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Expansion of the Universe - Closed Universe

  1. Dec 22, 2012 #1
    The Universe is expanding at an increasing rate, and from what I've read there is still much debate around its fate. Will it continue to expand forever, slow to a constant rate of expansion, or slow and fall back in on itself?

    Why can we not yet prove that the Universe will eventually fall back in on itself? Gravity is a function of mass so if at some point in the future more mass exists than space in the universe it will begin to decelerate its expansion and eventually fall back in on itself. So we just need to be able to answer the following question; Is the formation of matter occurring faster than the formation of empty space? Or, Is the universe becoming more dense? And of course, is it becoming denser at a constant rate, increasing rate, or decreasing rate. The conundrum eventually becomes; how many integrals of this thought process are required to reach a constant?

    If everything started as a huge cloud of gas following the big bang it stands to reason the universe is getting more dense, because it certainly isn't gas now (of coarse there's still the issue of how fast is empty space being created). I'm just surprised we haven't thought of a way to prove a constant exists that supports the theory that the universe is becoming more dense and will eventually collapse back in on itself.
  2. jcsd
  3. Dec 22, 2012 #2


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    I mean you no disrespect, but this is all so wrong I hardly know where to start.

    Yes, you do have it right that the universe is expanding at an increasing rate but how you can use this to conclude that the universe is becomeing more dense and should collapse is completly beyond me.

    As far as current evidence goes, the univese is becoming MUCH less and less dense and will continue that exponentially. Even if the "big rip" scenario does not take place, and many physicists say it is unlikely, the very best that will happen is that each galaxy will become isolated in its own observable universe until it eventually goes cold and even black holes evaporate.

    If the "big rip" does take place then everything down to fundamental particles will separate and eventually become their own observable universe although by that time I'm not clear that "observable universe" will be a particularly meaningful phrase since things won't be radiating photons towards each other.
  4. Dec 22, 2012 #3


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    Matter isn't being formed anymore except in extremely tiny amounts that have no consequence to the universe as a whole. Practically all matter in the universe today was formed in the early universe as Hydrogen and Helium. Once stars formed they began to convert these two light elements into heavier ones. All elements heavier than Helium are the result of nuclear fusion inside stars. (Except for trace amounts of some lighter elements like lithium that were produced in the early universe as well)

    So mass is not being created, the universe is expanding, and the gravitation between distant parts of space falls off as things recede from each other.

    Locally things get denser, as gas clouds collapse into stars and planets and such. However when you look at the observable universe as a whole you will see that density is far far less than it was in the early universe thanks to expansion.

    Also, you seem to think space is "being created". This isn't really true. All that is happening is that objects are receding from other objects. Saying space is being created brings up other issues. For example, what is space? If it can be "created" it must be something right? Well, we don't know. There isn't an easy answer for this.
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