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Observable Universe vs. HyperUniverse

  1. Jul 22, 2006 #1
    I've noticed recently that many posts are veering towards the question of just how much of our total Universe can we observe. So I thought a new thread could address this question.

    1) What is the radius of the observable Universe?

    2) What are our best guesses as to the radius of the entire Universe?

    3) Every year, more and more fringial quasars come into our observable view because their light finally reaches our telescopes. Does the HyperUniverse (the part beyond our observable horizon) expand faster then the rate at which our observable horizon expands? In other words, does the HyperUniverse create more unseen 'objects' every year than our observable Universe shows to us?

    4) WMAP suggests the Universe is accelerating in its expansion, at what velocity is this expansion taking place at?

    5) If an observer could sit comfortably in a spaceship on the observable Universe's horizon and look outwards at the HyperUniverse, would he continually see new point sources of light 'blip' into existence as the light from 'objects' in the Hypersphere finally reach his eyes?

    6) Forgive me if this question is dumb. Would 'objects' in the HyperUniverse gravitationally effect 'objects' just inside our Observable Universe Horizon, or would these gravitational effects simply not occur since they also propogate at C?

    Thanks. I hope this thread can serve as a rallying point for amateur cosmologists like me to gather expert opinions from PF members like SpaceTiger and others. We love our questions, don't we!
  2. jcsd
  3. Jul 22, 2006 #2


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    I think you are smart. So see what you can get from this

    Inflation and the Cosmic Microwave Background
    Charles H. Lineweaver (School of Physics, University of New South Wales, Sydney, Australia)
    34 pages, 13 figures, contributed chapter to " Proceedings of the New Cosmology Summer School" edt. M. Colless, to be published by World Scientific, 2003. Based on five lectures given at the 16th Canberra International Summer School "The New Cosmology" held 3-14 February at the Australian National University. A version with full resolution images is available at this http URL
    Report-no: UNSW 03/05/1-1
    "I present a pedagogical review of inflation and the cosmic microwave background. I describe how a short period of accelerated expansion can replace the special initial conditions of the standard big bang model. I also describe the development of CMBology: the study of the cosmic microwave background. This cool (3 K) new cosmological tool is an increasingly precise rival and complement to many other methods in the race to determine the parameters of the Universe: its age, size, composition and detailed evolution."

    1. he explains a commonly used distance scale (comoving distance)
    IIRC the current particle horizon (radius of obs.) is around 47 billion LY.
    check the number, it could be 42 or something

    that is to say that the absolutely most distant crud that we we could be getting some kind of radiation from it at this moment is currently 47 billion LY from us.

    (that crud would have sent out the signal way long ago when it was more amorphous but by now it would have coagulated into galaxies and stars, so we are asking what are the most distant galaxies that we could be getting radiation not from them exactly but from their ancestral precursor crud that eventually formed them. is that clear or is that confusing?)

    2. I don't see any need for the word "hyperU" in your post. you are just talking about our universe----the part we see already and the part whose light hasnt got here yet.

    there is no estimate that I know of about the radius of the full universe. one can try to BOUND it. Like one can say it is AT LEAST such and such.

    I guess the radius pretty clearly has to be at least 100 billion LY but it could just as easily be a billion billion LY, or infinite.

    Humans have no good handle on it. I am going by the paper by Scott that SpaceT suggested----it says that USING WMAP DATA AND THE USUAL MODEL the full universe volume must be at least tenfold bigger than the observable volume.

    well, the observable volume radius is say 47, so the full volume radius has to be at least twice that, or 100, if the volume is going to tenfold. right? (two cubed is around ten)

    this is all very order of magnitudey, because IMHO people simply dont know. we dont even know if it is finite----it might be infinite----how are you going to find a 'best estimate' of a number you dont even know if its is finite?
    Last edited: Jul 22, 2006
  4. Jul 22, 2006 #3
    Well someone positioned near our horizon of the observable universe obviously has a different observable universe and thus also a different horizon.
  5. Jul 23, 2006 #4


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    About 46 Gly. Note that this is the particle horizon, i.e. the current distance of a massless particle sent from our position at t = 0.

    This can be only estimates of a lowest limit in its size. According to Neil Cornish this is about 39 Gly (24 Gpc in diameter). Note that there is no contradiction in the fact that this number is less than the particle horizon. So IMO your term "hyperuniverse" is not a very good one.

    Yes, the number of objects in the observable universe increases as long as the universe is not in a de-Sitter expansion phase. Such a phase will happen in future, asymptotically when t [itex]\rightarrow \infty[/itex], when 100% of the energy density will be dominated by the cosmological constant. Currently about 73% of the energy density is dominated by the cosmological constant. In case of a de-Sitter expansion it can be shown that the radius of the observable universe is located at a constant comoving distance. Since objects remain always at the same comoving distance from us (neglecting peculiar speeds), there will be no objects entering the observable universe in future, although the radius of the observable universe will always increase as a proper distance, Dp = Dc a(t).

    Considering a deceleration parameter of about q = - 0.6, the acceleration of expansion is about 1.3 × 10-15 km / s2 per Mpc. Moreover, the speed of expansion is 71 km / s per Mpc (the value of the Hubble parameter).

    I guess yes, similar to a ship moving through the ocean.

    Objects located outside our observable universe are causally disconnected from us.
    Last edited: Jul 23, 2006
  6. Jul 23, 2006 #5


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    Looks like some good answers so far. Just to make sure we're on the same page, there are no new objects appearing in the "hyperuniverse", as you call it. They're all already there, just spreading apart due to expansion. However, there are new parts of the "hyperuniverse" becoming observable to us as time goes on. These parts first appear to us in the microwave background (if we're observing with ordinary light). When they first appear, it will be at a time when they haven't yet formed compact objects (like stars, galaxies, etc.). However, they will be seen doing so if we continue to watch them throughout the universe's evolution.
  7. Jul 23, 2006 #6
    Mm... but the things outside the Earth's observable universe isn't outside say... the observable universe of the Crab Nebulae.. that means that if a supernovae went off just beyond the Earth's observable universe but within the Crab Nebulae's observable universe, the supernovae's shockwave may reach the Crab Nebulae one day right? What I don't get is... can't the Crab Nebulae be disturbed, shake and give off a tiny ripple of a gravitational wave or something and that can reach the Earth? (obviously we're not gonna detect such a small thing from the Crab Nebulae, but say the effects are large and we can detect it) So... my question is, can something outside our observable universe affect us?
  8. Jul 23, 2006 #7
    thanks hellfire,
    just to confirm, the above means that:
    exapnsion is happening at the same rate as the rate of enlarging of our obvservable universe?
  9. Jul 23, 2006 #8


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    OK, my answer was not correct at all. The observable universe is not a limit of causal binding.

    Strictly speaking, the objects that can causally affect us are part of our past light cone. It comprises objects as they were in past, emitting light that is reaching us now.

    The observable universe, however, is a spatial hypersurface at the current cosmological time. It comprises objects as they are now. Some of them may emit light that will reach us in future, others may not because space expansion may carry that light always farther away from us.

    This limit is given by the event horizon. It gives us the border in the current spatial hypersurface of the objects that might affect us in future.
  10. Jul 23, 2006 #9


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    Sorry Jenny, I deleted the answer you are quoting.

    The observable universe is the current volume (with objects as they are now) of our past light cone (with objects as they were when they emitted the light we are receiving now).
    Last edited: Jul 23, 2006
  11. Jul 23, 2006 #10
    Thanks, that makes a lot more sense now.
  12. Jan 14, 2008 #11
    IMO i believe that the universe is 4D and that we perceive only 3D but we feel the affects of time(4th dimension) so since theory says that an infinite number of 3d objects can fit into a 4d object it would seem that the universe is infinite in our perception...never ending. But should we be 4D it would not be infinite because we would be able to get to the end....though we would also be perceived as infinite.
  13. Jul 28, 2009 #12
    Do you consider the universe to span a 4d space or to be a 4d object within 4d space? (I'm new to cosmology and just stumbled across this thread and it's pretty interesting to say the least.)
  14. Aug 2, 2009 #13
    If you assume the universe is becoming full on de Sitter, then q = -1, and the acceleration rate is (c^2)/R
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