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Infinite universe

  1. Dec 24, 2012 #1
    Hi, Merry Christmas everyone!

    If universe is infinite doesn't that violate the second thermodynamics? Because that means there would infinite amounts of matter and/or energy in the universe?
    Even if all the stars would come to end up like neutron stars white dwarfs or black holes which would happen after an infinitely long time.
    Looking at such a universe wouldn't it be like the ultimate perpetual motion device?

    Oh and another question.We have found out for quite a while now that the universe is expanding but if we go on and assume that the universe is finite then into what is it expanding?
    How come something expand into nothing, obviously for something to expand there either has to be something already there for that something to expand into or the universe while expanding is creating the space into which it expands but that would set a limit and when that limit would be reached it could expand no more if we take on to think that we have just some finite amount of matter in the universe then I guess that matter can't expand forever can it?
  2. jcsd
  3. Dec 24, 2012 #2


    Staff: Mentor

    I don't think you can say the universe is infinite if it comes from a big bang and that that created the spacetime we all know and love.
  4. Dec 24, 2012 #3
    I'm not saying it is or it is not as I don't know I can just have my opinion to what I believe more based on what my logic thinking tells me and that would be that the universe is not infinite but still that doesn't make my question less interesting I hope?

  5. Dec 24, 2012 #4


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    What does an infinite amount of matter and energy have to do with thermodynamics?

    It is not expanding into anything. Or more accurately it isn't required to be expanding into anything. First, the term "expanding" means that everything WITHIN the universe is getting further away from everything else unless bound together. The current model of the universe is based on General Relativity, and according to GR math the universe can have expansion without needing to expand into anything at all. Think of it this way, if the universe IS infinite, what would it be expanding into then? Still nothing either way.

    The problem here is that the math of GR has no problem with dealing with an expanding universe. Mathematically it makes sense. However translating that mathematical language into a more intuitive one that is easy for people to accept is VERY difficult. This is similar to the problem in Quantum Mechanics in that many of the results simply don't seem to make sense to us.
  6. Dec 25, 2012 #5
    "What does an infinite amount of matter and energy have to do with thermodynamics?"

    Ups I'm sorry when I said this I was thinking of a finite universe which couldn't have infinite amounts of energy /matter in it.

    Ok so according to GR the universe isn't expanding into anything outside it's own "matrix" But then it means that it can't expand forever as when expanding there will be a point when the clusters /galaxies what not, will be so far away from each other that there will be no force or bond to push them even further?
    Ok I guess I should ask like this does according to GR the expansion can go on forever and into infinity just like the center of a black hole is a singularity to GR math a point with infinitely/absurdly small?
  7. Jan 8, 2013 #6
    I belive what makes the universe infinite is because... the explosion that made this universe, created the very time we exist in so theoreticly the explosion contenues to happen indefinantly. Therefore the explosion is allways exploding somewhere in time. So as long as there is energy from the explosion affecting our realm we have an ever growing universe...
  8. Jan 8, 2013 #7
    Well every explosion has a finite amount of matter that makes that explosion.If we say the universe came from the big bang then I believe it had some large but still finite amount of matter in it and if that is the case then how can this matter later on expanding become infinite starting from a finite amount?
    I guess some would go on and say that there was a singularity at the start but even in black holes we know that no matter how insanely dense the singularity is it still has some finite amount of matter in it. (A super tightly packed suitcase is still just a finite not infinite suitcase)

    The scientists these days have came to a conclusion that the universe would logically end in a heat death ,if the matter/energy would be infinite as @starhawk suggests then i guess there could be no heat death?
    As long as I know a finite amount of energy/matter cant have forever on going explosions as that would require infinite amounts of energy/mass?

    Infinite itself sounds like a tragedy for the human mind and I believe for math equations especially.
  9. Jan 8, 2013 #8


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    The Big Bang was not an explosion in space. Any thoughts you are basing off of that view WILL be wrong and will do nothing but confuse you.
  10. Jan 8, 2013 #9
    Well I think the confusion arises when we use the word explosion.I have to admit I use the word because that's the easiest way of saying it (maybe not the most educational)
    and the other reason would be that it is hard to think of another name to a expansion faster than that of the shockwave of a thermonuclear weapon.

    The thing I'm confused about and you didn't mention anything in your last post either is ,did the space after the very first moments of singularity created itself from nothing inch by inch while expanding rapidly or did it just expand from a very compressed unknown to us state into something less compressed and colder with more thermodynamic equilibrium in it's regions?
    But if it's the later case or something similar to it then I have a question how can something expand into nothing when there wasn't even a vacuum ? The only logical outcome i can think of is that matter while expanding had to create the space/place into which it could expand then,? just like a man has to build a house for him to be able to live in one.
  11. Jan 8, 2013 #10


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    The universe doesn't expand into empty space. The stuff in the universe just spreads farther apart. There is already (nearly) empty space inside the universe. The empty space between galaxy clusters gets bigger, or in other words, the galaxies move apart from each other. Thus, the universe expands.

    Thermodynamics doesn't say anything about the universe being finite or infinite.
  12. Jan 8, 2013 #11


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    You may already know this, but I feel the need to post it in case you or someone else doesn't.

    Expansion is not like a shockwave. A shockwave from a blast is a entirely different. From wiki:

    A shock wave (also called shock front or simply "shock") is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium (solid, liquid, gas or plasma) or in some cases in the absence of a material medium, through a field such as the electromagnetic field. Shock waves are characterized by an abrupt, nearly discontinuous change in the characteristics of the medium.[1] Across a shock there is always an extremely rapid rise in pressure, temperature and density of the flow.

    There is no abrupt change anywhere in the universe whose cause would be expansion, no wave passing through anything. Expansion is, quite simply, the increase in distance between objects in space. Expansion causes objects to move apart from one another at an increasing velocity as the distance between the objects increases. One cannot assign a speed to the expansion because it is not movement itself, it merely causes movement. Instead it has a RATE. The rate of expansion is approximately 74 km/s per megaparsec in distance. If the distance between two objects is 2 megaparsecs, then they will be moving away from each other at 148 km/s or so. At 4 megaparsecs it's 296 km/s. This is a smooth increase in recession velocity, not an abrupt one.

    Unknown. We don't even know if there was a real singularity. IF the universe is infinite in size, then it has always been so, even at the first moment we can call "after creation" or whatever. BUT, even if the universe is NOT infinite, we still don't need to say the universe is expanding into anything. The math that General Relativity uses says nothing of the sort. It simply says things will move away from each other due to the metric expansion of space.

    That's because you are thinking it all came from a singular point and expanded outwards from there into "something" or "nothing". This is absolutely not true. A singularity simply means that our math broke down and started giving us infinities as answers, which means the math no longer works and the answers are meaningless. So we can't even talk about the singularity because we practically know the math is meaningless at that scale. And as I explained above, the expansion is not a shockwave, it is not expanding into anything.
  13. Jan 8, 2013 #12
    Well as for the shockwave I was not thinking that it is the same to the universe expansion as in the case of a blast I realize those are different situations I just used the word to describe the process that I wanted to describe.let's call it a bad analogy :)

    Singularity or not but one thing's for sure if matter started to move farther apart then that leads to conclusion that once it was closer , much closer, very close but how close and what goes on when being that close we don't know for sure I guess.
    Last edited: Jan 8, 2013
  14. Jan 8, 2013 #13


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    Two things.

    1. Inflation is different than "normal expansion" that is called the Metric Expansion of Space. Once inflation was over with, normal expansion began. This rate of expansion was MUCH less than during inflation.

    2. Normal expansion was actually a larger rate in the past and was slowing down until recently when it started to increase again. The expansion rate is not only increasing, the rate of increase is also increasing. This is the "accelerating expansion" referred to.
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