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B Doesn't there have to be more than one observable universe?

  1. May 16, 2018 #1
    I was just curious because if space can expand faster than light, doesn't that mean there will be a lot of space that we just can't see? Do objects just vanish because we can't see them?

    For instance, if a hypothetical alien lived in MACS0647-JD galaxy which is 13.3 billion light years away, wouldn't it's observable universe extend into space outside of our observable universe? Wouldn't that mean observable universes are infinite and share the same age? Is there anything that says space can't extend past where we can observe based on the speed of light?
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  3. May 16, 2018 #2


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    An observable universe is just a patch of a larger whole that any particular observer can see. There are as many observable universes as there are observers (so, infinite).
    Each observable universe is finite in extent.
    But if the observers are not far from one another, like two people on Earth, then the difference in what they can see is imperceptibly small. So when we talk about the observable universe, we usually mean the extent of the larger universe that people on Earth can see.

    That number is the time it took its light to reach us times the speed of light. It's not really distance in any common sense of the word. If you could stop the expansion today and take a ruler to measure where it is, you'd measure a significantly higher number. Distances in cosmology are a bit wonky, is what I'm saying.

    If you've seen it once, then you'll keep seeing it.
  4. May 16, 2018 #3


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    Yes. No.

    I'll tackle the second one first. Whenever an object crosses a horizon, an observer who doesn't cross that horizon never actually sees the crossing happen: instead, they see an image of the object get redshifted more and more as time goes on. This is true whether you are talking about the event horizon of a black hole or the cosmological horizon. In essence, what happens is that the finite number of photons which were emitted by the object before it crossed the horizon get spread out in time infinitely into the future. Eventually those photons will redshift so much that they can't be detected, but there is no sudden disappearance.

    The first question can be complicated when you start considering theories beyond the standard model, but in the simplest models the answer is simply yes: there's plenty of stuff beyond the observable universe.

    This argument doesn't imply an infinite universe. It does, however, imply that there is no boundary to the universe. This could be the case for a finite universe that wraps back on itself (e.g. spherical or toroidal shape). Or it could be infinite.
  5. May 16, 2018 #4
    Thanks and does this mean if an object redshifts out of view it's just in a universe we can't observe? Is there any evidence as to where this ends if it ends? Wouldn't this mean there will always be objects in our universe that exists in an observable universe that extends well beyond our own observable universe and wouldn't this go on ad infinitum unless there was some way to stop expansion?
  6. May 16, 2018 #5


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    "In a universe we can't observe" isn't a well-defined statement. The problem is that the bare term "universe" means "all that exists". Nothing that exists can be in another universe, because the universe is everything! A more precise statement is that they are outside of our observable universe, and our observable universe is only a fraction of the overall universe.

    Also, for a sense of scale, it will take a couple trillion years for galaxies to redshift so much their light becomes undetectable.
  7. May 16, 2018 #6
    That's what I meant. There's an overall universe or overall space with pockets of observable universes within that overall space. When you look at the 46.6 billion light year radius of the observable universe, Would an alien in MACS0647-JD galaxy see it's observable universe as 46.6 billion light years in each direction and would some of the space of it's pocket observable universe extend into a space outside of our pocket observable universe?

    We know that two galaxies about 4,200 megaparsecs apart will be moving away from each other faster than the speed of light. This is about 13.7 billion light years. So there's more than enough room for objects to be more than 4,200 megaparsecs away from each other in there own observable sphere.

    This is speculation but the age of the universe might point to the time we separated from another observable universe that was just like or similar to ours. I just read a really good paper:

    From Planck Data to Planck Era: Observational Tests of Holographic Cosmology
    Niayesh Afshordi, Claudio Corianò, Luigi Delle Rose, Elizabeth Gould, and Kostas Skenderis
    Phys. Rev. Lett. 118, 041301 – Published 27 January 2017


    An article about the paper said this:


    Wouldn't you expect data from the observable universe we're born from in this noise from the CMB because if this information is on the event horizon of a black hole from that universe, wouldn't it show up in the noise?
    Last edited: May 17, 2018
  8. May 17, 2018 #7


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    Yes, there's nothing controversial about that. Mostly because the other observable universes are just more of the same. Again, there's as many observable universes as there are observers.
    This picture shows the situation you're describing:
    observable universes 1.png
    The patches depicted above grow with time, while A and B recede from each other.

    You're putting too much weight on the Hubble distance - the distance where recession velocities reach the speed of light.
    1. This number is only coincidentally close to the age of the universe times the speed of light
    2. It doesn't mean that galaxies beyond it are unobservable

    Causal patches of the universe can and do separate, but it's not equivalent to galaxies disappearing from sight, nor does it happen at one particular time (it's an ongoing process).

    This post discusses how and in what sense do causal patches of observable universes separate, using light-cone graphs as a visual aid:
  9. May 17, 2018 #8
    That's not true, is it? Due to the accelerated expansion of the universe, distant objects are moving outside our observable horizon all the time. Unless I am mixing up different cosmological horizons, those can get confusing. I don't think so though. In the far distant future our local group of galaxies, or at least the largest gravitationally-bound structure that we are part of, will be all that can be seen from the Milky Way. The rest of the galaxies will have accelerated away from us so much that they are beyond our observable horizon. They will have red-shifted into nothingness from our perspective.

    Edit: Ok looking at the link you posted, it is the particle horizon you are talking about yes? This is a pretty loose definition of "will keep seeing it" I think. Light is quantized, so the number of photons arriving per second from these highly redshifted objects gets smaller and smaller. Eventually there will in fact be a time when no more photons arrive, and any "stragglers" will be so red-shifted that we could never detect them anyway.
    Last edited: May 17, 2018
  10. May 17, 2018 #9


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    The point is, even though the signals we'll be receiving will become more and more redshifted, there'll never be a time when we'll stop getting them. Crossing the cosmic event horizon now does not make the earlier light sent from the object disappear. The light from the horizon crossing event will arrive at the observer only after infinite time.
    It's the same effect as with an event horizon of a black hole, that kimbyd mentioned in her post #3. There's some more about it in the post about causality and light-cones I linked above.

    So the objects in question are never unobservable in principle, only in practice - due to redshift and faintness of the signal.
  11. May 17, 2018 #10
    Thanks for the response and thanks for the visual. That's exactly what I'm saying with Barbara and Adam.

    I do disagree on your second part though and I agree with kurros.

    Okay, now I see you were talking about the particle horizon. I agree with that.
  12. May 17, 2018 #11
    Well, again only from a classical perspective. With quantized photons there will eventually be a time when the probability of one photon from an object arriving in the next trillion years will be vanishingly small. A finite number of emitted photons is stretched over infinite future time.
    Last edited: May 17, 2018
  13. May 17, 2018 #12


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    Again, that's the 'in practice' part. Even classically, given enough time any light wave will become so redshifted, that you'd need an impractically large detector to see it. But it doesn't mean the signal isn't there.

    I'm talking about the meaning of the event horizon. If the worldline of an object ever was within the event horizon of an observer, the event of the object's crossing of the horizon will be observed only after infinite time.
    Particle horizon is more in line with what the OP is talking about when he asks about observable universes.

    Ok. What part exactly do you disagree with, and why?
  14. May 17, 2018 #13
    It does in the quantum case though. In the example I gave, there is probably no signal at all, not even one photon. That is literally zero signal, unless you happen to get extremely lucky. So "probably zero signal" is philosophically quite different to "an extremely weak signal" I would say.

    Though I am not sure if there is some super bizarre quantum argument to save things here, something about how the wavefunction is still here even though it has an extremely low amplitude...
  15. May 17, 2018 #14


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    I don't disagree with that. The point I'm making is that with signals sent from within the event horizon you can in principle get 'extremely lucky' and observe that one photon. This is not true for signals from without the event horizon.
    It's good to focus on the classical aspects here, since the expansion of the universe is based on a classical theory of gravity, and all the standard definitions and nomenclature used - such as the meaning of various horizons and the observable universe - does not include quantum effects.
  16. May 17, 2018 #15

    We don't know. It's possible that the bubble we are in is finite. In some inflationary theories, the bubble of "normal vacuum" is finite and its border is expanding into "false vacuum" all the time. If this is the case, we are far away from that border, likely many times farther than the size of the observable Universe (otherwise, if we would be close to the border, CMB would have a measurable dipole component).
  17. May 17, 2018 #16

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    What @Bandersnatch is true (modulo redshift).

    This is a myth. Once on our past lightcone, always on our past lightcone, even in universes that have accelerating expansion. As time passes, some objects that are not our past lightcone move onto our past lightcone, but, once on, no object moves off our past lighcone, i.e., the amount of stuff in the observable universe increases with time.
  18. May 17, 2018 #17
    Here's an interesting article I just read that touches on this.


    I think it stands to reason that there's this overall space filled with observable pocket universes that share the same physics. Hawking's latest paper tries to reduce the physics of these pockets and I think that's on the right track. I think if you want universes with all of these different physical laws you need to look for evidence that supports the string theory landscape and 10^500 false vacua. It goes onto say:

    That's crazy that we can only reach 3% of the galaxies in our observable universe today. That doesn't bode well for the fate of everything in our universe but then again maybe it does. Maybe the expansion of space eventually leads to the birth of new universes.

    This is why I do think it's more than a coincidence that the radius of the Hubble's sphere is close to the age of the universe in light of the recent published paper I linked to earlier called From Planck Data to Planck Era. Maybe the age of our universe is when we were born from another universe that was like ours. Maybe a black hole formed in that universe and out of the other end we were born and this is why there's this data in the noise of the CMB. Maybe that information came from a parent universe.

    That would mean you could get infinities within infinities of the same or similar things occurring if at the end of every universe or if every black hole can give birth to other universes.
  19. May 17, 2018 #18


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    Hawking's last paper discusses a completely different situation of pocket universes nucleated from eternal inflation. The 'there's more observable universe beyond what we can observe' insight that you've been focusing on in this thread is unrelated. The entire universe composed of all such observable patches forms just a single pocket in the eternal inflation landscape.

    It clearly is, if you look at the maths of where it comes from.
  20. May 17, 2018 #19


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    The string theory landscape isn't necessary. The actual features that are necessary are two-fold:
    1) There have to be multiple metastable low-energy states which lead to different low-energy laws of physics. The string theory landscape satisfies this, but it isn't the only possibility. These states need to be metastable with long lifetimes as otherwise they'll all decay into whatever the lowest-energy state happens to be, leading to the same physics everywhere.
    2) A way for the system to explore a large number of allowable states, such as spontaneous symmetry breaking. It may not be sufficient to merely have a theory which permits different low-energy laws, not unless there's mechanism within the theory to explore the parameter space.

    Fortunately, point (2) almost always holds for any theory which satisfies (1).
  21. May 17, 2018 #20
    I understand Hawking is talking about Eternal Inflation and Holographic Cosmology, that's why I said he's on the right track when it comes to the physics of these pockets in any theory. I think it easier to separate a multiverse of universes that share the same or similar physics vs. ones that have different physics in each pocket. Hawking's said:


    I think this is on the right track whether it's Hawking's talking about reducing eternal inflation to a timeless state or what I'm talking about here.
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