand and started to accelerate the expansion of space. This is an example of what I call redistribution. I assume that the total energy resource of the universe has not increased since the BB.

That isn't what "redistribution" means. Redistribution means that the unobserved parts of the universe are behaving differently from our part: ours is epanding while parts that we don't see are contracting. It isn't generally acceptable in science to conclude the opposite of what your data is telling you!

Wiki (Metric Expansion of Space): “the way we define space in our universe in no way requires additional exterior space into which it can expand since an expansion of an infinite expanse can happen without changing the infinite extent of the expanse. All that is certain is that the manifold of space in which we live simply has the property that the distances between objects are getting larger as time goes on.”

Wiki is saying that for practical purposes we can regard the universe as infinite. I prefer to say that the universe is everything, but I don’t want to quibble. Using Wiki’s formulation, I conclude that we are both right: you are right that the universe appears to be expanding and I am right that it is not getting bigger.

No, what you are saying is self-contradictory; if it is expanding, then it is getting bigger. Those two words mean the same thing.

And you need to read the rest of the section you quoted because no, expansion can happen whether the universe is finite or infinite.

 

[Johninch]

Thanks very much for all the explanations.

I come to the conclusion that the scientific consensus is that the totality of the universe is getting bigger in the spatial dimensions and as a result is moving from a high average density of energy + matter to a lower average density of energy + matter, irrespective of whether the universe already started as being infinitely large or not.

If this is a correct statement of the consensus, I have no further questions.

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[Drakkith]

Thanks very much for all the explanations.

I come to the conclusion that the scientific consensus is that the totality of the universe is getting bigger in the spatial dimensions and as a result is moving from a high average density of energy + matter to a lower average density of energy + matter, irrespective of whether the universe already started as being infinitely large or not.

If this is a correct statement of the consensus, I have no further questions.

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That is correct.

 

[George Jones]

Thanks very much for all the explanations.

I come to the conclusion that the scientific consensus is that the totality of the universe is getting bigger in the spatial dimensions and as a result is moving from a high average density of energy + matter to a lower average density of energy + matter, irrespective of whether the universe already started as being infinitely large or not.

If this is a correct statement of the consensus, I have no further questions.

Yes, but if dark energy really is equivalent to a cosmological constant (it may well not be), then the density of dark energy is constant. In this case, even if the scale of the universe grows without bound, the energy/mass density of the universe will approach this (the value for dark energy) finite, non-zero value, even though the density of matter and radiation will both approach zero.

 

[Johninch]

Yes, but if dark energy really is equivalent to a cosmological constant (it may well not be), then the density of dark energy is constant. In this case, even if the scale of the universe grows without bound, the energy/mass density of the universe will approach this (the value for dark energy) finite, non-zero value, even though the density of matter and radiation will both approach zero.

Interesting point, although it appears to fall within my statement which merely says that the density of energy+matter is diminishing. I understood this to be the same as saying that the average density of the total mass in the universe (which includes DE) is diminishing. Further I am assuming that DE has positive mass, even if it exerts negative pressure.

This point was discussed in https://www.physicsforums.com/showthread.php?t=114495

I have the following questions to the above thread (which is closed), in which you participated:

#5 Garth: “If the concept of the total mass of a volume of DE is difficult to define then just work with density and pressure. If DE is the cosmological constant, ω=−1 , then as the universe expands the negative pressure will produce more 'total mass' as the density remains constant and the volume increases.”

If the mass of DE increases, where would the additional mass come from?


#6 Pervect: “while it's true that the universe doesn't have a mass, it's a rather unsatisfying explanation to give when someone asks about the mass of dark energy.”

If the universe doesn’t have a mass, how can it have a density?

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[ebos]

I would assume that the further one looks out towards the "edge" the closer one gets to the "beginning. No matter which direction you look out into space, the earlier you will see. So the 'edge' is the beginning with us seemingly at the center. This 'center' is everywhere since the 'big bang' started everywhere. Also, since space is curved, the radius of the observable universe is equal to its circumference. Hey, it works for me but since we in no way can ever understand these concepts (never say never) it will probably always remain a mystery. better to just go out and have a nice cold beer. Once in a while it is just OK to say, "I don't know".

 

[ebos]

Oh, I forgot your original question. The light would eventually return to its source but because space-time expands faster than the speed of light in a vacuum that won't happen either because it could never out-run the expansion of space-time..

 

[davenn]

I would assume that the further one looks out towards the "edge" the closer one gets to the "beginning. No matter which direction you look out into space, the earlier you will see. So the 'edge' is the beginning with us seemingly at the center. This 'center' is everywhere since the 'big bang' started everywhere. Also, since space is curved, the radius of the observable universe is equal to its circumference. Hey, it works for me but since we in no way can ever understand these concepts (never say never) it will probably always remain a mystery. better to just go out and have a nice cold beer. Once in a while it is just OK to say, "I don't know".

did you notice that it's been over 3 yrs since this thread was last responded to ?

 

[ebos]

did you notice that it's been over 3 yrs since this thread was last responded to ?

Yes, my head is still sore where I just slapped it.. I noticed that about 10 secs after I posted. Hey, I've got Oldtimer's Syndrome. Wait till you're 62

 

[davenn]

Yes, my head is still sore where I just slapped it.. I noticed that about 10 secs after I posted. Hey, I've got Oldtimer's Syndrome. Wait till you're 62

Am only a handful of yrs behind you, I know the feeling wellDave

 

[phinds]

Am only a handful of yrs behind you, I know the feeling wellDave

Ah you youngsters, always complaining. I'm way older and I still have a mind like a steel trap and a memory like an elephant. Now if I could just remember where I put the peanuts.

 

[davenn]

Ah you youngsters, always complaining. I'm way older and I still have a mind like a steel trap and a memory like an elephant. Now if I could just remember where I put the peanuts.

hahaha

 

[Dale]

Thread closed. You are welcome to start a new one.