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Expanding universe

  1. Jan 14, 2012 #1
    My question may be stupid, but as we look out into the universe aren't we seeing into the past? For instance let's say we are looking at a group of stars and they are 3 million light yrs away and moving away from us. Aren't they just moving away from us at that moment? Like the stars on the edge of the universe we really don't know if they are still there. True? Light years are a long time. They could have burned out a long time go and we have seen the result of that. Because the light hasn't gotten here yet. Could you help me with this and explain how we know for sure the universe is still expanding. Because we are always looking into the past. Thanks swimfit
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  3. Jan 14, 2012 #2


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    The only stupid questions are the ones not asked, or the ones not sprung from an inquisitive mind*.

    * Hot enough for ya? Workin' hard or hardly workin'?


    True. But what do you think is going to happen to them? We're pretty comfortable with the evolution and lifetime of stars.

    A light year is a distance, not a duration. The distance from Sol to the nearest star is 4.38 light years.

    True. There have been a few events that we have recently witnessed of starts going supernova in other galaxies. We see it now though it happened millions of years ago.

    We know only what we observe and we build predictive models based on our observations. We have no models that predict that the galaxies will come to a screeching halt for some reason.
  4. Jan 14, 2012 #3
    To clarify slightly if a star is a light year away we see the light that it emitted about a year earlier and I still find a year a pretty long time although in some areas of physics it must be considered minute.

    Also it is not clear that the universe has an edge. I think current data actually agrees with the flat space model (so the spacetime of the universe is flat and infinite). In that case there is no edge. Also current data even agrees with the expansion of the universe becoming bigger and bigger. So it doesn' t only expand it keeps expanding faster. What is meant by the universe expanding by the way is that all distances becomes bigger so not that the 'edge' moves farther away.

    Even in the closed model where space would eventually collapse on itself there is no edge just as the earth as a 2 dimensional surface (please read flatland!) doesn't have an edge it goes on and on in circles.

    Based on the theory of general relativity (which is confirmed by experiments on earth as well as in space) we interpret the data we get to get these results, but of course everyone should keep an open mind nonetheless (nudge nudge wink wink weird neutrino boys etc.)
  5. Jan 14, 2012 #4


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    The observable universe certainly does. I saw nothing in the OP's message about that needed correcting on that account.
  6. Jan 15, 2012 #5
    Thanks Dave for your explanation. I've seen shows that show the universe like a balloon. What is driving the expansion the big bang? Plus I've seen shows on dark energy could that be driving the expansion? Plus that the expansion is speeding up too. I just thought that they were seeing the universe many years in the past. And thought that it ws still speeding up. It is Truly amazing how wonderful the universe is. It would seem that if the expansion was drivin by the big bang that the expansion would start slowing down, so something must be driving the expansion. The lack of gravity outside of the universe. Thanks again for your explanation. Swimfit
  7. Jan 15, 2012 #6


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    There are TWO things going on. The expansion of the universe, which is a result of the big bang, and was for some time expected to slow down and possibly even reverse.

    Some years ago, it was discovered to physicists great surprise that it is not only NOT slowing down, it is accelerating. THIS is caused by what we call "dark energy" (which is shorthand for "we don't have a clue")

    There IS no "outside" of the universe
  8. Jan 16, 2012 #7
    This is what I meant by ther is no edge.
  9. Jan 16, 2012 #8
    If there is no outside of the universe, then what are expanding into? From what I've seen on tv about M theory that maybe multi universes out there. I know it is just a theory just like string theory. Swimfit
  10. Jan 16, 2012 #9


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    We're not expanding into anything. The unverise is everything. If there were anything to be expanding into, it would be part of the universe and we wouldn't be expanding into it. It's hard to get you head around at first, but it is the way it is.
  11. Jan 16, 2012 #10


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    If you were standing on the surface of the Earth, and you could throw a baseball 25,000 miles, you could play catch with yourself. If, while you were playing catch with yourself, the surface of the Earth doubled in extent (so that the Earth's circumference were now 50,000 miles), you would have watched the Earth's surface expand by 100% in every direction, yet the surface would not be expanding "into" anything. The surface is all there is, and it has simply doubled in extent.

    The same thing can happen to our universe, except in 3 dimensions. It can grow in extent without having to have anywhere to "grow into".
  12. Jan 16, 2012 #11
    Think about the balloon you saw. it's 2 dimensional surface is expanding, when the balloon is blown up, in the sense that 2 points on it keep getting further and further apart from each other. But there is no 1 dimensional boundary begin pushed into anything else.

    Maybe think about the expanding more as stretching.
  13. Jan 17, 2012 #12
    Ok back to the beginning. Where was the singliarity? If there is nothing beyond the universe. Did it appear out of nothing. And if it did what is this nothing? Is the universe expanding behind space time? Swimfit
  14. Jan 17, 2012 #13
    The singularity was everywhere! the universe was the singularity and then started to expand. So the big bang happened everywhere.

    These are very difficult questions since a part of the big bang theory is that we cannot derive any information about 'before' the big bang. So phsyicswise time starts at the big bang, we cannot actually (at least for as far as we know now) know anything about the physics before the big bang or what there was.

    It is the Spacetime itself that is expanding.
  15. Jan 17, 2012 #14
    Yes space time expanding and the galaxey's are going along for the ride. Like Spots on a balloon and as you blow it up the spot the spots get further apart. Right? But the the universe is taking up more space as it expands correct? And for the sake of argument we will assume that beyond the universe is nothing and the expanding universe is creating more space time. Because the universe is all that there is as far as we know. I'm sorry I'm like a dog with a bone. Is the fabric of space the same as space time or dark matter or something else. Swimfit
  16. Jan 17, 2012 #15
    You have to be carefull here. The Universe takes up all space at all times (and all times at all spaces). So it becomes unclear when you say it takes up 'more' space. The only thing we can say is that everything (like galaxies and stars) seem to be drifting further and further away from everything else. This is what is meant by expanding the distance between two things is becoming larger. When you phrase that like taking up more space it seems like there was some space left to take up, this is not true.

    Also the Universe is defined to be everything that exists. So the universe is certainly all that is or ever will be, however we will probably never be able to describe all of it (actually I think you can prove that).

    Tha fabric of space is not really an official term but more like a neat way to visualize what mass does to spacetime in general relativity.

    When we watch certain spining galaxies from earth and we compute the mass it has from all the visible matter (visible with any kind of telescope) we end up with not even nearly enough to account for the fact that it can stay together (it basically needs enough centripetal force). There are also other experiments that compute the visible mass and then via some other way compute the mass a galaxy should have and there is huge discrepancy. The explanation for this is: "There is something that creates enough gravity to account for the discrepancy". This something is what we call dark matter.

    Actually no one is completely sure what dark matter is. There are even theories that it is just very dim baryonic matter. In fact I think most people now say that the dark matter in the universe actually is partly this and partly that. Anyway it is not in anyway space, it is matter, it does however give curvature to spacetime (which is what in general relativity means gravity).
  17. Jan 17, 2012 #16


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    I'd suggest just forgetting about this "singularity". Just realize the universe was in a very hot very dense state and expanded from there. The singularity is simply where our models stop working because we don't know how everything works at those energy levels.

    No. Unlike the balloon, the universe is thought to contain ALL of spacetime. There is nothing "outside" of the universe. Realize that we aren't saying there CANNOT be anything outside the universe, but that the current model does not require anything to be outside of the universe for it to expand into.

    As has been said, the "fabric" of spacetime is simply a metaphor to help people understand it. The actual model is purely a mathematical description of how spacetime behaves.
  18. Jan 17, 2012 #17
    I agree of course with this. However probably because my native language is dutch in which universe translates to "heelal" which literally translates back to "whole-everything" I tend to just view the universe as everything, the things we can account for now and the things we don't know about yet. Of course it doesn't usually pay to act like you know what you don't yet.
  19. Jan 17, 2012 #18
    Ok I think I understand now. The universe is everywhere and the only thing really expanding is the physical matter in the universe. Then why don't they say that. Everyone keeps saying the UNIVERSE is expanding!!! I was taking you at your word. Swimfit
  20. Jan 17, 2012 #19


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    Because without having an expansion of space, it doesn't fit. Simply having a ballistic expansion rate wouldn't give us the results of our observations that we see. For example, the redshift we see from far away galaxies doesn't fit with normal doppler shift according to my knowledge.
  21. Jan 17, 2012 #20
    Space is expanding. Matter is moving apart, or trying to, from other matter because of this. Gravity counters the effect.
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