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I Where galaxies are now or is there a universal 'now'?

  1. Aug 17, 2016 #1
    No matter what direction I point my super duper telescope, I will observe galaxies that are barely 500 million years old in every direction some of them being located 12+ billion light years away. So 2 galaxies on opposite side of the Earth should be twice 12+ billion light years apart. In other words, close to 25 billion light years from each other. However 12+ billion years ago the universe was much much smaller and much denser with the galaxies much closer together. Definitely not 25 billion years apart. That would be like saying the early universe was billions of times the size of our present-day universe. So I'm trying to visualize this scenario. I have a few ideas but lack the understanding to follow them through. I know it must have something to do with the universal 'now' and our 'now' and the famous Phinds balloon analogy. What would be really cool is if someone could draw a picture of how we see it and how it actually is since I know now that those are two completely different viewpoints. Hope I didn't lose anyone because it is hard to be clear when you're as confused as I am. I'll be keeping a regular eye on the comments to further explain any misconceptions. Thanks a million - this has disturbed me for some time.
     
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  3. Aug 17, 2016 #2
    Some of your post is hard to follow. However, there is no universal "now". This is covered by Special Relativity, the relativity of simultaneity. Different observers will disagree on whether two event are simultaneous. Over large distances time and space become interwined, to simplify, with spacetime intervals. We can't see something as it is "now". Even something a foot away is about one nanosecond away in time. So what does now mean?
     
  4. Aug 17, 2016 #3

    Fervent Freyja

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    It's alright, even the experts seem to be confused... Wonder if any of them could answer this?
     
  5. Aug 17, 2016 #4
    I'm by no mean an expert, but what I have come to understand is that GR implies there is no such thing as 'universal now'.
     
  6. Aug 17, 2016 #5

    Fervent Freyja

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    Of course not. Isn't it more a question of how much the distances between galaxies have changed, don't the rates vary with different galaxies?
     
  7. Aug 17, 2016 #6

    phinds

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    That's a common misconception. Actually, if you could completely stop all motion and measure where everything is in our "now" you would find that the current radius is about 47billion light years. You are conflating the age of the universe with the size of the observable universe but doing it that way overlooks that it's all been expanding all that time.
     
  8. Aug 18, 2016 #7

    Orodruin

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    This is a situation where SR does not apply and you really need GR. However, the same thing holds true in GR and is even more involved. There is no universal way of assigning a "now". What is usually implied ehen you see such statements as Phinds' above is defining "now" as using the same time since the Big Bang for s comoving observer (essentially an observer at rest relative to the CMB), but this is not the only possible definition. In fact, this definition deviates from what would be the "natural" definition of "now" if you look at the SR definition.
     
  9. Aug 18, 2016 #8
    So let's buy a super double duper telescope (SDDT) which can peer right back to the singularity. I know this is a contentious issue but for my thought experiment I think I'm allowed to pretend it's a one-dimensional object. I find it's 13.8 BLY distant. I turn the old SDDT 180 degrees and peer into the past and find the hypothetical singularity again. Also 13.8 billion light years. So now, without taking inflation into account I can safely conclude that the one dimensional hypothetical singularity is, at very minimum, 27.6 BLY - away from itself. Or if I point my SDDT at any point in space I will again find my singularity but now I'll need some trig to figure out how far it is from itself. In other words, the singularity can be anywhere between 0 and a minimum of 27,6 LY away from itself.
    Now that this event happened 13.8 BYA, I understand that things have changed a lot since this event. So can I not focus my SDDT a little closer, say 13.7 BLY or 13.6 BYA etc. etc. to watch what happens there and then? Which brings me to my question - what happens? I would almost think this disproves a point-like singularity but I'm obviously missing some clarification. To me this shows that the singularity could NOT have been one-dimensional but instead have appeared everywhere probably at once. Now I'm not a scientist (well I'm an anthropologist/sociologist and a writer) so I've probably missed a huge point or two or even three. This is where I need your help, my gurus and oracles. If you can keep the explanations or the links visual I would certainly appreciate same. I understand this is a complicated and lengthy question so please bear with me.
    Thanks again, Erik aka ebos.
     
  10. Aug 18, 2016 #9

    Fervent Freyja

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    What does @Orodruin think about this number?
     
  11. Aug 18, 2016 #10

    Orodruin

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    No you are not. You seem to have a fatal misunderstanding in what the singularity is. It is not an object, it is a moment in time (one which is usually not included in space-time).

    The Big Bang occured everywhere at once. As has already been alluded to, you also need to get rid of the notion that space is something static and that what you see that is 13 billion years ago is 13 billion light years away. It is not the case, as space itself is getting bigger (I am here selecting a very particular perspective, but this is something we probably do not want to get into now).

    You should beware of taking visuals too far in these issues. They are just analogies of what is going on, intended to explain more or less what it is all about. They are not intended to be used as a starting point for drawing your own conclusion. This is what we use mathematics for.
     
  12. Aug 18, 2016 #11
    I understand the BB happened everywhere at once. I also understand that the singularity is a moment in time. I'm just throwing a bunch of what-ifs into the mix. However, from reading many other respected cosmologists and astro-physicists, many of them do believe that the universe started at a point in space-time. But that's their problem.
    However, as I said, when we look in all directions with this futurisric telescope we WOULD see this 'singularity' or proto-universe before inflation in every direction we looked. Wouldn't this be evidence of the BB starting everywhere instead of one point in space-time? Because then obviously it doesn't occupy only one point in space. We can see it everywhere we look!
    The visuals of Flatland and the balloon analogy have helpd me tremendously. So if I can throw another thought experiment out there; What could we see if all the galaxies were instantly magically transported to their actual 'now' location from our point of view because, obviously, they are already there? I understand there is no 'now' in space but if I could travel at lighspeed to observe the locations of all the galaxies NOW, what would it compare to what I can observe from Earth? Would all the faraway galaxies in time and space be no longer observable? Which brings me to one last question: If I look in every direction today and observe the beginning of the Universe every where I looked, then aren't the observable Universe and the whole Universe the same thing? In other words, I'm able to see every thing this side of the BB because the BB is the outer perimeter of what I can see and also the outer perimeter of the Universe. Only when we reset everything to this actual 'now' (an impossibility) then many galaxies would no longer be observable. I got to go to sleep...
     
  13. Aug 18, 2016 #12
    Whoops! And I also realize that because of inflation these distances are only a minimum and are actually a lot larger.
     
  14. Aug 18, 2016 #13

    Chronos

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    Permit me to suggest a reading selection that may shed some light on your questions; https://arxiv.org/abs/astro-ph/0310808, Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe
     
  15. Aug 18, 2016 #14

    phinds

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    Just to be sure you are clear, this is NOT a contentious issue, it is impossible. Google "surface of last scattering". I assume that you are positing this impossibility, not because you are not clear on its impossiblity but because you want to talk about things all the way back to t=0, but I just want to be sure.
     
  16. Aug 18, 2016 #15
    The relativity of simultaneity derives from SR without reference to GR. Sure, GR has further implications, but the R of S follows from the Lorentz equations. Every spacetime interval is seen as different for different observers, so now is meaningless especially for great distances/intervals.
     
  17. Aug 18, 2016 #16

    Chalnoth

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    There's really no confusion here. It's just difficult to get across some of these concepts without resorting to lots of math.

    There are two general points related to the question in the OP:
    1. There is no unique definition of "now" at different points in space. If you have two different events, happening at two different places and two different times, and those events happened far enough away in space that light could not have passed between those two events, then it is possible to write down a coordinate system where the two events happened at the same time. It's also possible to write down a different coordinate system where they don't happen at the same time. In short, "now" makes sense when things are located close to one another. As things get further and further apart, "now" makes less and less sense.

    2. It turns out that while there is no unique definition of "now", there is a very convenient one: the cosmic microwave background. It is possible to define a "now" everywhere in the universe where "now" is defined as the time in every location in space where a specific CMB temperature is measured. For example, we measure the CMB to be about 2.725K in temperature right now. We could say that "now" on a far-away galaxy is whenever an observer on that galaxy is able to measure the same temperature. This turns out to be very convenient for writing down simple equations to describe our universe, but it doesn't have any fundamental meaning: the choice of "now" is still a choice we make for our own convenience, not a fundamental property of the universe.
     
  18. Aug 18, 2016 #17

    Orodruin

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    Yes, SR is enough to realise that "now" is not universal. However, you cannot always use the usual SR way of foliating space-time in a curved space-time and our universe is curved, in particular at large distances. Also note that there is nothing forcing you to use the regular foliation based on equal time coordinate in Minkowski space. You can pick any foliation as long as the simultaneities are spacelike hypersurfaces.
     
  19. Aug 20, 2016 #18

    Grinkle

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    My very lay perspective -

    Suppose I know the speed of light and I know the distance of some far away object and I know that objects motion relative to me. Then I can model where the object is relative to me at the time I receive a signal from it. I can't know that the model is correct because I can't get signals to confirm it, but still, if I want to define my now as the time I am receiving signals from the outside world, I can at least make some intuitive sense of the feeling that stuff is happening even though I can't access information about it (the sun is still shining 'now' even though I can't confirm that for 8 more minutes, give or take). Other observers doing the same thing at different places far removed from me will generate a different model and there is no saying one model is more correct than any other model. That doesn't trouble me. I think my instinct that even though I can't prove it for 8 minutes, in some sense that matters fundamentally to me, the sun is shining 'right now' is a meaningful statement.
     
  20. Aug 20, 2016 #19

    Bandersnatch

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    I'm pretty sure the confusion in the OP was not as much about relativity of simultaneity and the definition of now, but more about visualising proper distances to sources of signals sent and received in an expanding universe (i.e. distance then vs distance now vs light travel time) - but I see he managed to get himself banned in the meantime, so eh.
     
  21. Aug 20, 2016 #20
    Sure. The smaller the distance, the more meaningful "now " becomes. If someone 10 feet away fires a gun at me, the relativity of simultaneity is not going to be of much help. :frown:
     
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