Is the backward extrapolation of the expansion of the Universe valid?

  • #26
timmdeeg
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What I don't like about the raisin bread analogy: It is a 3D space that is in the larger environment. The bread - unlike the universe - has a boundary. The balloon analogy avoids that.
I don't like it too, I'm missing in addition the free fall. Therefore I prefer the picture that the galaxies are just falling away from each other. :biggrin:
 
  • #27
PAllen
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I don't like it too, I'm missing in addition the free fall. Therefore I prefer the picture that the galaxies are just falling away from each other. :biggrin:
The balloon analogy also fails to capture the free fall in vacuum feature of mathematical model. In one case you are led to see dots carried by stretching plastic, in the other raisins carried by expanding dough. In reality, galaxies are, indeed, in free fall and are not carried by anything.

The closest analogy I know of to capture this is only 1 x 1 finite model. Imagine a cone pointed downward, missing its tip. The circles around it represent space, vertical axis is time. Then galaxies are following straight lines away from the missing tip. The straightness implies free fall, the collection of lines captures initial conditions, the geometry of the cone captures that there is simply enough room for the straight motion to continue. The expansion is then just a statement about the shape of the manifold and the initial conditions. But then you have to somehow imagine these features being true for more dimensions with spatial slices always infinite. I don’t know of any simple way to picture this.
 
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  • #28
timmdeeg
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The closest analogy I know of to capture this is only 1 x 1 finite model. Imagine a cone pointed downward, missing its tip. The circles around it represent space, vertical axis is time. Then galaxies are following straight lines away from the missing tip. The straightness implies free fall, the collection of lines captures initial conditions, the geometry of the cone captures that there is simply enough room for the straight motion to continue. The expansion is then just a statement about the shape of the manifold and the initial conditions. But then you have to somehow imagine these features being true for more dimensions with spatial slices always infinite. I don’t know of any simple way to picture this.
On the other side this analogy seems a lot more abstract than the raisin bred analogy. Couldn't it be that if somebody understands and likes it he then would prefer to imagine the reality, the galaxies going further and further away from us?
 
  • #29
PAllen
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On the other side this analogy seems a lot more abstract than the raisin bred analogy. Couldn't it be that if somebody understands and likes it he then would prefer to imagine the reality, the galaxies going further and further away from us?
As long as you can imagine this being similarly true for every galaxy being 'us', then that is fine. Also, that there is and never was any 'center', and that spatial extent is and always was infinite.

Note, that in my 1x1 analogy, the spatial slices are without boundary or center at all times, but finite. Any perceived center is in the embedding space, not the toy universe.
 
  • #30
timmdeeg
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As long as you can imagine this being similarly true for every galaxy being 'us', then that is fine. Also, that there is and never was any 'center', and that spatial extent is and always was infinite.
The first means the cosmological principle. But isn't the latter an assumption?
 
  • #31
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The first means the cosmological principle. But isn't the latter an assumption?
Not an assumption, quite. It is a match of GR to observations, such that either observations have large errors we don’t know about, or GR is wrong at large scales.
 
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  • #32
timmdeeg
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Not an assumption, quite. It is a match of GR to observations, such that either observations have large errors we don’t know about, or GR is wrong at large scales.
Yes, agreed, thanks. As far as I am aware of we have no indication suggesting that "GR is wrong at large scales" though.
 
  • #33
I am totally confused. I feel like I am in a converstion with Zathras.."He is the one! But he is not the one!"

As best as I can tell, you are saying that even before the universe as we knew it existed there was a void, but technically that void is the universe. And since the void is endless, the universe is endless and has no real border. At some point or other, SOMETHING at some random location in the void heated up really intensley and spewed out our universe (as we know it)...then it cooled down a bit, slowed down a bit, and then at some point sped up again. Wherever that occurred, matter must have been really dense. And All the galaxies are spreading out relative to each other, and as they spread out, the density of the universe gets redistributed. When the universe stops expanding..does that mean the density has equalized across the universe? I have more questions, but this seems like my major sticking point at the moment. And I know I keep getting pulled away from this for long periods of time..but with the quarantine on, I promise I'll try to keep an eye on this post and respond quickly..
 
  • #34
timmdeeg
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When the universe stops expanding..does that mean the density has equalized across the universe?
The density was and will be always equalized across the universe if you think of scales large enough.

Our observational data suggest, that our universe ("spewed out" in your terminology) consists of two kinds of ingredients, the dark energy (the cosmological constant resp.) which "works" gravitationally repelling and other stuff, matter and radiation which work gravitationally attractive. The density of the first is according to our model constant over time, the second gets thinned over time due to the expansion of the universe. At present our model says that our universe will expand for ever. This means that matter and radiation will be negligible far in the future.
 
  • #35
The rest of your questions seem to stem from your idea of these "expansion rings", which as you now know, is flawed.

So, in the wake of your updated idea of cosmological expansion, what further questions do you have?
Um, no; I was very happy with the original responses, it was someone else who started the whole 'expansion rings' thing.

The only problem I now have about the big bang expansion is the oversimplification presented by the science media; and that's probably acceptable in the same way that 'the force of gravity' is still constantly referred to; sometimes even in the very same programs that are discussing General relativity!

The other question I was referring to was on this thread, and it seems that there now has been some traffic on that thread too. So I'll take up any further problems I have with it there.

Although a skim-read seems to indicate that I won't need to; you folk are pretty good at this stuff!

Cheers,
P
 
  • #36
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As best as I can tell, you are saying that even before the universe as we knew it existed there was a void, but technically that void is the universe.
Not really. Here's where it's important to understand the implications of what's in the title of the thread. The backwards extrapolation of a model built up by observations. We observe that there are galaxies and stars and other bits of matter in all directions. We also observe that there is a redshift associated with practically all observable galaxies. This redshift, along with other observations, such as the makeup of the particular spectra of different galaxies and their brightness, size, and shape support the idea that almost all of the matter in the universe is receding from us, and the further away it is, the faster it recedes. So we build up a model of the universe in which this expansion is a fundamental part of.

From this it makes sense to extrapolate backwards, in which case all matter was closer together in the past. In other words, if we run the clock backwards, all matter gets closer together.

But, we run into a problem. Our observations can only see out to a certain distance, both in terms of physical distance and in terms of time since light travels at a finite speed. In other words, it takes time for light to travel from one point to another. There is a 'barrier' of sorts that we can't see past using light or other forms of electromagnetic radiation. This barrier is called the surface of last scattering and it represents the point in time in which the universe transitioned from being filled with a hot, ionized plasma to a cooler, un-ionized gas. Because plasma strongly absorbs EM radiation, there is simply no light in existence from before this time, as any light emitted from before this time would have been absorbed. The first EM radiation that was emitted that was able to freely travel through space for large distances without getting absorbed has been heavily redshifted and is now in the microwave region of the spectrum. It's called the Cosmic Microwave Background, or CMB. The CMB is the oldest thing we can observe using EM radiation.

But, this hasn't stopped cosmologists from speculating on what the universe might have been like before this time. If we take our model and continue running the clock backwards we will find that the universe continues to increase in density and temperature as all matter and all the ambient radiation gets closer and closer together. As this matter gets closer together, the gravity between them gets greater too, and at a certain point the density becomes so great that the equations of General Relativity start producing infinities as solutions. These solutions are called singularities in math, and it is from this that the idea of a 'real' singularity comes from. 'Real' singularities probably don't exist, as history has shown that any time you get singularities in math they are either meaningless, or you're math isn't quite right for those conditions.

There are many different ideas about what the very early universe might have been like, and they range from an infinitely long history of being hot and dense, to being utterly devoid of all matter and radiation as we know them, to not existing at all in terms we are familiar with. It's very important to understand that none of these are supported by observations at this time, as we literally cannot make observations beyond the patterns and structure we see in the CMB.

At some point or other, SOMETHING at some random location in the void heated up really intensley and spewed out our universe (as we know it)...
Sort of. One of the possibilities is that some section of our universe underwent some sort of exotic phase change in a fundamental field, giving rise to matter, radiation, and possibly an extremely quick 'inflation' phase. But that's a very speculative idea at the moment.

then it cooled down a bit, slowed down a bit, and then at some point sped up again.
Yes, the cooling and slowing down part is correct. The accelerating expansion you've probably heard about is more aptly described as a particular way the expansion is slowing. Basically, it's not slowing down as quickly as we thought. If the rate of deceleration is high enough, the universe eventually stops and collapses back in on itself. If it's 'just right' the expansion continues forever, but it gradually slows, tending to zero as time tends to infinity. If the deceleration is not high enough, then eventually the expansion will actually speed up again and we run into an exponentially accelerating expansion, known as the 'big rip' scenario.

And All the galaxies are spreading out relative to each other, and as they spread out, the density of the universe gets redistributed. When the universe stops expanding..does that mean the density has equalized across the universe?
No, there can still be small-scale density fluctuations. What matters is the large scale. There needs to be enough matter and radiation for gravitation to counter both the inertia of the expansion and the repulsive effects of dark energy. Small-scale density changes don't matter.
 
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  • #37
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as we literally cannot make observations beyond the patterns and structure we see in the CMB.
The patterns in the CMB tell us a lot about what happened earlier. We also have the relative abundances of the different nuclides that formed just minutes after the Big Bang. In the future measurements of the cosmic neutrino background (from seconds after the Big Bang) and gravitational waves (potentially from even earlier times) might join.
 
  • #38
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The patterns in the CMB tell us a lot about what happened earlier. We also have the relative abundances of the different nuclides that formed just minutes after the Big Bang. In the future measurements of the cosmic neutrino background (from seconds after the Big Bang) and gravitational waves (potentially from even earlier times) might join.
Indeed. Excellent points.
 
  • #39
Not really. Here's where it's important to understand the implications of what's in the title of the thread. The backwards extrapolation of a model built up by observations. We observe that there are galaxies and stars and other bits of matter in all directions. We also observe that there is a redshift associated with practically all observable galaxies. This redshift, along with other observations, such as the makeup of the particular spectra of different galaxies and their brightness, size, and shape support the idea that almost all of the matter in the universe is receding from us, and the further away it is, the faster it recedes. So we build up a model of the universe in which this expansion is a fundamental part of.

From this it makes sense to extrapolate backwards, in which case all matter was closer together in the past. In other words, if we run the clock backwards, all matter gets closer together.

But, we run into a problem. Our observations can only see out to a certain distance, both in terms of physical distance and in terms of time since light travels at a finite speed. In other words, it takes time for light to travel from one point to another. There is a 'barrier' of sorts that we can't see past using light or other forms of electromagnetic radiation. This barrier is called the surface of last scattering and it represents the point in time in which the universe transitioned from being filled with a hot, ionized plasma to a cooler, un-ionized gas. Because plasma strongly absorbs EM radiation, there is simply no light in existence from before this time, as any light emitted from before this time would have been absorbed. The first EM radiation that was emitted that was able to freely travel through space for large distances without getting absorbed has been heavily redshifted and is now in the microwave region of the spectrum. It's called the Cosmic Microwave Background, or CMB. The CMB is the oldest thing we can observe using EM radiation.

But, this hasn't stopped cosmologists from speculating on what the universe might have been like before this time. If we take our model and continue running the clock backwards we will find that the universe continues to increase in density and temperature as all matter and all the ambient radiation gets closer and closer together. As this matter gets closer together, the gravity between them gets greater too, and at a certain point the density becomes so great that the equations of General Relativity start producing infinities as solutions. These solutions are called singularities in math, and it is from this that the idea of a 'real' singularity comes from. 'Real' singularities probably don't exist, as history has shown that any time you get singularities in math they are either meaningless, or you're math isn't quite right for those conditions.

There are many different ideas about what the very early universe might have been like, and they range from an infinitely long history of being hot and dense, to being utterly devoid of all matter and radiation as we know them, to not existing at all in terms we are familiar with. It's very important to understand that none of these are supported by observations at this time, as we literally cannot make observations beyond the patterns and structure we see in the CMB.



Sort of. One of the possibilities is that some section of our universe underwent some sort of exotic phase change in a fundamental field, giving rise to matter, radiation, and possibly an extremely quick 'inflation' phase. But that's a very speculative idea at the moment.



Yes, the cooling and slowing down part is correct. The accelerating expansion you've probably heard about is more aptly described as a particular way the expansion is slowing. Basically, it's not slowing down as quickly as we thought. If the rate of deceleration is high enough, the universe eventually stops and collapses back in on itself. If it's 'just right' the expansion continues forever, but it gradually slows, tending to zero as time tends to infinity. If the deceleration is not high enough, then eventually the expansion will actually speed up again and we run into an exponentially accelerating expansion, known as the 'big rip' scenario.



No, there can still be small-scale density fluctuations. What matters is the large scale. There needs to be enough matter and radiation for gravitation to counter both the inertia of the expansion and the repulsive effects of dark energy. Small-scale density changes don't matter.
OK..so, now here is my question. Suppose we refer back to either the balloon or raisin bread meteaphor. Lets say that we we tracking the expansion of the dots in a computer model using a graphic representing the metaphor. Lets say for argument sake that we decided that while the universe is the void and everything in it, for now we are just interested in the known universe.. Basically, everything up to whatever galaxy we consider to be the furthest out or closest to the edge of our known physical universe. If you track that galaxy from the first trackable moment of time (the beginning of the cmb), to the moment before when we predict the first scenario might occur (everything being pulled back in by gravity) . Now, lets say we set the model to track how far that galaxy moves every billion years. Here is my question. If we treated its trajectory like a radius, and imposed the "expansion rings" I talked about earlier. The computer graphic makes it so that Each billion years illustrates another ring. If that galaxy was roughly 100 percent into its radius after the first billion years, would each billion years it stay at the end of its radius? Or would it lose/gain ground. Now..I want to be clear here..I am not asking if it moves the same distance every time. I'm asking if it makes to the same distance in time for each billion year moment. Let me ask this another way, if a galaxy is suppose to reach a certain point every billion years, and its speeding up. Does that mean it keeps getting further past the last point it should have reached faster and faster each billion years? Or doe sit mean it keeps reaching that spot that it suppose to reach faster and faster each billion years?
 
  • #40
Drakkith
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Now, lets say we set the model to track how far that galaxy moves every billion years. Here is my question. If we treated its trajectory like a radius, and imposed the "expansion rings" I talked about earlier. The computer graphic makes it so that Each billion years illustrates another ring. If that galaxy was roughly 100 percent into its radius after the first billion years, would each billion years it stay at the end of its radius? Or would it lose/gain ground.
I admit that I don't know for certain, as I am very, very far from an expert in this area. The problem is that that the recession velocity of objects is not linear, neither over time nor over distance. Instead, the recession velocity of an object increases as it gets further away from us. So while the expansion rate is decreasing over time, the recession velocity of an object might still be increasing over that same time period.

If I had to give an answer I'd bet that the the recession velocity of any galaxy, as long as it is far enough away to be gravitationally unbounded with us, has been growing and will continue to do so unless we have a 'big bounce' scenario.

Lets say for argument sake that we decided that while the universe is the void and everything in it,
Just FYI, currently accepted models of the universe have no void, no 'outside of the universe', no extra dimensions, no boundary, no edge, etc. Our best understanding is that the universe is either:

1. Infinite, which means that you can travel in any direction and never come back to your original location nor encounter a boundary.
2. Finite but without a boundary. This means that you can travel in any direction without ever encountering a boundary, but you will eventually loop back around to your starting point without having to turn around or make course changes. It's the 3D version of taking a great circle path all the way around the 2D surface of a sphere.

Note that neither of these require that the universe be embedded in higher dimensions. In other words, we don't need to exist in 4+ spatial dimensions in order to loop back on ourselves. Other possibilities exist, such as a finite and bounded universe, but are not believed to be likely for a variety of reasons.

This appears to be a conceptual sticking point for you (and many, many others), and it is extremely important that you understand it if you want to learn how expansion correctly works.
 
  • #41
timmdeeg
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If you track that galaxy from the first trackable moment of time (the beginning of the cmb), to the moment before when we predict the first scenario might occur (everything being pulled back in by gravity) .

Let me ask this another way, if a galaxy is suppose to reach a certain point every billion years, and its speeding up. Does that mean it keeps getting further past the last point it should have reached faster and faster each billion years? Or doe sit mean it keeps reaching that spot that it suppose to reach faster and faster each billion years?
Your reasoning refers to the dynamical development of the universe.

https://www.semanticscholar.org/paper/Expanding-confusion:-common-misconceptions-of-and-Davis-Lineweaver/b4158c898cf809e1225cbb00492a1d704279359a/figure/0

To clarify this issue please look at this spacetime diagram which shows dotted worldlines since t = 0, our worldline vertical at 0 proper distance and those of galaxies moving away from us whereby their proper distances are increasing according to the expansion of the universe. Proper distance means the distance at a fixed moment of time. A galaxy which we observe with a cosmological redshift of e.g. z = 3 is now 20 Glyr away from us. You can trace back this galaxy and e.g. see that it was 10 Glyr away from us 5 Gyr years after the big bang (t = 0). Moreover this diagram shows at what time the light we observe was emitted. Look at the galaxy z = 1, its worldline is crossing the light cone 5 Gyr at about t = 5 Gyr and the distance then was a little less than 5 Glyr.

Note to talk about recession speeds at cosmological distances is ambiguous. Its not possible in the sense of Special Relativity, because there exists no global inertial frame of reference in curved spacetime. So its better to talk about increasing distances between comoving objects (like galaxies which are far away from each other).
 
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  • #42
jbriggs444
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Let me ask this another way, if a galaxy is suppose to reach a certain point every billion years, and its speeding up. Does that mean it keeps getting further past the last point it should have reached faster and faster each billion years? Or doe sit mean it keeps reaching that spot that it suppose to reach faster and faster each billion years?
The galaxy is not ever moving. It is essentially stationary always. The raisin bread analogy correctly has the raisins stationary within the dough.
 
  • #43
timmdeeg
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The galaxy is not ever moving. It is essentially stationary always.
But would you agree that this is a coordinate dependent view? As far as I can tell fixed coordinates means the FRW view.
 
  • #44
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But would you agree that this is a coordinate dependent view? As far as I can tell fixed coordinates means the FRW view.
Sure. Every object is moving at every sub-light speed if you are free to adopt a coordinate system where it is doing so.

If you decide not to fix coordinates, you have no defined speed. If you do fix coordinates, there are not many reasonable choices.
 
  • #45
But you can use the red shift to tell that something got further away from you. And you can tell that the red shift behind you might not be as much as the red shift in front of you. From your point of view, you are further away from the galaxy behind you, and the galaxy in front of you is further away. Even if you didn't know the actual measurments, that creates a pretty graphic representation that galaxies are moving. For the purpose of my question (so far), it doesn't really matter how or why it moves..the fact is, if its further away from us than the last time we checked, its moved. And the fact remains that every galaxy is moving in a certain direction. You can claim there is no center...but if they are all moving away in a relatively straight line, then if you traced them back, you'd have a pattern that looks like they are going back to the center...even if we didn't know where the ACTUAL center is. When I talk about the "known universe" I am basically saying, "the furthest galaxy out" is the physical border of our "known universe"..gravity extends a beyond that and without matter further out, it probably fizzles at some point, and that would be the actual border of our "known universe". There might be much much much much..infinitely more void than that beyond. But most of our physics probably don't extend into that void without matter being there somewhere. Here is my question..assume that what I just defined as the known universe duplicates itself differently somewhere else in the void. Is that also part of our universe? Or would we consider that a different universe?
 
  • #46
Ibix
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And you can tell that the red shift behind you might not be as much as the red shift in front of you.
No - the universe looks the same everywhere. Anywhere you stand, everything is moving away from you and has the same redshift profile we see on Earth.
You can claim there is no center...but if they are all moving away in a relatively straight line, then if you traced them back, you'd have a pattern that looks like they are going back to the center...
This is true if you pick a finite group of points and pick one to call 'motionless'. Then all the points move away from the one you called motionless. But you can pick any point to call the motionless one and it will always look like the centre of expansion, except that with a finite group of points there will be ones nearer or further from the edge. But an infinite group of points has no edge, and you still find that any point sees every other point moving away from it. So there genuinely is no criterion for calling any point the centre - they're all the same.
without matter further out,
There's matter everywhere. That's the point. Edit: or, more precisely, the average density of matter on the 100MPc scale and above is uniform - obviously there's local variation.
 
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  • #47
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I think the "rings" idea is tripping you up. Using your "rings" analogy, tree rings once set are stationary and new ones are added on the outside of the tree each year. If we put imaginary rings in the universe, these "rings" you speak of are ALL expanding, there are no stationary rings.

Additionally tree rings only expand outwards from the center each year, the imaginary universe rings you are referencing are all expanding at the same time and each one is expanding in all directions.

There is no "addition to the rings" like a tree. Pick any piece of "space", make this piece any size, place this piece anywhere in the universe. No matter which piece you decide to pick it will be constantly be expanding in all directions.

If you can grasp this concept you will realise the whole concept of "rings" is utterly meaningless. This is also why you cannot run time backwards and find "the center," it just doesn't exist.
 
  • #48
I think the "rings" idea is tripping you up. Using your "rings" analogy, tree rings once set are stationary and new ones are added on the outside of the tree each year. If we put imaginary rings in the universe, these "rings" you speak of are ALL expanding, there are no stationary rings.

Additionally tree rings only expand outwards from the center each year, the imaginary universe rings you are referencing are all expanding at the same time and each one is expanding in all directions.

There is no "addition to the rings" like a tree. Pick any piece of "space", make this piece any size, place this piece anywhere in the universe. No matter which piece you decide to pick it will be constantly be expanding in all directions.

If you can grasp this concept you will realise the whole concept of "rings" is utterly meaningless. This is also why you cannot run time backwards and find "the center," it just doesn't exist.
Yes, I have said before that I realize there are no actual rings, and that we are just using the rings for our own visual context. I get that the universe is just one entity that is growing. I am one person that is growing. But you can look at pictures of me when I was 10 year younger, and 10 years younger than that, and so on until I am a baby. I think that you guys are getting so hung up on insisting that the math works one way, that you are leaving behind the way every day practical people think and measure. You can insist all you want there there are no actual rings. But as long as time exists..so do the rings.. If there were no rings, there would be no time. Or more specifically, no way to track time.

to put it another way, without time matter can't change..and we wouldn't have any way of quantifying how much of a change there was. Without matter changing, there would be no way to detect time when something changes or telling how long it took. Spacetime is literally the thing that determines any type of change. Every moment of existence the universe refreshes itself and becomes a new universe. Each universe is a ring in the onion of the total spacetime of our universe.

The thing I've really been trying to figure out all of this time, is which is more important to the speed that we observe a galaxy..the straight distance and impetus that a galaxy has, or the amount of density of matter that expands? Is it a cannon shot? Or a toilet bowl thats filling back up?
 
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  • #50
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The original question of this thread has been answered, and it is now veering off into personal speculation. Thread will remain closed.
 

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