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

[PhysicalAardvark]

TL;DR Summary

The universe is expanding, so at one point in the past it must have been all concentrated into a single point. But is this really an accurate observation?

Summary: The universe is expanding, so at one point in the past it must have been all concentrated into a single point. But is this really an accurate observation?

Hi,

This is my second probably naive question that's been on my mind as a lay-scientist for a long time (the other will hopefully appear as 'Dark energy noob question').

The reasoning behind the Big Bang seems to be as follows; The universe is expanding in all directions. So at some point in the past it must have been smaller. So at some point way, WAY back, it must have come together at a single point.

Now, this may be completely valid, but given that the remote galaxies we are talking about here are billions of light years distant, how accurately do we need to know that the are all accelerating exactly away from each other to ensure that they all came from a single point?

If all matter had originated in a volume rather than a point, wouldn't we still see exactly the same behaviour, to the levels of accuracy that we can measure?

Thanks,

P

 

[Orodruin]

You have a common lay misconception of the Big Bang being a form of explosion. This is not the case. The expansion is space itself expanding. The Big Bang occurred everywhere, it is just that ”everywhere” was smaller.

 

[PhysicalAardvark]

I did know this, but hadn't put two and two together in terms of the origins of the universe. So thanks for that; it makes a lot of sense.

So I'll re-phrase my question. Is there still the concept that at one point in time "everywhere" was infinitesimally small? And is this necessarily the only conclusion?

P

 

[weirdoguy]

If the Universe is infinite now, it was also infinite during the big bang. Observable Universe was way smaller then now.

 

[PhysicalAardvark]

Touche.

I guess I still have this concept of everything being compressed, somehow. The 'Hot Big Bang', after all. Where there is too much energy in the space it occupies for the single particles we now know to even exist in that form. And then things 'spread out' and 'cool down' so that matter can condense. This is certainly the picture painted in most popular press I see.

But if there has always been infinite space, why were these transitional phases needed? And why is inflation needed to explain rapid expansion if the universe has always been infinite anyway?

Thanks for your answers, folks, I've already progressed my understanding more in an hour than I could have in months trying to study it for myself.

P

 

[PeterDonis]

Moderator's note: Moved thread to the Cosmology forum.

 

[mathman]

Back to a single point is an overstatement. The universe was very small, but the origin is unclear.

 

[Khashishi]

Our models obviously break down at some point, but it's not clear where. It doesn't really make sense to have infinite density.

 

[mfb]

But if there has always been infinite space, why were these transitional phases needed? And why is inflation needed to explain rapid expansion if the universe has always been infinite anyway?

Even in an infinite universe it makes sense to talk about the scale factor - how distances change over time. While the mathematical volume is always "infinite" there is a real change in the universe over time.

It is possible (and usually expected) that the density was never really infinite, but that the universe started in some really compact version with an extremely high but finite density. How compact? We don't know. The Planck density would be a plausible number but it could be something else as well.

We do know that it expanded at least from the earliest point on where we know the laws of physics, because we can observe the results of this early expansion (e.g. in the ratio of hydrogen to helium to lithium any similar measurements).

 

[MainFragger]

I kind of have an oddball question about this. If the universe was expanding in the same amount every year, eventually the universe would be very misshapen. Because the amount of space being generated would not complete the next concentric circle of expansion. From what I have read, space expands, but the amount of actual matter remains roughly the same, and each concentric ring expands to the point where it has more space per mass, and thus a lower density per expansion ring on the outer edges.. If we are measuring these rings in individual years, what allows the expansion ring to generate enough space per year, when it has to push all of the existing space out to fill the inner most ring, but then also provide enough space for each additional ring to completely expand within a year? Every time the universe expands, it basically has to generate enough space to expand each previous year of space to fill a bigger concentric ring for the next year, plus the inner most ring, and push the outer most ring further out, and expand how much space IT contains. That's a HUGE amount of space generated per year, and it gets bigger each year. Aaaaaaand...it has to complete all of those expansions exactly within a year.. so, it has to generate much more space per year, and do in it faster to be able to fill the appropriate amount of void or space within that year. I guess my over all question is, if we assume the expansion of the universe is an autonomous process, then phyisically, doesn't it have to be designed to fulfill that process physically. Doesn't this technically mean that the universe has to be physically designed in such a way that this expansion could only work this way, or the whole thing would either not expand right or at all or hyperexpand, or expand unevenly?

 

[PeterDonis]

If the universe was expanding in the same amount every year, eventually the universe would be very misshapen.

Not if it is expanding at the same rate in all directions at every point, which it is (at least on average, which is how we model it).

rom what I have read, space expands, but the amount of actual matter remains roughly the same, and each concentric ring expands to the point where it has more space per mass, and thus a lower density per expansion ring on the outer edges

I have no idea where you are getting this from, but it's wrong. The average density of the universe (on large enough scales so that our global model is a good approximation) is the same everywhere. The expansion is not a process of adding "rings"--the universe is not a volume with a boundary that is adding "more space" at the boundary.

 

[MainFragger]

Annd? Now that you've told me what its not, how about explaining what it is? I am always up for learning more about this..

 

[PeterDonis]

Now that you've told me what its not, how about explaining what it is?

Have you read the earlier posts in this thread? Particularly the ones by @Oroduin and @mfb.

What other sources have you looked at?

 

[mfb]

Concentric rings around what? There is no special place in the universe. There are no outer edges or inner regions either.

If the universe was expanding in the same amount every year

It wasn't, by any metric, but this doesn't matter here.

 

[diogenesNY]

@MainFragger May I suggest that you take a gander at the linked article:

https://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf
This is a paper by Charles Lineweaver and Tamara Davis (two rather credible sources), entitled _Misconceptions About the Big Bang_. It was published as an article in Scientific American some years ago, and in my opinion, one of the most readable, yet fact laden and informative short pieces on the subject. Give it a read, and I predict you will spend the next hour or so half vocalizing "wow... hmmmm... I never really... but wait, so that's how..."

Its a fun read. Check it out.

And for extra credit, do a little search engine research on the origin of the _term_ 'Big Bang'.

diogenesNY

 

[MainFragger]

I have read that article, and it doesn't really change what I was asking. Sorry it took me a while to get to it it, I do contract work, and that keeps me moving around and busy a lot. The point is, at some point or other the universe started out as something really small, and is growing and growing at an exponential rate. Its not a perfect 3D orb, but close enough for the purpose of general description of my question. There are no physical concentric lines laid out in the universe, in the same way that there is no physical lines between Pennsylvania and New Jersey, New York, Delaware, West Virginia, and Ohio. Which is to say, yes, there's not a physical line, but if we wanted to map something out, we imagine the lines. The universe is predicted to be about 13.8 billion years old, so if you wanted to keep it simple, small and digestable, you could say that every billion years is a "ring" in the expansion of the universe (like rings in a tree). I like even numbers, so I am just going to round that off to 14 rings. At one point the universe was just what we would call the center...but then at some point it expanded enough that the the center became big enough to consider more than one part or ring. In this case, a billion years seems like a good place to start for a segment of our radius line, as well as however many light years that is in distance.

That having been said, my question is.. As the universe expands..if something were pushing it to expand, it would need a certain amount of pressure and perhaps matter at the center to keep pushing the segments in the center out to the next area/ring of expansion. If it used the same amount of pressure or mass as before, it would probably fall flat of completing that next ring in time for the next billion years. The simplest way to think of this is that each new area of space that has expanded has to actually be bigger than the area before it. If you used the same everything within that second billion years to achieve that, it wouldn't work, you'd get something that is mis-shapen and incomplete. So, whatever mechanism is causing the universe to expand, has to have enough energy to get the next ring exactly right..not too much, or it would be bigger, and not too little or it would be smaller and in both cases not properly formed.

So, what is the mechanism of expansion? Because if it isn't a living thing that can accommodate and adjust for inaccuracies, the consistently even growth of the universe points to a universe designed to keep its shape regardless of what happens next without any kind of thought process or intervention. Its is designed to keep its design and growth rate consistent without anyone's help.

My second question is related to the speed of the expansion of the universe. I have read that the closer to the further edges you get to the faster the galaxies are moving away. But, what I don't fully understand is, if the galaxy is moving away from you faster, and it is say 70% into the light year radius line of that area of the universe, when it expands to the next expansion ring, does it end up at the same 70% mark? Or does it shift a little. Also, When you say that the galaxies expand away from each other faster, is that because it reaches the edge of its billion year area faster? Or because it reached and surpassed its previous position from the previous ring faster? Because 70% of each ring that is further out is further than the previous 70 per cent, but also, may not line up perfectly with the radius line.. because it may be more important that it moves to 70 perceent of the space created in each new expansion vs. 70 percent of its straight trajectory.

 

[PeterDonis]

at some point or other the universe started out as something really small

No, it started out as something really dense and really hot. In our best current model, the universe is spatially infinite, so it wasn't "small" at any time.

and is growing and growing at an exponential rate

It is now, but it wasn't until a few billion years ago. Before then the expansion was decelerating, not accelerating.

At one point the universe was just what we would call the center...but then at some point it expanded enough that the the center became big enough to consider more than one part or ring.

No. As @Orodruin already pointed out way back in post #2, this is not how the expansion of the universe works. The universe is not expanding into a pre-existing space, and there is no "center".

if something were pushing it to expand

There isn't. The expansion is due to inertia: at the end of inflation, the universe was in a very hot, dense, rapidly expanding state, so it kept expanding by inertia.

Until a few billion years ago, the expansion decelerated because the gravity of the matter and radiation was slowing it down. Now the expansion is dominated by dark energy, which causes it to speed up. Those things affect the rate of expansion, but the universe would still be expanding anyway.

the consistently even growth of the universe

It hasn't been "consistently even". See above.

I have read that the closer to the further edges you get to the faster the galaxies are moving away.

No, the farther you get from us the faster the galaxies are moving away relative to us. But that is only relative to us. There is no "edge".

 

[DaveC426913]

An oft-used example to look at how the universe expands is the "raisin bread" analogy.

Consider a spherical ball of raisin-filled dough in the oven. The raisins are essentially evenly distributed throughout the dough.

The dough expands equally, at every point. Any two adjacent raisins are moving away from each other at the same speed as any other two adjacent raisins.

Raisins that are far apart from each other move away from reach other faster.

In fact, the rate at which any two raisins move apart from each other is directly proportional to the distance between those two raisins.

Thus, even when the ball of dough is two or three times its original size, the raisins are still distributed evenly.

 

[mfb]

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.

 

[Orodruin]

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.

Well, to some extent. It has different issues, such as the existence of a higher-dimensional embedding space.

 

[PhysicalAardvark]

Sorry it took me a while to get to it it, I do contract work, and that keeps me moving around and busy a lot.

Me too.

The point is, at some point or other the universe started out as something really small,

No; I get this now (as a result of asking this question).
The universe seems to have started everywhere, and all at once. All of that is expanding simultaneously.
I've paid very specific attention to 'modern' science documentaries on the topic; and they mostly agree with this.
Astonishingly, 'The Wonders of the Universe' with Brian Cox completely agrees with this idea. I'd listened to it a dozen or more times, yet somehow missed this crucial point. The universe did not start at a point. It started everywhere, and all of that is expanding.
But there are still mainstream science documentaries (Jim Al-khalili; looking at you) where it is explicitly stated that the universe started at an infinitesimal point.
I still have had no answers whatsoever about my more serious question, though.

 

[DaveC426913]

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.

Certainly. Which is why the analogy only goes so far. I really just wanted to address the OP's assumption that expansion occurs somehow heterogenously.

 

[PeterDonis]

I still have had no answers whatsoever about my more serious question, though.

What "more serious question" has not been answered?

 

[PAllen]

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.

Well, you can remove the boundary. Imagine the bread is infinite in all directions at all times. Then, to help mental intuition, you may envision the expansion stepwise. You imagine the infinite bread as first broken into an infinite collection of finite loaves, double each one's volume, put them back together. Repeat. Somehow this was the first way I could succeed in picturing the expansion without running into mental obstacles like how do other pieces get out of the way fast enough. Then, over the balloon analogy, this has the advantage of being 3-d and never finite, rather than 2-d and always finite.

 

[DaveC426913]

I still have had no answers whatsoever about my more serious question, though.

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?

 

[timmdeeg]

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.

 

[PAllen]

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.

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.

 

[timmdeeg]

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?

 

[PAllen]

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.

 

[timmdeeg]

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?

 

[PAllen]

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.

 

[timmdeeg]

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.

 

[MainFragger]

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..

 

[timmdeeg]

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.

 

[PhysicalAardvark]

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

 

[Drakkith]

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.

 

[mfb]

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.

 

[Drakkith]

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.

 

[MainFragger]

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. Let's say that we we tracking the expansion of the dots in a computer model using a graphic representing the metaphor. Let's 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, let's 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?

 

[Drakkith]

Now, let's 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.

 

[timmdeeg]

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%3A-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).

 

[jbriggs444]

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.

 

[timmdeeg]

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.

 

[jbriggs444]

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.

 

[MainFragger]

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?

 

[Ibix]

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.

 

[MikeeMiracle]

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 realize 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.

 

[MainFragger]

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 realize 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 that's filling back up?

 

[PeterDonis]

Thread closed for moderation.

 

[PeterDonis]

The original question of this thread has been answered, and it is now veering off into personal speculation. Thread will remain closed.