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Big Bang Singularity

  1. Mar 17, 2015 #1
    Question TO: Physics Forums – March 17, 2015

    Various sources I have read talk about the big bang origin as a point singularity. Here is one example:

    [Quoted from The Beginning of Time, a lecture by Stephen Hawking (1996).

    At this time, the Big Bang, all the matter in the universe, would have been on top of itself. The density would have been infinite. It would have been what is called, a singularity.
    . . .
    Therefore, to understand the very high-density stage, when the universe was very small, one needs a quantum theory of gravity, which will combine General Relativity with the Uncertainty Principle.]

    Since the currently most commonly accepted cosmological model is spacially flat and isotropic, it must also be infinite and have infinite mass-energy. I cannot understand how the point singularity at time zero with zero volume and infinite mass-energy can in an infinitesimally short time become an infinite space, with its infinite mass-energy uniformly filling all of this infinite space.

    Can anyone explain this to me?

    It seems to me to be geometrically more logical space is, and always has been infinite since the big bang. Then the singularity also occupies at time zero all of infinite space, and at every point of this infinite space there is a finite mass-energy with an infinite mass-energy density. What do you think of this alternative?
     
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  3. Mar 17, 2015 #2

    marcus

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    I don't think your quote from Hawking says there actually WAS a "singularity". You only quote an excerpt, not the full passage. It sounds to me as if he is saying that the conventional picture of singularity is not something that happens in nature and so we need a better theory.

    Specifically a quantum theory that would include the GOOD results of General Rel, but would also satisfy the Heisenberg Uncertainty principle of quantum mechanics.

    That would be a QUANTUM version of GR, and it would not have a singularity.

    Basically "singularity" means a failure site of a man-made theory, that shows where the theory breaks down and is inapplicable. there have been singularities in other theories and physicists have fixed them by improving or replacing the theory that developed such glitches.
     
    Last edited: Mar 17, 2015
  4. Mar 17, 2015 #3

    marcus

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    Buzz, the finite vs infinite question is a separate issue.

    A singularity can have infinite extent---a manmade theory can fail over a broad front, it doesn't necessarily have to fail only at a single point.

    "singularity" does not mean "single point". One of the meanings used to be "oddity" "bizarre unusual behavior" irregularity....peculiarity.
    So crazy weird as to be a once in a lifetime occurrence...
    Mathematicians took the word over to describe where mathematical functions blew up and stopped giving meaningful numbers. It wouldn't have to be at a single isolated point. Physicists took the word over from them. In 1900 Max Planck cured a singularity in the black body radiation curve.
    Around the same time Bohr saw how to cure a singularity in the theory of the hydrogen atom, and atoms in general.
    Curing singularities is part of physics history going way back.

    If Hawking gave you the impression that the Big Bang failure was only at a single point then he was being sloppy in his writing. He was writing for laypeople and couldn't be bothered to explain. We don't know for a fact that the U is spatial infinite but if it is, and we model it with GR in the usual way, then as you go back in time the theory fails across a broad front---you get meaningless results and infinities as you approach the start of expansion everywhere over a broad infinite extrent. With the classical (non quantum) theory the start of expansion completely fails to even be defined.

    So you get a lot of people working on QUANTUM GR and quantum cosmology these days. On various ways to get rid of the Big Glitch at the start of expansion. Quite a few of the proposed solutions involve a rebound from a prior contracting phase. But there's actually a whole menu of different proposals, that differ in the details they predict and will eventually have to get sorted out and tested.
     
    Last edited: Mar 17, 2015
  5. Mar 18, 2015 #4

    Chronos

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    The short version is, it is a mathematical artifact. That is how we know the underlying theory is incomplete,
     
  6. Mar 18, 2015 #5
    Thank you Marcus and Chronos. As I interpret your answers, you agree with my alternative regarding the GR characteristics of the singularity not being a point. I undertstand about the efforts of theoretical physists to invent a theory of quantum gravity (QG). The reason I posted my questionto the GR forum rather than the Cosmological forumis I wanted to avoid the issues of QG in seeking an answer.
     
  7. Mar 18, 2015 #6
    Hawking disproved this idea and admitted his mistake later on in his life, if the Universe is infinite then it must have stayed infinite, it expands into itself, a singularity has zero volume which violates this idea.
     
  8. Mar 18, 2015 #7

    phinds

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    No, a singularity does not have "zero volume". A "singularity" is just a place where the math model breaks down. It can be infinite or zero or whatever result is non-physical, thus leading to the use of the term singularity (which does NOT mean "point"). Did you not see Marcus's post explaining this?
     
  9. Mar 18, 2015 #8
    So the Universe did begin in a singularity, I do not think that that is the universe did arise from a singularity because "the singularity arises from using Einstein’s equations for gravity and we know Einstein’s equations are not sufficient", "they aren’t able to describe certain extreme gravitational phenomena".
     
  10. Mar 18, 2015 #9

    phinds

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    Well, the universe "began in a singularity" only in the sense that the universe began in a place/way that our math models don't describe, but his does not imply that it began at a point. You seem to persist in using the term "singularity" to have some meaning other than "the place where out math models break down and give unphysical results".
     
  11. Mar 18, 2015 #10
    The Big Bang model is not really a model of the "Bang" (though Bang is not a good way to describe it) itself, it is a model of what happens afterwards. Anything before it would be ignored because it would have no observational consequence. Our theories/mathematics, I agree, reach a point where they can only explain what happens afterwards. And if I am correct you clearly responded to my mistake (I admit) by responding: "No, it did NOT (the universe beginning in a singularity) and that in fact is the point of this thread."
     
  12. Mar 18, 2015 #11

    martinbn

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    There are no singularities. When people say that there is a singularity, that's just a sloppy expression for "the space-time is singular". And a singular space time is a space-time with incomplete geodesics (or a variation for other types of world-lines). I think that most people think that there are no singularities in the real world. But what is absolutely sure is that there are no singularities in the theory, in the mathematical model. There are no places where something is infinite, nor places where the math models break down. In the flat FRWL model the space-time as a manifold is [itex]\mathbb R^3\times\mathbb R_{>0}=\mathbb R^3\times (0,\infty)[/itex] plus a certain matric. Notice that [itex]t=0[/itex] is not part of the manifold and thus there are no singularities in the space time.
     
  13. Mar 18, 2015 #12

    phinds

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    I agreed w/ all that.
     
  14. Mar 18, 2015 #13
    I appreciate all the contributions to this thread. Perhaps my motivation for starting this thread was confusing. First, I agree that the arguments are convincing against the GR singularity actually correctly modeling the time zero state of the universe, or even correctly modeling the state during the Planck time period (t < 5.4 x 10^-44 secs) . That is not the point of this thread. My question is about the cosmological solution to the GR equations at time zero.
    As I understand the contributors who responded to that question, they agree that an infinite universe at time greater than zero implies that the universe at time zero must also be infinite, not a point singularity.

    I have found many supposedly good sources saying that the GR cosmology involves a point singularity at time zero. Here are a few examples:
    [
    Gravitational singularity (Wikipedia)
    Extrapolating backward to this hypothetical time 0 results in a universe with all spatial dimensions of size zero, infinite density, infinite temperature, and infinite space-time curvature.

    Initial singularity (Wikipedia)
    General relativity is used to predict that at the beginning of the Universe, a body containing all mass, energy, and spacetime in the Universe would be compressed to an infinitely dense point.

    The Big Bang: What Really Happened at Our Universe's Birth?
    by Mike Wall, Space.com Senior Writer (October 21, 2011)
    http://www.space.com/13347-big-bang-origins-universe-birth.html
    Traditional Big Bang theory posits that our universe began with a singularity — a point of infinite density and temperature whose nature is difficult for our minds to grasp.

    http://www.redorbit.com/education/reference_library/space_1/universe/2574628/big_bang/
    Extrapolating the history of the universe backwards using current physical models leads to a gravitational singularity, at which all distances become zero and temperatures and pressures become infinite.

    Singularity - University of Virginia
    http://www.astro.virginia.edu/~jh8h/glossary/singularity.htm
    In classical general relativity, a location at which physical quantities such as density become infinite. Another definition is a point in spacetime where timelike worldlines end (or begin). Singularities can be initial singularities (such as the big bang itself) or ending singularities, such as at the center of a black hole, or the big crunch.
    ]
    I have been unable to find any internet source that explains the time zero state of the GR cosmology correctly. Those sources that do not make the point singularity mistake leave the topic ambiguous by simply referring to a singularity. I would hope that knowledgeable physicists might make an effort to correct these mistakes by communicating with internet sources to which interested lay readers look for correct science.
     
  15. Mar 18, 2015 #14
    Thank you for your response. Can you post a citation to a source explaining Hawking's correction to his earlier mistake?
     
  16. Mar 18, 2015 #15
    That's because we know that GR leads to a physically impossible result if we try to extrapolate to time zero.
    Since that can't be correct, we know that something else must be going on, and it probably involves quantum physics.
    There are competing theories out there, any of which or none of which might be correct
     
  17. Mar 18, 2015 #16

    Drakkith

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    Okay. That's like asking what the value of X2 is at zero when you've restricted your domain to (0,∞) then?
     
  18. Sep 15, 2015 #17
    I agree "we know" that what GR Cosmology says about time zero is physically impossible, and I agree that it is physically impossible.

    As I understand it, GR Cosmology actually does say something specific about the extrapolation to time zero. There are two cases:
    1. For a universe that is infinite after time zero, GR Cosmology extrapolates that at time zero: Everywhere in the infinite universe the temperature and density are infinite. It does not say that "all distances become zero", as I quoted above from http://www.redorbit.com/education/reference_library/space_1/universe/2574628/big_bang/.
    2. For a universe that is finite after time zero GR extrapolates that at time zero: The universe is a single point at which temperatue and densitiy are infinite.

    Does anyone disagree that it would be beneficial to lay readers looking for an understanding about GR Cosmology if the various sources I listed in my post #13 were modified to make clear what GR Cosmology actually says about the extrapolation to time zero?
     
  19. Sep 15, 2015 #18

    Chronos

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  20. Sep 19, 2015 #19

    bcrowell

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    No, this is just completely wrong, both in terms of physics and in terms of history. The physics that you're describing incorrectly was understood correctly by about 1930, which was before Hawking was born.
     
  21. Sep 19, 2015 #20

    bcrowell

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    This is wrong, or at best misleading. Cosmological models based on classical GR don't include t=0.
    This is wrong. Cosmological models based on classical GR don't include t=0.
    This is wrong. Cosmological models based on classical GR don't describe the big bang singularity as a point.
    This is wrong. Cosmological models based on classical GR don't include t=0. Distances *approach* zero in the limit as t *approaches* zero.
    This is wrong. Singularities in classical GR are not points or sets of points in spacetime.

    What's gone wrong here is that a bunch of people have tried to explain the big bang singularity for a lay audience, and in the process they've oversimplified the ideas. Some of these people are probably journalists who don't have a deep background of physics, and they themselves don't know better. Others may be physicists, but they've oversimplified for a lay audience. On WP, it's probably a case where the text evolved over time, with edits by various people of various levels of knowledge.
     
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