The Universe shouldn't look like it does if it is expanding

  • Thread starter dedwarmo
  • Start date
  • #1
9
0

Main Question or Discussion Point

If the Big Bang is true then 14.7 billion years ago the universe was the size of a pea. We are seeing the most distant objects as they were 14 billion years ago. Shouldn't the most distant objects appear to be closer together than objects near us? Shouldn't the universe be denser the further back in time you look?

Suppose Galaxy A is 14 billion light years away to the east of us and Galaxy B is 14 billion light years away to the west. 14 billion years ago they would have been very close together. We are seeing them now as they were 14 billion years ago, yet they appear to be 28 billions light years apart.
 

Answers and Replies

  • #2
9
0
It's obvious I'm a lay person and my knowledge extends to what you can learn on the Discovery Channel. Please be kind.
 
  • #3
phinds
Science Advisor
Insights Author
Gold Member
2019 Award
16,102
6,100
If the Big Bang is true then 14.7 billion years ago the universe was the size of a pea. We are seeing the most distant objects as they were 14 billion years ago. Shouldn't the most distant objects appear to be closer together than objects near us? Shouldn't the universe be denser the further back in time you look?

Suppose Galaxy A is 14 billion light years away to the east of us and Galaxy B is 14 billion light years away to the west. 14 billion years ago they would have been very close together. We are seeing them now as they were 14 billion years ago, yet they appear to be 28 billions light years apart.
Ah ... you misunderstand (but I think you knew that). The OBSERVABLE unverse is approximately 47 light years in radius, which is the distance to the CMB (look up "surface of last scattering"). You also need to study up on the expansion of the universe.

Try this for a start:

www.phinds.com/balloonanalogy


EDIT: also, do NOT take TV physics seriously. They get an awful lot right, but they get quite a bit wrong, some of it incredibly wrong, and you'll never know which is which is that is your primary source of learning.
 
  • #4
9
0
Ah ... you misunderstand (but I think you knew that). The OBSERVABLE unverse is approximately 47 light years in radius, which is the distance to the CMB (look up "surface of last scattering"). You also need to study up on the expansion of the universe.

Try this for a start:

www.phinds.com/balloonanalogy


EDIT: also, do NOT take TV physics seriously. They get an awful lot right, but they get quite a bit wrong, some of it incredibly wrong, and you'll never know which is which is that is your primary source of learning.

Regardless of what the actual numbers are shouldn't the older objects appear closer together than younger objects?
 
Last edited:
  • #5
phinds
Science Advisor
Insights Author
Gold Member
2019 Award
16,102
6,100
Regardless of what the actual numbers are shouldn't the older objects appear closer together than younger objects?
I am at a total loss as to how you could think that.
 
  • #6
9
0
I am at a total loss as to how you could think that.
My conclusions are based on my assumption that objects in space on the grand scale are relatively evenly distributed.

Suppose you have Objects A and B that are 2 light years apart and moving away from each other at 1 light year per year. If they are one light year away then we are seeing them as they were 1 year ago (1 light year apart).

Now suppose you have Object C and Galaxy D that are 2 light years away. They will appear to us as they were 2 years ago. Suppose they are in reality the same distance apart as Objects A and B, but since they are farther away from us they will appear to be closer together because we are seeing them as they were 2 years ago whereas the closer objects A and B appear as they were 1 year ago.
 
  • #7
9
0
In other words pick two galaxies that are on opposite sides of the universe from us that are at the edge of the observable universe. At some point long ago they were very close together. We are just now receiving light from them as they were billions of years ago.

Maybe I'll have to answer my own question. The light from the time when the universe was very compact hasn't reached us yet. Maybe even the universe is expanding faster than the speed of light and we will never see the light from those early years.
 
  • #8
phinds
Science Advisor
Insights Author
Gold Member
2019 Award
16,102
6,100
My conclusions are based on my assumption that objects in space on the grand scale are relatively evenly distributed.

Suppose you have Objects A and B that are 2 light years apart and moving away from each other at 1 light year per year. If they are one light year away then we are seeing them as they were 1 year ago (1 light year apart).

Now suppose you have Object C and Galaxy D that are 2 light years away. They will appear to us as they were 2 years ago. Suppose they are in reality the same distance apart as Objects A and B, but since they are farther away from us they will appear to be closer together because we are seeing them as they were 2 years ago whereas the closer objects A and B appear as they were 1 year ago.
Ah, now I see what you are saying. Yes, you are correct but what's the point? Things that are the same distance apart subtend a smaller angle if they are farther away (and are equidistant, in pairs, from us). Is that in some way odd to you? It doesn't have anything to do with time, just distance, and adds nothing puzzling, that I can see, to a discussion of the expansion of the universe.
 
  • #9
phinds
Science Advisor
Insights Author
Gold Member
2019 Award
16,102
6,100
In other words pick two galaxies that are on opposite sides of the universe from us that are at the edge of the observable universe. At some point long ago they were very close together. We are just now receiving light from them as they were billions of years ago.

Maybe I'll have to answer my own question. The light from the time when the universe was very compact hasn't reached us yet. Maybe even the universe is expanding faster than the speed of light and we will never see the light from those early years.
You would really do well to read the FAQ in the cosmology forum
 
  • #10
9
0
Ah, now I see what you are saying. Yes, you are correct but what's the point? Things that are the same distance apart subtend a smaller angle if they are farther away (and are equidistant, in pairs, from us). Is that in some way odd to you? It doesn't have anything to do with time, just distance, and adds nothing puzzling, that I can see, to a discussion of the expansion of the universe.
I understand what you're saying about similar sized objects subtend a smaller angle the more distant they are. But there is also the factor that the farther way they are the longer light has had to travel to reach us. The farther we look away in the universe the farther back in time we are looking. Can we at least agree on that? And then I'll move on the point I am trying to make. Thank you for your patience.
 
Last edited:
  • #11
phyzguy
Science Advisor
4,509
1,449
There actually is an effect like what you are referring to. Suppose I take an object of fixed physical size, like a galaxy, and move it further away. Of course, as it moves further away, its angular size gets smaller. However, this is only true up to a point. Beyond a redshift of about 1 (which is a lookback time about halfway to the big bang), the object's angular size stops getting smaller and starts getting larger again. See page 5 ("Angular Diameter Distance") of this paper.
 
  • #12
Bandersnatch
Science Advisor
2,913
1,823
If I may, I believe I know wherein lies the source of your misunderstanding.

You think that two objects x distance apart from eachother, and seen by us as separated by some angle α on the sky, will increase the visible angle of separation due to the expansion of space.
In fact, they stay in the same points in the sky, even though the universe is expanding.

To see this, try drawing a triangle with an observer, and two galaxies at verteices A, B and C respectively.

The angle at vertex A is the angular separation of the galaxies B&C we observe on the sky.

Due to expansion of the universe, each of the distances(AB, AC & BC) is multiplied by the same factor.
Let's say the factor is 2(or ~1000 to match the actual difference between the age of last scattering and now). Each side of the triangle is 2(or 1000) times larger, but the angle at vertex A(and all the rest of the angles too) remain the same.

The triangle expands in all directions, producing a proportional(with the same angles) triangle to the one we first observed. The galaxies move away from each other, but move away from us as well, and at the same rate.

Therefore the galaxies on the night sky do not move across the sky due to the expansion of the universe. We see them as they were X years ago, but each remains in the same spot on the sky it's always been(barring proper motion, not related to the expansions of the universe as such).

edit: fixin' my Engrish
 
Last edited:
  • #13
9
0
Thank you all for your replies.

If I may, I believe I know wherein lies the source of your misunderstanding.

You think that two objects x distance apart from each, and seen by us as separated by some angle α on the sky, will increase the visible angle of separation due to the expansion of space.
In fact, they stay in the same points in the sky, even though the universe is expanding.

To see this, try drawing a triangle with an observer, and two galaxies at verteices A, B and C respectively.

The angle at vertex A is the angular separation of the galaxies B&C we observe on the sky.

Due to expansion of the universe, each of the distances(AB, AC & BC) is multiplied by the same factor.
Let's say the factor is 2(or ~1000 to match the actual difference between the age of last scattering and now). Each side of the triangle is 2(or 1000) times larger, but the angle at vertex A(and all the rest of the angles too) remain the same.

The triangle expands in all directions, producing a proportional(with the same angles) triangle to the one we first observed. The galaxies move away from each other, but move away from us as well, and at the same rate.

Therefore the galaxies on the night sky do not move across the sky due to the expansion of the universe. We see them as they were X years ago, but each remains in the same spot on the sky it's always been(barring proper motion, not related to the expansions of the universe as such).

This seems to make sense, but was not the older universe more dense than it is today? Maybe I should have framed this as a question about density. If the universe is uniformly dense then the following should be true:

*The portions of the universe that are 5 billion light years away will appear to have the density of the universe as it was 5 billion years ago.

*The portions of the universe that are 10 billion light years away will appear to have the density of the universe as it was 10 billion years ago (more dense that it was 5 billion years ago).

*And so on.

Is this not true in terms of density?
 
  • #14
phyzguy
Science Advisor
4,509
1,449
You think that two objects x distance apart from eachother, and seen by us as separated by some angle α on the sky, will increase the visible angle of separation due to the expansion of space.
In fact, they stay in the same points in the sky, even though the universe is expanding.
In fact the expansion of the universe DOES increase the visible angle of two objects a fixed comoving distance apart as they get further away. Please see the paper I linked in the post before yours.
 
  • #15
phyzguy
Science Advisor
4,509
1,449
Thank you all for your replies.




This seems to make sense, but was not the older universe more dense than it is today? Maybe I should have framed this as a question about density. If the universe is uniformly dense then the following should be true:

*The portions of the universe that are 5 billion light years away will appear to have the density of the universe as it was 5 billion years ago.

*The portions of the universe that are 10 billion light years away will appear to have the density of the universe as it was 10 billion years ago (more dense that it was 5 billion years ago).

*And so on.

Is this not true in terms of density?
This is definitely true. The average density of the universe increases as you go to higher redshift. But bear in mind that the universe is also getting more uniform as you look back. Today's universe is very "clumpy", with very dense regions, like galaxies, stars and planets, and very empty regions, like interstellar space and the voids between galaxy clusters. By contrast, if I look all the way back to the "surface of last scattering", which is the source of the Cosmic Microwave Background radiation at a redshift of about 1000, the universe was uniform to within about 50 parts per million. In between, the universe gets hotter, denser and more uniform as you look back. Attached is a slice of a simulation done in Korea (called the Horizon run), which shows this.
 

Attachments

  • #16
9
0
Attached is a slice of a simulation done in Korea (called the Horizon run), which shows this.
Nice PDF. I'm guessing the orange areas represent higher density and blue areas lower density.

Now the problem I have is that billions of years ago the size of the universe was smaller than it is today but the part of the universe that is labeled Horizon (Big Bang Surface) is larger than the portions that have a younger Lookback Time.

Maybe this Wikipedia article is describing the problem I have,

http://en.wikipedia.org/wiki/Observable_universe

"Though in principle more galaxies will become observable in the future, in practice an increasing number of galaxies will become extremely redshifted due to ongoing expansion, so much so that they will seem to disappear from view and become invisible. An additional subtlety is that a galaxy at a given comoving distance is defined to lie within the "observable universe" if we can receive signals emitted by the galaxy at any age in its past history (say, a signal sent from the galaxy only 500 million years after the Big Bang), but because of the universe's expansion, there may be some later age at which a signal sent from the same galaxy will never be able to reach us at any point in the infinite future (so for example we might never see what the galaxy looked like 10 billion years after the Big Bang), even though it remains at the same comoving distance (comoving distance is defined to be constant with time, unlike proper distance which is used to define recession velocity due to the expansion of space) which is less than the comoving radius of the observable universe. This fact can be used to define a type of cosmic event horizon whose distance from us changes over time; for example, the current distance to this horizon is about 16 billion light years, meaning that a signal from an event happening at present would eventually be able to reach us in the future if the event was less than 16 billion light years away, but the signal would never reach us if the event was more than 16 billion light years away."
 
  • #17
marcus
Science Advisor
Gold Member
Dearly Missed
24,738
785
Dedwar,
it's good you are reading stuff like this
http://en.wikipedia.org/wiki/Observable_universe
and what you just quoted about galaxies fading out in future is very interesting! but not relevant to what you are asking about.

I think I understand why you are puzzled. See if I am right: you think that at great distance the sky should have a lot of dots close together because the universe was denser so there should be more galaxies (more dots) per given square patch of sky.

You are puzzled, I think, because you expect the wrong thing. When we look far back in time we do not see dots, we do not see galaxies because there were no stars and no galaxies.
the farthest stuff we can see is dense hot gas that has not condensed to form stars yet
and its image, the image of this comparatively small dense hot sphere of gas is spread out over the whole sky and forms the backdrop to everything nearer and more recent that we see.

And you have already seen an image of this small dense hot sphere of gas, on the WIKIPEDIA page that you already gave a link to http://en.wikipedia.org/wiki/Observable_universe

It is the BLUE OVAL further down the page. It is mostly blue, with some blotches of green and orange.

It looks just the way it ought to look, given the known expansion of distances. How it looks was in fact predicted from the expansion model already in 1940s and 1950s, and it was first detected around 1970.

This is the oldest stuff we can see. And what we see it the glow of this oldest stuff, which at the time it emitted the light was around 3000 kelvin like the surface of a small to mediumsize star. Distances and wavelengths have expanded 1000-fold since the light was emitted (which happened around year 380,000 of the expansion).

This gas that we see glowing (in that Blue Oval on the wikipage) is BILLION TIMES DENSER than the average density of matter in space nowadays, even with all the galaxies.

think of a cubical box. if you shrink the distance along an edge by a factor of 1000, then the volume of the box goes down by a factor of 10003, which is a billion. So the density of stuff in the box goes up by a factor of a billion. that is the density of the hot gas that we see when we study the blue oval map of the ancient light background.

Of course this is the image of something that USED to be comparatively small, a few millions of lightyear radius back when it emitted the light we are now mapping, but in a sense it looks spread out because of expansion. It is spread out over the whole sky! And the distance to that original glowing gas has expanded 1000-fold so (even though it used to be close) it is now the most distant stuff we can see. Everything that was originally closer has long since cooled and condensed to form stars and galaxies etc etc. And the light it sent us when it was 3000 kelvin primordial hot gas has already passed us by. So we see that nearer stuff in later stages of development.

Think of the Blue Oval as a movie of ancient time that is projected out on a gigantic movie screen which is the whole sky. Read more about it. Wikipedia has links. this is the denser ancient universe that you have been asking to see, but it just doesn't look how you thought it *ought* to look (many galaxy dots close together) :smile:
 
  • #18
309
4
Dedwarmo, Allow me to refer back to some of the numbers used earlier. If we see two galaxies ~14bly away, and 1 ly apart then I have a triangle with one side =1ly and two sides = 14bly, and the galaxies are as they were 14b years ago (I think that you are ok with that). Suppose we could "stop" the universe (expansion, movements, everything) and measure the "proper" distances today - then the sides of the triangle would be 2x23.5 and 1x (1*23.5/14). So the distance 14by ago was only 1ly where as today it is ~1.8ly - i.e. it was denser in the past.

I am only a beginner myself, who also struggled with this question, so I hope this helped.

Regards,

Noel.
 
Last edited:
  • #19
9
0
Of course this is the image of something that USED to be comparatively small, a few millions of lightyear radius back when it emitted the light we are now mapping, but in a sense it looks spread out because of expansion. It is spread out over the whole sky!
Allow me to express what I see to be a paradox this way:

The farther out we look in space the farther back in time we go, yet the farther back in time we go the smaller the universe was. It seems to me as the apparent distances grow larger they should actually be getting smaller.

Maybe the problem is solved by saying these objects are actually much, much farther away than they appear to be. An object that is 13 billion light years away we won't see it's 2012 position until 13 billion years has passed.
 
  • #20
marcus
Science Advisor
Gold Member
Dearly Missed
24,738
785
Allow me to express what I see to be a paradox this way:

The farther out we look in space the farther back in time we go, yet the farther back in time we go the smaller the universe was. It seems to me as the apparent distances grow larger they should actually be getting smaller.
...
I get the impression that you are thinking carefully about the distances and angles involved here. You might very well enjoy learning more about cosmology. This next thing you say might not be the solution but is nevertheless a good line of thinking to pursue. Keep asking questions.

Maybe the problem is solved by saying these objects are actually much, much farther away than they appear to be. An object that is 13 billion light years away we won't see it's 2012 position until 13 billion years has passed.
I'm a bit sleepy now. Might get back to this tomorrow. There are a number of things you need to realize, steps you need to take. Maybe some of this you already know.

1. we don't see the whole, we only see the part that is currently observable (from which light has had time to reach us.

2. In cosmology most often the idea of distance is not light travel time but what is called proper distance. What you would measure if you could halt the expansion process NOW and have time to measure directly how far something is now. Hubble expansion law is stated in terms of proper distance. It's also defined for times in the past. How far something was at some definite moment in the past if you could have frozen the expansion, in order to have time to measure by some direct means: radar, yardsticks, whatever.

3. If an galaxy is 13 billion lightyears away NOW at this moment, and sends us a flash of light (maybe a star explodes...) then it will take much longer than 13 billion years for the flash of light to get here. Because distance expansion keeps it from getting here, retards its progress. It will eventually make it but it will take much more than 13 billion years to get here.

Because the distance to the galaxy is expanding at 13/13.9 times the speed of light.
13.9 billion years is a handy quantity called the "Hubble time" which makes it easy to compute how fast distances are expanding. A distance of 13.9 billion lightyears is currently expanding at exactly c. And other largescale cosmological distances expand proportionally.

4. By universe standards, 3/8 of a million years is YOUNG.
When the universe was young, all the matter we can now see was 1100 times closer to "us" (or to the matter which eventually became our galaxy, solar system, and us.
It was all around "us" extending out some 41 million lightyears. A hot (comparatively) dense ball of gas. Light from the outer layers of that ball is only just now reaching us. And it comes nearly equally from all directions in the sky.

I suspect if you keep thinking and asking good questions like what you already asked you will have fun learning about cosmology.
 
Last edited:
  • #21
If the Big Bang is true then 14.7 billion years ago the universe was the size of a pea. We are seeing the most distant objects as they were 14 billion years ago. Shouldn't the most distant objects appear to be closer together than objects near us? Shouldn't the universe be denser the further back in time you look?

Suppose Galaxy A is 14 billion light years away to the east of us and Galaxy B is 14 billion light years away to the west. 14 billion years ago they would have been very close together. We are seeing them now as they were 14 billion years ago, yet they appear to be 28 billions light years apart.
dedwarmo, I'm with you 100%. I'm no physisist, but think just like you. Maybe you can say we use common sense..?

You think that two objects x distance apart from eachother, and seen by us as separated by some angle α on the sky, will increase the visible angle of separation due to the expansion of space.
In fact, they stay in the same points in the sky, even though the universe is expanding.

To see this, try drawing a triangle with an observer, and two galaxies at verteices A, B and C respectively.

The angle at vertex A is the angular separation of the galaxies B&C we observe on the sky.

Due to expansion of the universe, each of the distances(AB, AC & BC) is multiplied by the same factor.
Let's say the factor is 2(or ~1000 to match the actual difference between the age of last scattering and now). Each side of the triangle is 2(or 1000) times larger, but the angle at vertex A(and all the rest of the angles too) remain the same.

The triangle expands in all directions, producing a proportional(with the same angles) triangle to the one we first observed. The galaxies move away from each other, but move away from us as well, and at the same rate.

Therefore the galaxies on the night sky do not move across the sky due to the expansion of the universe. We see them as they were X years ago, but each remains in the same spot on the sky it's always been(barring proper motion, not related to the expansions of the universe as such)..
Your triangle example holds true for something being seen in the same spot in the sky, but if this is what really occurs, they also will look smaller over time. Are we experiencing this? Having said that, if something in the sky is staying in a visible constant position, but is getting visibly smaller, how do we conclusively know that this object is not just moving away from us in a direct opposite direction..?

Attached is a slice of a simulation done in Korea (called the Horizon run), which shows this.
dedwarmo,I have figured out what this Horizon run should look like to explain how you're thinking. This colored cone should be doubled up, placing the wide orange areas so they touch each other. You will have both visual points at opposite ends, looking like a double cone. This way when you look toward the middle(wide spectrum) this is what you'e currently seeing in the sky. But, as you look beyond what we currently see (in the middle) going further back in time back to the beginning of the so called "Big Bang", you see everything comes closer together (more dense in space) as you're trying to describe in your initial post. Am I making sense..?
 
  • #22
DrChinese
Science Advisor
Gold Member
7,288
1,090
I'm no physisist, but think just like you. Maybe you can say we use common sense..?
That's funny: as if physicists lack common sense, or having more "common sense" than a physicist is a virtue when it comes to discussing physics.

On the other hand, you might also consider "using" the spellchecker. Would that be a "common sense" thing to do? :smile:
 
  • #23
phinds
Science Advisor
Insights Author
Gold Member
2019 Award
16,102
6,100
dedwarmo, I'm with you 100%. I'm no physisist, but think just like you. Maybe you can say we use common sense..?
Uh ... do you REALLY think that "common sense" is all that helpful when studying cosmology or quantum mechanics? "Common sense" is based on our direct experience and there are a great many things in cosmology and quantum mechanics which are SO far out of our direct experience that "common sense" is not only not helpful, it is downright detrimental to the learning process.
 
  • #24
On the other hand, you might also consider "using" the spell checker. Would that be a "common sense" thing to do?
I didn't use the advanced reply, so I never knew where the spell checker was. I found it and will now use it. Thanks.
You think that two objects x distance apart from eachother, and seen by us as separated by some angle α on the sky, will increase the visible angle of separation due to the expansion of space.
In fact, they stay in the same points in the sky, even though the universe is expanding.

To see this, try drawing a triangle with an observer, and two galaxies at verteices A, B and C respectively.

The angle at vertex A is the angular separation of the galaxies B&C we observe on the sky.

Due to expansion of the universe, each of the distances(AB, AC & BC) is multiplied by the same factor.
Let's say the factor is 2(or ~1000 to match the actual difference between the age of last scattering and now). Each side of the triangle is 2(or 1000) times larger, but the angle at vertex A(and all the rest of the angles too) remain the same.

The triangle expands in all directions, producing a proportional(with the same angles) triangle to the one we first observed. The galaxies move away from each other, but move away from us as well, and at the same rate.

Therefore the galaxies on the night sky do not move across the sky due to the expansion of the universe. We see them as they were X years ago, but each remains in the same spot on the sky it's always been(barring proper motion, not related to the expansions of the universe as such)..
Your triangle example holds true for something being seen in the same spot in the sky, but if this is what really occurs, they also will look smaller over time. Are we experiencing this? Having said that, if something in the sky is staying in a visible constant position, but is getting visibly smaller, how do we conclusively know that this object is not just moving away from us in a direct opposite direction..?Any thoughts on this..?


Attached is a slice of a simulation done in Korea (called the Horizon run), which shows this.

dedwarmo,I have figured out what this Horizon run should look like to explain how you're thinking. This colored cone should be doubled up, placing the wide orange areas so they touch each other. You will have both visual points at opposite ends, looking like a double cone. This way when you look toward the middle(wide spectrum) this is what you're currently seeing in the sky. But, as you look beyond what we currently see (in the middle) going further back in time back to the beginning of the so called "Big Bang", you see everything comes closer together (more dense in space) as you're trying to describe in your initial post. Am I making sense..?
Does my analagy make any sense..?
 
  • #25
phinds
Science Advisor
Insights Author
Gold Member
2019 Award
16,102
6,100
Your triangle example holds true for something being seen in the same spot in the sky, but if this is what really occurs, they also will look smaller over time. Are we experiencing this? Having said that, if something in the sky is staying in a visible constant position, but is getting visibly smaller, how do we conclusively know that this object is not just moving away from us in a direct opposite direction..?[COLOR="Red"]Any thoughts on this..?[/COLOR]
I have no clue what you mean by the bolded part

I think you have neglected to take into account cosmological time scales. That is, it would likely take a minimum of millions of years to actually observe something looking smaller over time. If it's, say, 5 billion LY from us, then in one human lifetime it basically isn't going to move.

Certainly, an object that is, say, 5 billion light years from us will look WAY smaller when it is 10 billion LYs away but it will be at the same point in the sky (more or less) because EVERYTHING (outside out local galactic group) is moving directly away from us.
 

Related Threads on The Universe shouldn't look like it does if it is expanding

Replies
1
Views
1K
Replies
4
Views
793
  • Last Post
Replies
6
Views
2K
Replies
2
Views
1K
Replies
20
Views
6K
  • Last Post
2
Replies
31
Views
1K
Replies
10
Views
3K
Replies
19
Views
428
Top