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At what speed is our universe expanding?

  1. Oct 9, 2015 #1
    I know this question is quite odd as we're still researching on this, but is the universe speeding up, aka, accelerating? Or is the speed constant? Or is it slowing down?

    Thoughts?

    (PS - I don't know what the prefixes B I A means, I have chosen a random A, please consider if it's wrong, am new here).
     
  2. jcsd
  3. Oct 9, 2015 #2

    Chalnoth

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    The rate of expansion is currently slowing down, and appears to be approaching a constant value. This is referred to as an accelerated expansion because the distances between objects in our universe are accelerating.

    This is because the expansion is defined as velocity per distance. Currently the value is close to 70km/s/Mpc. Galaxies that are 100 megaparsecs away average 7,000 km/sec recession velocity. Galaxies 200 megaparsecs away average 14,000 km/sec recession velocity, and so on. So as galaxies get further away, they speed up. While the expansion rate is dropping slowly, it isn't dropping quickly enough that galaxies fail to speed up as they move further from us.
     
  4. Oct 9, 2015 #3

    phinds

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    No, we are not still researching the fundamental fact of an expanding universe that is expanding at an accelerating rate. The objects in the universe (at very large scales, like above the size of a galactic cluster) are moving away from each other at an accelerating rate. The farther away things are from each other, the faster they are receding from each other so there is no single rate.

    That is, we ARE researching all details of cosmology, but those fundamental facts are not in question.
     
  5. Oct 10, 2015 #4
    I didn't mean that we are researching whether the universe is expanding or not, I actually meant that we are researching on whether the rate of expansion is increasing or decreasing or is constant, like you said :D.

    Thanks for the help sir :).
     
  6. Oct 10, 2015 #5
    As you said the expansion rate is dropping, does that mean the universe is preparing for a Big Crunch? I am using the layman's term, but I am sure you get it. Is the universe indicating that the end will be a rolling back of everything, like the theory of Big Crunch?
     
  7. Oct 10, 2015 #6

    marcus

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    As far as we can tell, a collapse or "big crunch" is not indicated.

    According to the simplest best-fit model (called LCDM) which cosmologists use, the expansion will continue indefinitely---simply at a lower percentage rate.

    Of course we cannot know the future because we do not completely understand nature's laws, we can only make the simplest best model that we can, and fit it to the observational data. and continue to try to make the model better.

    So far, the model shows that distances are increasing at a percentage rate of 1/144 % per million years.
    And this percentage is slowly decreasing in a way that suggests that it will eventually level out at around 1/173% percent per million years.
     
  8. Oct 10, 2015 #7
    Neither some day in the future? No indications?
     
  9. Oct 10, 2015 #8

    marcus

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    That is right. No indications! The standard cosmic model does a good job of fitting the data and it indicates that in the long run distances will be expanding at a steady rate of about 1/173 of one percent, per million years.

    Of course in the future we could get new observations from new instruments and learn new laws of nature. Then we might be able to improve the consensus model, and it might say something different. But so far we have to live with the model (based on Einstein's General Relativity, the so far best theory of how gravity and geometry interact with matter) that gives the best fit to the data we have so far.

    You may not LIKE the conclusion of continued expansion. You may WANT to have a big crunch for some emotional, poetical, philosophical reason. But so far, with our best GR-based model, fitted to the huge mass of observational data, this is not indicated.

    People speculate some about alternative models which might give a different result, they imagine, invent variants, but so far they don't get a better fit to the data and there is no general acceptance of any of the alternatives. You just have to be patient. Maybe the standard model will be changed in the future, when people know more.
     
  10. Oct 10, 2015 #9
    Fair enough! That helped :). I have been studying about how the universe might end, since many months, and I do know about the concept of the closed universe, open universe, and another one which I currently don't remember the name.
    But if am not wrong, the universe is not infinite. To make it expand forever, you'd need an infinite amount of energy which is not the case. Hence at one point or the other, the universe will come to an end, whether it's by the freezing of the universe due to continued expansion or the Big Crunch or whatever, that's what my understanding of the studies say.
    Thoughts?
     
  11. Oct 10, 2015 #10

    marcus

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    No.
    According to GR the distances can continue to expand indefinitely without a continuous input of energy.

    Why? According to what law of nature? It is a human prejudice that things must have a beginning and an end. Because we individuals are born and then die we tend to think everything must be like that :oldbiggrin:
    and because we build things and they eventually fall down...
    The Universe does not have to work like that.

    GR is 100 years old, our theory of gravity/geometry-and-matter-interaction. For 100 years people have tried to show it is wrong. They have thought up tests. GR has passed all the empirical tests so far.
    And according to GR a spatially finite universe can expand indefinitely without an input of energy.

    Of course that means it gets cool and dark---little by little it gets colder and darker as distances expand. But it does not END. It just continues expanding and getting colder and darker and emptier.

    If you don't like this, then
    1. you have to invent a theory that passes empirical tests better than GR, agrees with present-day observations better. Many people have tried without success.
    2. or you have to wait patiently and use the existing model, until somebody else develops a replacement for GR that works better and gains acceptance by the scientific community.

    You cannot simply impose your own human preconceptions on nature.
     
  12. Oct 10, 2015 #11
    All right, I get that. Intelligent discussion there!

    So when things start to cool down, would life sustain or it means an end to human race?
     
  13. Oct 10, 2015 #12

    marcus

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    now you are asking a question that comes under biology, evolution, technology, artificial intelligence---not cosmology

    The human species is only some tens of thousands of years old. Several other humanoid species (e.g. Neanderthal) have already gone extinct. Species tend not to last timespans like a BILLION years. Whether plant or animal, in time they get replaced by other species.

    Evolution depends on adaptation and reproductive fitness. Possibilities are endless. It is even possible that self-manufacturing artificial intelligent life-forms eventually proliferate, if they have a higher fitness in the prevailing conditions after all the stars have died out.

    I'm not especially interested in that kind of speculation. It is not cosmology.

    If we are not talking cosmology, then I will be quiet and let others carry on the discussion.
     
  14. Oct 10, 2015 #13
    That small! I had no idea.
     
  15. Oct 10, 2015 #14

    marcus

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    Yes! I think it's good to have a sense of the quantities---imagine 1/144 of one percent per million years. It helps me anyway.

    And imagine a distance of size 14.4 billion light years, between two galaxies.

    What is 1/144% of 14.4 billion? That is how many light years that distance is increasing, per million years.

    (it works out to one light year per year :oldbiggrin:)
     
  16. Oct 10, 2015 #15

    phinds

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    Here's how I remember it: (this is from my article on the balloon analogy linked to in my signature)

    Expansion, even with acceleration, is so staggeringly slow on small scales that it might as well not be happening. Over cosmological distances, it has a huge effect, but here's my favorite analogy to show local effect. Even though the universe is expanding, it's still going to be hard to find a parking place. This is just a simple-minded way of thinking about the local effects of the expansion. If you could go out into intergalactic space and magically draw a set of parking place lines, it would be about TWENTY BILLION YEARS before they had moved far enough apart to let you park a second car. Now, I'm willing to circle the block a couple of times to get a parking place, but twenty billion years is just too much. I'd be late for the movie.
     
  17. Oct 11, 2015 #16

    timmdeeg

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    I suspect you are confusing expansion with expansion rate. The meaning of those two terms is much different.
    Expansion means growing distances between comoving things (galaxies). Over time those distances can grow decelerated, accelerated (then we talk about accelerated expansion) or exponentially.
    Whereas expansion rate is "defined as velocity per distance", as Chalnoth has explained in #2. Consequently, in case the expansion rate is constant, the universe expands exponentially (as it did during inflation in the very early universe). Imagine two points on an expanding balloon: their distance will double from time interval to time interval.
    So, if the expansion rate is dropping doesn't mean that the universe starts to contract, as you may have been reasoning. It means that the universe expands less faster than exponentially.
     
  18. Oct 11, 2015 #17
    Thanks everyone. I am more clear now :).
     
  19. Oct 13, 2015 #18
    Timmdeeg, you mentioned an expanding balloon. Does this inverse square law graphic help to explain how the observable universe is expanding with more of it becoming unobservable day by day? A tiny sphere representing the O.U. on the left most surface becomes a much bigger sphere by the time it reaches the right most surface. but the increase is slowing down I believe? Incidentally, does this inverse square law approximate at all to the measured rate of expansion over time and if not what is the approximate relationship?

    https://en.wikipedia.org/wiki/Inverse-square_law#/media/File:Inverse_square_law.svg

    I just read that the expansion is actually accelerating so I guess inverse square law isn't right.
     
    Last edited: Oct 13, 2015
  20. Oct 14, 2015 #19

    timmdeeg

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    Yes it isn't. I tried to picture the extremely rapid (exponential) expansion of the universe during inflation.
     
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