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Questions about the structure of the universe

  1. Apr 19, 2014 #1
    In general, what's a good way to understand the structure of the universe?

    I'm especially curious about the nature of the edge of the universe. Relativistic concepts dealing with the expanding universe confuse me greatly.

    Thanks for your time!
  2. jcsd
  3. Apr 19, 2014 #2
    First off there is no edge of the universe, what physicists refer to as our universe is in reference to our observable universe. We simply have no means of measuring beyond that so make no definite statements. It could be infinite or finite.

    here is an article covering expansion and redshift,
    https://www.physicsforums.com/showpos...6&postcount=10 [Broken]

    here is one on Universe geometry


    here is what observational cosmology covers

    "What we have learned from Observational Cosmology

    my signature contains more articles to understand basic cosmology at the

    is a manual for the

    lightcone calculator showing the expansion history and future expansion of the universe

    those should get you started on the hot big bang model represented by [itex]\Lambda[/itex]CDM model
    Last edited by a moderator: May 6, 2017
  4. Apr 20, 2014 #3
    I have a lot of probably misconceptions about the edge of the universe.

    We have an edge of our observable universe, but no "new" things will ever be observed, correct? That means our universe is expanding at least the speed of light? So, thinking with the Lorentz transformations, the edge of our observable universe, from our perspective, is standing still in time. Which already doesn't make sense to me. But I can move on from here: this portrays an onion layer structure to our universe. From any one perspective, anywhere in space, we observe a bubble of space-time?

    What significance even comes from the universe being infinite or finite if we're already existing in an inescapable bubble?
  5. Apr 20, 2014 #4


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    The universe does not expand at the speed of light. This is a very common misconception. Instead objects can recede from each other at light speed and beyond. This is a consequence of Hubble's Law which says that recession velocities scale linearly with separation.

    The edge of the observable universe is determined by how far light has traveled since the big bang. The size of the observable universe is increasing at greater than light speed because photons receding from Earth are traveling at speeds greater than c on account of the expansion. There is no problem with special relativity here, because locally photons always travel at the speed of light -- any superluminal recession velocity observed is due to the expansion of space and is not constrained by Einstein's speed limit.
  6. Apr 20, 2014 #5
    Not quite, "the greater the distance the greater the recessive velocity" V=HD. This is a observer location distance dependent relation. If you change observer to the edge of the observable then the expansion is the same as here and you would see a different region for the observable universe. What constitutes your observable universe depends on your location.

    edit just saw Bapowell's reply
  7. Apr 21, 2014 #6


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    Observing the universe from a different reference frame provides no additional information. Viewed from another galaxy 'now' you would merely see a universe older and cooler than the MW [by as many years as its light travel time]. A view 'then' [when photons we now observe were emitted] provides a view of the universe as it appeared when younger and hotter than 'now' It otherwise has absolutely no effect on the extent of the observable universe.
  8. Apr 21, 2014 #7
    How does recession verse expansion make any difference? (edit) Recession allows for infinite centers kind of view, while expansion dictates a single origin, right?

    If we existed on those universes as seen by us 'today,' then we would be however many billion years younger. But, what if we existed on those universes 'now,' without the looking back in time effect. We would still see a 14 billion year old universe, right? Just.. shifted?

    But in order to reach that universe in that time, we would break the speed of light, which is an impossibility. So, our particular inertial frame is finite (but infinitely expanding), right?

    edit edit: Wouldn't this mean that, for our frame, an infinite universe might as well be considered to be compressed into the shell of our observable universe? Which is standing still in time?
    Last edited: Apr 21, 2014
  9. Apr 21, 2014 #8
    recessive velocity is a measurement of the rate of expansion and is observer and distance dependant. For the reasons already provided.

    there is no way to observe the entire universe without looking back into time, the redshift is included in our measurements, see the redshift and expansion article I posted above.

    no this makes little sense, we cannot determine if our universe is infinite or finite. We can only see a finite portion (The observable universe) Those observations include the speed of light + expansion.

    Think of it this way a star emits light say 13 billion years ago. The universe was smaller then, as the light approaches us the universe expands both ahead and along the path of the light beam. However the rate of expansion is LOCALLY miniscule to the beam of lights speed. So it keeps approaching us, and decreasing the distance between us and the beam. As the expansion occurs the increase in distance along the path of the beam already travelled causes the frequency of the light beam to decrease (Cosmological redshift). However expansion ahead of the beam, does not affect the beam, other than to increase its distance between us and the beam.

    As the beam approaches us its already travelled part of the way, as it approaches us its recessive velocity decreases. The closer the light beam gets the less recessive velocity it will have, (assuming you had some magical means to measure the leading edge of the beam lol) also due to the less distance between us and the beam the less the rate of expansion will delay it from arriving. However the expansion history during its travel will have already reduced the frequency.

    edit just a side note, the amount of expansion per megaparsec is miniscule, when people say the expansion is faster than the speed of light they are referring to a far larger unit of distance, for example the amount of expansion between us and the CMB is 3C roughly, however per megaparsec its far far slower than the speed of light [itex] H=67.3 km/s/Mpc[/itex] so [itex]H=67300 meters/sec/Mpc[/itex] as opposed to [itex]c\ =\ 2.99792458\ \times\ 10^{8}\ m\ s^{-1}[/itex] one Mpc is [itex]3.08567758 × 10^{22}[/itex] meters
    Last edited: Apr 21, 2014
  10. Apr 21, 2014 #9


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    Hubble's Law, [itex]v = Hr[/itex], relates the recession velocity of a distant object, [itex]v[/itex], to its distance from us, [itex]r[/itex]. The expansion rate is given by [itex]H[/itex]. From this, you can see that for a given expansion rate, the recession velocity depends on separation. There is no origin necessary: imagine blowing up a balloon and consider the balloon's surface. Where is the center? There is none -- all points on the surface uniformly recede from all others with a speed proportionate to their separations.
  11. Apr 22, 2014 #10
    Let our universe have two objects. Consider "us" to be at rest.

    As the second object "moves" away, we take the size of the universe to be the space between the two objects.

    I'd prefer this to be imagined on a 1D line, but I don't know if that breaks things.

    Where is the difference between expansion and... recession? Space is created between the two objects rather than considering space to be traversed by the "moving" object? Space can be created in amounts that exceed the distance light can cover in the same interval? This feels like a wrong view...
  12. Apr 22, 2014 #11
    recessive velocity is a measurement of expansion, so in that sense there is no difference. the rate of expansion per Mpc is 67.3 km/s/Mpc. Does this sound like its anywhere close to being faster than the speed of light? I explained earlier that the points where we describe recessive velocity as being faster to the speed of light is due to a far larger unit of distance. Which quite frankly is a poor descriptive as it does depend on the unit of measure, Ie a Very large unit

    The point where this occurs is called the hubble sphere, and its a cumulative of the expansion per Mpc between us and the Hubble sphere. (67.3+67.3+67.3......keep adding up till it finally exceeds light speeds value lol.) However the rate of expansion in the individual Mpc's between us and the Hubble sphere is still 67.3 km/s/Mpc.
  13. Apr 22, 2014 #12


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    Space expands. Objects in space recede on account of the expansion.
  14. Apr 22, 2014 #13


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    Recession and expansion are the same. Space is not a thing and does not stretch or expand, it's just that things IN space get farther apart (see the link in my signature). This is called recession. MOVING is a whole 'nother thing. This gets a bit weird because we are using English language to describe stuff that it wasn't quite made to describe. Things cannot move greater than the speed of light but they can, and do, recede at any speed. Objects at the edge of our observable universe are receding from us at about 3c but their proper motion relative to us is negligible by comparison.

    Chronos's comment about "no new information" is misleading. If you could magically move instantaneously to a galaxy 5 billion light years away, you would as he says see an observable universe that has the same characteristics as ours in that your observable universe would be the same radius as ours. You WOULD, however, see things in that observable universe that are not in our observable universe and you would see the things in our observable universe (those that you could see anyway) as being, as Chronos says, a different age that than what we see them as being.
    Last edited: Apr 22, 2014
  15. Apr 22, 2014 #14


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    The reason that I draw a distinction between expansion and recession is because the former is a rate, determined by [itex]H[/itex] that applies to the growth of length scales in the universe (I say space expands, but I mean that distances increase). Meanwhile, points in space recede from each other on account of this expansion with a speed given by Hubble's Law. In words, then: recession speed = expansion rate x separation.
  16. Apr 22, 2014 #15
    Okay, I think I have a more accurate picture now, however this new picture is equally confusing.

    So, space is created? Is there any way to better understand this concept? I haven't touched quantum mechanics yet.

    In other words, the balloon model is serious business. We're inside an inflating balloon and cannot leave our little subset of space. The balloon might have a center with respect to the 'actual' edge, but we'll never know, and, for us, it is a meaningless question.

    I'm wanting to view the universe as a lower dimensional bubble contained in something else. Such as a soap bubble in a bucket of water. The 2D-ish soap bubble thins out in a uniform manner as it is absorbed into the 3D bucket. Is their any actual logic to this kind of perspective?

    I'm wanting to use this perspective because the idea of an infinite expansion seems as arbitrary to me as claiming to be the center of the universe.

    edit: The big bang implies a point, right? But this doesn't imply an actual center for the universe?
    Last edited: Apr 22, 2014
  17. Apr 22, 2014 #16


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    I'm not sure whether you've got it or not, but I think your wording is confusing. The fact that no matter where we are in the universe we are at the center of our own OBSERVABLE universe has nothing whatsoever to do with the balloon analogy. We are not inside an inflating balloon because a balloon has a material, physical boundary (edge) whereas the observable universe has a horizon but there is nothing material marking it. The balloon analogy is best understood if you read my discussion of it linked to in my signature. The "balloon" actually goes away in the most realistic version of the analogy, as I explain there.

    Space is not created, things IN space just get farther apart.
  18. Apr 22, 2014 #17


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    No, nothing is inside the balloon. We are on the 2D surface -- it is analogous to the real 3D space of the universe. The balloon nicely illustrates that no one single point on the balloon is central, however, from its vantage point, it sure appears to be. As long as the balloon is homogeneously inflated, all points recede from all others according to Hubble's Law.

    No, the big bang implies a beginning (a point in time only).
  19. Apr 22, 2014 #18


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    Fair enough. Thanks for pointing that out.
  20. Apr 22, 2014 #19
    http://www.astro.ucla.edu/~wright/balloons.gif just a visual aid

    edit:not a very good one though lol tends to have the directions of expansion wrong I'll see f I can locate a better one.

    this one isn't bad they give three different animations


    ignore the descriptive explosion though lol, some of their wording on the site isn't precise, other descriptive's are just plain wrong. So just use the animations lol

    ignore this as well

    "The idea is that we live in a universe with three spatial dimensions that we can perceive, but that there exist "extra" dimensions (maybe one, maybe more than one) that contain the center of the expansion. Just like the two-dimensional beings that inhabit the surface of the balloon universe, we cannot observe the center of our universe. We can tell that it is expanding, but we cannot identify a location in our 3D space that is the center of the expansion."

    its too bad the article itself is so poorly worded, the animations I wanted to post merely as a visual aid showing 1d rubber band analogy, raisin bread/balloon analogy and the redshift. Its the only site I found that had all 3. Unfortunately I couldn't just copy the animations. So ignore all the descriptives on the site, far too many errors
    Last edited: Apr 22, 2014
  21. Apr 22, 2014 #20
    My wording is very poor, thank you all for helping me fix it. I'm trying to focus on one thought at a time. Sorry for not replying to everything that's being said!

    This particularly is trouble for me:

    I asked two cosmology professors "does space grow" today. I got both a yes and a no.

    I guess the next step is to ask for more detail? Here's my thinking:

    Things become further apart if space in between them increases. The increase in space of the universe isn't explained by relative velocities alone. So we have extra space that was never traversed. Therefor... empty space grows..?

    But this also confuses me because the idea of space growing is rather crazy as well. What happens to space that is grown inside a galaxy? Galaxies are held together with a ridiculous amount of energy. Does grown space leak out like some kind of gas? Problems! Haha, maybe that would make sense. Galaxies leaking space and all.

    edit: I like that a lot actually. Is matter turning into space a thing that's been studied?
    Last edited: Apr 22, 2014
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