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Thread: Doubts in big bang theory.

  1. #1 Doubts in big bang theory. 
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    We know that at the time of big bang the density of our universe was infinite, this clearly shows that the volume of our universe must have been compressed to zero ,how can the universe be compressed to zero volume , zero volume objects never exist, but how is it possible in the case of our universe.


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    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by krishnabharadwajr View Post
    We know that at the time of big bang the density of our universe was infinite, this clearly shows that the volume of our universe must have been compressed to zero
    The big bang theory does not say that the universe was infinitely dense or zero size. It just says that it was hotter and denser than it is now. You cannot go back to time zero; our theories no longer apply that early. We know this because you get infinity as a result.


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    NO i have read that the universe was infinitely dense in the book "stephen hawking brief history of time" and thats not only the place you can see it even in wikipedia and other places . I am sure that its infinity.
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    Quote Originally Posted by krishnabharadwajr View Post
    NO i have read that the universe was infinitely dense in the book "stephen hawking brief history of time" and thats not only the place you can see it even in wikipedia and other places . I am sure that its infinity.
    We do not yet have a theory which covers what happened at, and immediately after, the Big Bang. We get infinities if we extrapolate low-energy models to the Big Bang, but that does not mean that the universe was infinitely dense at the BB, because those models don't apply to that energy domain.

    You can't taken everything literally that you read somehwere, you need to consider the context of what is being said as well.
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    Brassica oleracea Strange's Avatar
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    You just need to make sure that you do not mistake popular science books and simplifications in Wikipedia for science.

    Actually, Wikipedia is usually quite good on physics, so I would be surprised if it said the universe started out infinitely dense. OK, here is what it says:
    Extrapolation of the expansion of the Universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past.[18] This singularity signals the breakdown of general relativity.
    Big Bang - Wikipedia, the free encyclopedia

    Read that carefully: it says that extrapolating that far back indicates that the theory breaks down; i.e. it is not a valid result. That is what "singularity" means.
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    If it is not true, that's always what i expect to be like. But i had doubts since i have read them on popular books and sources. But i do suggest u guys to go through it once, i am 100 percent sure that i have read the same thing which i have reproduced as a doubt.
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    But i do suggest u guys to go through it once, i am 100 percent sure that i have read the same thing which i have reproduced as a doubt.
    I have no doubt that is what you've read, because popularizations of scientific theories are always simplified, and often overlook details.

    If you use the equations of General Relativity to extrapolate back to the beginning of the Big Bang, you get the answer of infinite density and zero volume. That does not mean there was actually a singularity, it means that the equations of GR do not work when you are dealing with an object that small.

    Alan Guth's Inflationary theory says that, rather than the infinitely dense singularity, the entire universe could have initially consisted of a mass of about a couple of ounces in a volume about the size of a proton. While this is an extremely high density, it is not infinite.
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    Then for how much time do you think that this ball of high density existed, as it suggests high density say's that it will be highly unstable.
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    Quote Originally Posted by krishnabharadwajr View Post
    Then for how much time do you think that this ball of high density existed, as it suggests high density say's that it will be highly unstable.
    That's a very debatable subject. According to BB theory, space/time began with the initial expansion. Before that, there was no universe for there to be time in. But as I said, it's a hotly debated subject. (i.e. nobody knows).
    Its the way nature is!
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    Then if space and time did begin with the initial expansion it clearly shows that time must be proportional (directly or inversly) to some quantity at that instant .
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    Quote Originally Posted by krishnabharadwajr View Post
    Then if space and time did begin with the initial expansion it clearly shows that time must be proportional (directly or inversly) to some quantity at that instant .
    No it doesn't. That's a completely unsupported conclusion.
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    Then how can one say, how time actually began.
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    Someone else asked about time in another thread. You might also look into that thread and the resources provided by another member. His answer was more cautious...
    But it's a start.
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    k i will go through it . But why cannot time be proportional to something at the time of big bang as it is the start of everything .
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    Quote Originally Posted by krishnabharadwajr View Post
    NO i have read that the universe was infinitely dense in the book "stephen hawking brief history of time" and thats not only the place you can see it even in wikipedia and other places . I am sure that its infinity.
    If Hawking said that then he's wrong. He was using classical lmechanics and attempting to apply it in the quantum regiem. That's not the correct way to do things. Other texts on GR will explain the problems with Hawkings assertions such as Ohanian and Ruffini's new text.
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    Even if the ball of mass wasn't infinitely dense, if it was smaller than its own Schwartzchild radius it should have remained clumped together as a big black hole.

    Space would need to have been expanding very, very fast in order to unclump it. Not just to break up the initial clump, but to break it all up into small enough pieces that the pieces themselves weren't all black holes in their own right.

    Otherwise it wouldn't matter how big the universe expanded. All the matter would have stayed in one spot.

    Or is there some theory that explains that?
    Some clocks are only right twice a day, but they are still right when they are right.
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    Quote Originally Posted by kojax View Post
    Even if the ball of mass wasn't infinitely dense, if it was smaller than its own Schwartzchild radius it should have remained clumped together as a big black hole.

    Space would need to have been expanding very, very fast in order to unclump it. Not just to break up the initial clump, but to break it all up into small enough pieces that the pieces themselves weren't all black holes in their own right.


    Or is there some theory that explains that?
    One theory is that before 10-43 seconds gravity did not exist as a separate force, but was unified with the strong and electroweak into one force.

    But until there's a working theory of quantum gravity, it's all speculation.
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    Quote Originally Posted by kojax View Post
    Even if the ball of mass wasn't infinitely dense, if it was smaller than its own Schwartzchild radius it should have remained clumped together as a big black hole.
    That would be true if it was a ball of "stuff" in space.

    A black hole requires a difference in gravitational potential. If the entire universe is the the same (high) density and there is nothing to be "outside" then it isn't a black hole.

    As the expansion of space is described by the same (consistent) theory that describes gravity, I don't see how gravity and expansion can be in conflict.
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    Quote Originally Posted by kojax View Post
    Space would need to have been expanding very, very fast in order to unclump it.
    Is there any model that allows for that?

    Lambda-CDM model - Wikipedia, the free encyclopedia
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  21. #20  
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    I am afraid that I didn't see anything in that article which would explain it. However I was forgetting the curved nature of space. If space is very much like a globe in four dimensions, so if you start walking in one direction and keep going long enough, you ultimately find yourself back where you started, then the matter that originally was all clumped together would have been equally attracted in all directions.

    There's no center of mass, and therefore no net gravitational force.

    Imagine if we took a whole lot of magnetic buckeye balls and spread them out evenly over the surface of a globe. They'd be attracted toward each other, but each buckeye ball is being pulled to an equal degree North as South, and an equal degree West as East, with the net result being that the buckeye ball stays glued to the surface of the globe, but otherwise isn't pulled across that surface in any particular direction.
    Some clocks are only right twice a day, but they are still right when they are right.
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