Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The limit of the observable universe

  1. Nov 4, 2011 #1
    I have heard it said that galaxies exist which are beyond the observable universe because the expansion causes them to be receding at super-luminal velocity. How can this be? We can see all the way back to the surface of last scattering, when the universe was just dense plasma. The limit to the observable universe cannot be both.

    Is the observable universe limited by an expansion event horizon or an opaque primordial plasma?
  2. jcsd
  3. Nov 4, 2011 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    It can't. Well...

    The observable universe is the bit you can see. There exist galaxies et al beyond this because light from those galaxies has not reached us yet. It follows that someone in Andromeda can see bits of the Universe we cannot see and will not for two and a half million years. (The light has reached them, but they are 2.5mil ly away.)

    Each year we can see 1ly further in space, but no further back in time because the bit of the Universe right here is a year older.

    Back to that "Well..."
    ... some people argue the Hubble expansion can result in superluminal relative speeds because it is a geometric expansion of space rather than the usual idea of speed involving travel in space. Considering that the limit of observable universe is retreating from us at the speed of light, then this suggests that some galaxies will never be observed (they are moving faster than that wrt us even though their speed wrt to objects closer to them is not faster than light.)
    This paper argues that the Hubble expansion is only good for nearby objects.
    Last edited: Nov 4, 2011
  4. Nov 4, 2011 #3


    User Avatar
    Science Advisor
    Gold Member

    The surface of last scattering [CMB] IS the limit of the observable universe. Your basic premise is correct - it is obviously illogical to suggest we can ever observe entities older than the universe itself.
    Last edited: Nov 4, 2011
  5. Nov 4, 2011 #4

    "As expected, Hubble’s receding velocity can be
    thought to give the relative velocity between two
    close comoving objects. It is also possible to
    reinterpret (13) by reversing the logic. Since
    Hubble’s receding velocity is supposed to give
    the relative velocity, it is natural to define vrel
    by parallel-transportation along the straight line
    joining two comoving objects at a fixed time, at
    least for nearby objects. For larger distances,
    one can imagine infinitesimally separated comoving
    observers placed in between A and B. Nearby
    observers can measure their relative velocities at
    a fixed time. From that information, relative velocity
    between distant objects A and B can be
    determined by integration, which indeed corresponds
    to parallel-transportation along the finite
    line-segment and thus we obtain (11). Consequently,
    vrel can be seen to generalize the usual
    concept of relative velocity in a cosmological context.
    In summary, we believe that the best way to
    resolve concerns about superluminal expansion
    speeds is to emphasize that Hubble’s law does
    not make sense for large distances. We showed
    that if the time derivative of the distance between
    two objects is naively identified as the relative
    velocity, then faster than light speeds can
    also be found in special relativity. Therefore,
    we need to be careful in determining the correct
    physical meaning of a mathematical quantity in
    a relativistic theory, which is also the main issue
    with Hubble’s law. These examples can be used
    to convince students that there is nothing wrong
    with a naive superluminal expansion speed since
    it has nothing to do with relative velocity or as
    a matter of fact it has no direct physical significance.
    Moreover, we pinned down the correct
    differential geometrical meaning of the Hubble’s
    receding velocity as the rapidity of a local
    Lorentz transformation. With the derivation of
    this last result, there must not arise any further
    issue with faster than light expansion speeds."

    Just as soon as I had began to accept the unlikely possibility of superluminal velocities then some one suggests it is a fallacy.

    Do we all agree that the matter which emitted the CMB was moving away from us at almost at the speed of light? Objects further away from must surely have to be travelling away from us faster than this? Also I believe that the dark energy expansion makes the relative velocities higher. Perhaps a deeper understanding of 4D spacetime and relativity can solve this?
    Last edited: Nov 4, 2011
  6. Nov 4, 2011 #5

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    But is that what "observable universe" usually means in this context?

  7. Nov 4, 2011 #6
    The current comoving distance to the particles which emitted the CMBR, representing the radius of the visible universe, is calculated to be about 14.0 billion parsecs (about 45.7 billion light years), while the current comoving distance to the edge of the observable universe is calculated to be 14.3 billion parsecs (about 46.6 billion light years),[1] about 2% larger.


    A 2% delta is a reasonable approximation?
  8. Nov 4, 2011 #7
    Essentially it's not so much that the galaxies themselves are moving at superluminal speed but rather that spacetime is itself expanding everywhere which causes things far away from us to recede at superluminal speed.

    I think that's the layman doesn't really explain anything explanation :P
  9. Nov 4, 2011 #8


    User Avatar
    Science Advisor
    Gold Member

    Welcome to PhysicsForums, Stimpon!

    Here is a good article that covers this subject:

    http://space.mit.edu/~kcooksey/teaching/AY5/MisconceptionsabouttheBigBang_ScientificAmerican.pdf [Broken]

    Keep in mind that new evidence is coming in all the time, and a lot of new science in this area has appeared in the past 20 years.
    Last edited by a moderator: May 5, 2017
  10. Nov 4, 2011 #9
    In this thread it was said that space has no objective existance itself, and that space is not created the same as ponderable matter.


    Therefore presumably all we can say is that our measurement of the distance between two very distant galaxies is increasing. And for galaxies beyond our observable universe the distance is increasing so fast that light from the distant galaxy will no longer be able to reach us. Why this happens is unknown, but the effect has been given the name dark energy.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook