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Expanding universe

  1. Dec 20, 2007 #1
    didn't know where to post this. i recently learnt that the universe is expanding and it is doing this symmetrical... and all points are accelerating away depending on their distance in space. ok this is ok but then i thought about it alittle bit... doesn't this imply that a galaxy far enough away is travelling at speeds greater than the speed of light? well yes... so i went looking for an answer turns out that relativity only applies to objects travelling through space. cool.

    but say that this distant galaxy were travelling away from us faster than light... would we ever even see it?
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  3. Dec 20, 2007 #2


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    Excellent questions!

    Yes that's right! There is a tricky issue in defining what is meant by speed. Obviously a speed is a distance traveled in a time, but in General Relativity there are different ways to define both of those things, so there are different 'speeds' that you can express for the same situation. The short answer to your question is still yes, but there are some interesting caveats (see some of the papers I reference below).

    You're on the right track, but it's not an issue of relativity not applying, but rather than the results of general relativity in an expanding universe can be a little tricky to get your head around, especially if you've been thinking in terms of special relativity only.

    Yes we can! http://www.astro.princeton.edu/~aes/AST105/Readings/misconceptionsBigBang.pdf" [Broken]
    is a pop science article (from Scientific America) written by two professional cosmologists that explains all of this very well.

    Hope that helps!
    Last edited by a moderator: May 3, 2017
  4. Dec 20, 2007 #3


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    The light from such a distant galaxy would travel through space , which itself is expanding, so the light is carried along with the cosmic expansion towards us.

  5. Dec 21, 2007 #4
    if 2 points in space are moving at relavistic speeds (or at the speed of light) away from each other wouldnt they be cut off from each other as light(and other forces) would never reach them from each other
    would they form 2 different universes
    Last edited: Dec 21, 2007
  6. Dec 21, 2007 #5


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    Hi cosmic.ash and welcome to these Forums!

    As I have just said it is possible to see some galaxies and quasars that are actually moving away from us at over the speed of light.

    These are galaxies beyond the point where the recession velocity is greater than c but nearer than our Particle Horizon.

    This is because their motion is largely due to the cosmological, or Hubble, expansion of space. They are being carried along with it, away from us. But similarly so is the light that is travelling in our direction from them.

    At a certain stage of those photons' journey the recession velocity drops to less than c and they continue to complete their journey to us, albeit very red shifted.

    Photons that leave a luminous object beyond our Particle Horizon never reach that stage in the journey and so we cannot see them at the present time.

    If the luminous object is between the Particle Horizon and the Event Horizon of the observable universe then the photons will eventually reach us at some time in the future.

    If the luminous object is beyond the event horizon then they will never reach us.

  7. Dec 21, 2007 #6
    thanks for that article Wallace cleared up alot of things.

    i'm just curious as to why the spead of the photon receding will suddenly be below c and therefore allow it to continue its journy to earth?

    EDIT: Or is it because since the light is moving through space at c and away from us at c that the distance between earth and the photon will decrease until its small enough that its no longer receding?
  8. Dec 22, 2007 #7
    thats the point
    as light will never reach us nor will other influences making it a different universe (unaffected by ours)
  9. Dec 22, 2007 #8


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    a partition of the universe like that----using each observer's horizon----would lead to a certain amount of confusion.

    there are galaxies we are looking at right now, which we are studying today, that according to you would not be in our universe. because if someone there tried to send us a signal TODAY it would never reach us.

    they are part of our causal past, we study them, they have affected us and will continue to for some time to affect us, but according to you they would not be part of our universe.

    so your definition makes the idea of universe kind of awkward.

    another awkwardness is that you are proposing to divide the universe (as we usually think of it) into two parts, where people out at the boundary would not notice anything. for them everything looks more or less like it does to us-----they have a different horizon. so your division is OBSERVER-DEPENDENT. in principle every observer has a different universe which is his (seen from the standpoint of his galaxy)

    linguistically it would not be very practical to talk like that, we'd have to rewrite all the books that use the word universe, and things would be harder to say, sentences would get longer and so on.

    astronomers already have terminology to designate the PARTS of the universe (1)those which HAVE affected us and from which weve gotten light signals etc and also (2)those in which events happening NOW can in FUTURE influence us and also (3) those receding faster than light by Hubble expansion.

    These are 3 different horizons. (1) is called the particle horizon, (2) is called the cosmological event horizon, and (3) is the Hubble radius.

    1. the particle horizon distance is around 47 Bly if I remember right
    2. the event horizon distance is around roughly 16 Bly
    3. the Hubble radius is roughly 13.5-13.8 Bly, these are just estimates.

    a neat fact is that a galaxy can be beyond the Hubble radius, that is TODAY receding faster than c, and can today send us some light, and as long as the galaxy is not outside the 16 Bly event horizon that light will still eventually reach us! This is a fact that gives some people trouble when they first encounter it. they think that the Hubble radius and the event horizon should coincide but they don't coincide----16 > 14. Just a detail though.
  10. Dec 22, 2007 #9


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    Just want to take a wack at that "detail", by re-posting this...
    The key here, I think, is "2 points in space moving at relativistic speeds." The object at a great distance may be moving away from us at a speed that is greater than c relative to us, and our space, but of course, it is travelling < c relative to its own space. The same expansion of space that eneables the object to travel at a speed that appears to exceed c also enables the photon emitted from that source to travel back toward us at a speed that is > c relative to our area of space, but exeactly c relative to its local space, right?
  11. Dec 22, 2007 #10


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    Lurch, about that detail, there are various ways to explain it and Wallace has IIRC several times, maybe Pervect, maybe several others. can't keep track. it keeps having to be explained. You obviously have a way that works for you.

    the way I frequently think of it (not necessarily better than others) is that the Hubble parameter KEEPS DECREASING. the value H0 is merely its current value 71 km/s per Mpc, but it has been tens and hundreds of times larger in the past.

    now the HUBBLE RADIUS is just c/H
    that is what you get if you solve for the distance something has to be today in order for its recession speed, today, to equal c. It is highschool algebra.

    So as H keeps getting smaller, the hubble radius c/H keeps reaching out farther and farther.

    So suppose a galaxy is today just outside of our Hubble sphere, that is it is receding faster than c, and suppose that today it sends us a photon.
    If that photon can only hang in there and not get swept back TOO much, then eventually our Hubble sphere will expand out to it and engulf it.
    and obviously any photon inside will eventually reach us.

    To recap: any photon aimed at us that is already inside will get to us, because inside the recession is <c so light always wins

    But a photon aimed at us that is just a little outside will ALSO because it is basically standing still from our perspective. it's progress towards us is canceled by the recession, so it stays the same distance or gets swept back very gradually,
    and the Hubble sphere is expanding, so it may at some point engulf the photon, and then it is inside. So it is cool from then on.

    the Hubble sphere has expanded dramatically----by orders of magnitude. in some sense that is the key.
    in the future it is expected to expand by a healthy percentage but nowhere near as much.
    If I remember H is expected to go down from 71 to around 50 or so (don't recall exactly) so the Hubble radius c/H would increase proportionately......which is why the cosmological event horizon is some 16 Bly, SOMEWHAT larger than the hubble radius of 13-14 Bly, but not like an order of magnitude larger.

    I'm just being sketchy because I think you have already assimilated a lot of this. I recall you were already at PF back in 2003 and 2004. Right? I remember talking astronomy with you back then.
    Last edited: Dec 22, 2007
  12. Dec 25, 2007 #11
    ok well could someone show a model for why the hubble distance expands? i would be very greatful :D
    Last edited: Dec 25, 2007
  13. Feb 14, 2008 #12


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    Hello Sorry, somehow I missed this question. But I liked one of your posts on another thread and happened to look up to see what other posts, and here is this really good unanswered question!

    the mathematical model that applies here is a very simple differential equation called the Friedmann, which is basic to all cosmology. It is a simplified version of the Einstein field equation, the main equation of GR, basically our law of gravity. Friedmann found it around 1923.

    All cosmology is based on the (two) Friedmann equation(s)

    the gist of one of the two equations is that the SQUARE OF THE HUBBLE PARAMETER IS PROPORTIONAL TO THE total ENERGY DENSITY.

    Now you know that in the standard model the dark energy density is estimated at 73 percent of the total and the dark energy is CONSTANT. that is a premise and it fits the data remarkably well. they are always checking it as new data comes in.

    all the other energy is THINNING OUT obviously. So the energy density is destined eventually to go down to 73 percent of its present!

    So by proportionality, the square of Hubble is destined to decline and level off at 73 percent of present.

    So the Hubble is destined to go down to sqrt(0.73) of its present value and level off, asymptotic.

    My intuition is that you are following this and you also see how beautifully simple it is.

    And the HUBBLE RADIUS IS THE RECIPROCAL of the hubble parameter so it is destined to go up to and approach asymptotically a length which is 1/sqrt(0.73) of its present length.
  14. Feb 15, 2008 #13
    ohhhh ok. thanks:D
    posted this awhile ago and there was no response. that helps loads though thanks :D
  15. Jul 9, 2008 #14
    Above is all fine. Historically GRT was essentially complete, but had a cosmological constant. Then came Hubble and his redshifting of galaxies, intrepreted as expansion of universe. So the cosmological constant was dropped. Thus Hubble expansion is not part of GRT. In GRT one can have affine parameterization along a null geodesic, with a finite velocity of a photon. Yet the Hubble expansion (also a local description) can have superluminal stretching of the manifold (continuum i.e. inbetweeness). So pseudo-Riemannian spacetime manifold has a finite velocity for photon; while Hubble expansion of overall manifold, as well as for a local patch, can have a superluminal stretching. This always sounds somewhat strange (a koan), that the same manifold can have two such apparently disparate conceptual descriptions.
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