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

  1. Aug 29, 2012 #1
    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.
  2. jcsd
  3. Aug 29, 2012 #2
    It's obvious I'm a lay person and my knowledge extends to what you can learn on the Discovery Channel. Please be kind.
  4. Aug 29, 2012 #3


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    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:


    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.
  5. Aug 29, 2012 #4

    Regardless of what the actual numbers are shouldn't the older objects appear closer together than younger objects?
    Last edited: Aug 29, 2012
  6. Aug 29, 2012 #5


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    I am at a total loss as to how you could think that.
  7. Aug 29, 2012 #6
    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.
  8. Aug 29, 2012 #7
    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.
  9. Aug 29, 2012 #8


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    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.
  10. Aug 29, 2012 #9


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    You would really do well to read the FAQ in the cosmology forum
  11. Aug 29, 2012 #10
    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: Aug 29, 2012
  12. Aug 29, 2012 #11


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    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.
  13. Aug 29, 2012 #12


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    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: Aug 29, 2012
  14. Aug 29, 2012 #13
    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?
  15. Aug 29, 2012 #14


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    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.
  16. Aug 29, 2012 #15


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    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.

    Attached Files:

  17. Aug 29, 2012 #16
    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,


  18. Aug 29, 2012 #17


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    it's good you are reading stuff like this
    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:
  19. Aug 29, 2012 #18
    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.


    Last edited: Aug 29, 2012
  20. Aug 30, 2012 #19
    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.
  21. Aug 30, 2012 #20


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    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.

    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: Aug 30, 2012
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