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Why the early universe didn't collapse into a black hole?

  1. Sep 7, 2011 #1
    So, if our observable universe was less than a centimeter across, why didn't it collapse into a super black hole? The gravitational field of all that matter... inflation seems weird when you think that it could become into a big black hole (and it didn't; right?)
  2. jcsd
  3. Sep 7, 2011 #2


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    Welcome to PF shazdeh. Your question is addressed in the Cosmology Forum FAQ:

    https://www.physicsforums.com/showthread.php?t=506992" [Broken]
    Last edited by a moderator: May 5, 2017
  4. Sep 7, 2011 #3
    It did not collapse because the spatial condensation expansion rate was so great in the production of the supporting 4-D ball.
  5. Sep 7, 2011 #4


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    It did not collapse into a black hole for reasons clearly outlined in the above referenced FAQ.
    Last edited: Sep 7, 2011
  6. Sep 8, 2011 #5
    Thank you for the answer!
    I read it again, and again, and again, and I didn't get it! :)

    I'm missing some points in here; there was space in singularity, wasn't it? Quoting SciAm June 2008, page 53, "at the time of inflation, our observable universe was less than a centimeter across." That's space, no?
  7. Sep 8, 2011 #6


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    Gravity was ineffective for a brief time after the BB.
  8. Sep 8, 2011 #7


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    Yes, that's the size of the observable universe -- the patch of spacetime within which all events are causally related. However, the actual universe is a patchwork of these causal regions. The big bang was not a localized event -- it happened everywhere at once. Keep in mind, that the big bang model does not address the initial singularity -- it is believed that the singularity is signalling a breakdown of the mathematical theory at this time, rather than representing something physical. So the big bang is the cosmological model that describes how the early universe expanded from a hot, dense phase into a cooler, less dense phase. This expansion, by all accounts, has been uniform and isotropic because the energy density is uniform and isotropic. An energy density of this form does not give rise to a gravitational field, and therefore does not yield black hole solutions.
  9. Sep 8, 2011 #8
    Shazdeh: Welcome to Physicsforums.....

    try reading the FAQ several more times..slowly..maybe over a few days...understanding the terminology and the concepts is not easy...especially when you are getting started....for example "tidal forces" is NOT an obvious concept..Wikipedia for one explains it....

    from the FAQ:
    "...In a homogeneous cosmology, symmetry guarantees that tidal forces vanish everywhere, and that any observer at rest relative to the average motion of matter will measure zero gravitational field..."

    So despite what might be commonly thought, there was relatively little gravity....a rough analogy might be falling through to the center of the earth...the further you fall from the surface the more gravity pulls you out as well as in....at the exact center you are pulled equally in all directions....

    Another analogy: in outer space, you virtually "float" because you are pulled weakly but about equally in all directions by gravity....

    yes....there was enough energy to cause incredibly rapid expansion....but no one knows just what caused the big bang itself...what was the nature of the initial instability....
    Last edited: Sep 8, 2011
  10. Sep 8, 2011 #9


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    Indeed, but I think it's important not to put too much focus on the amount of energy, or the rapidity of the expansion, or the strength of gravity. It's really a matter of symmetry -- a uniform spacetime of critical density will expand (or contract) -- it will not form a black hole.
  11. Sep 8, 2011 #10
    Thanks for your help! Those analogies were awesome!
    To wrap my head around it, here's a question: if we could by any mean open a well deep to the heart of the earth, and if I'd jump into it, would I reach the center of the earth? Because, "the further you fall from the surface the more gravity pulls you out as well as in", does this mean in a point along my trip the gravity pull from above and below would become equal and I'd stop? Sorry if it sounds a silly question, but I'm new to these! :)

    One more question, the ideas discussed here, like "there was relatively little gravity", are there mathematical proves behind it? It's just that I get convinced when I know there's some math behind it! :)
  12. Sep 8, 2011 #11


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    There will always be more mass "below" you than "above" you as fall towards the center of the earth. As Naty1 says, only at the very center will the net gravitational force be zero.
  13. Sep 8, 2011 #12
    If the hole is completely straight through the Earth, inertia would tend to carry a body on past the center where the gravitational force is zero.and into an oscillation mode through the center. But the rotation of the Earth would keep the body scraping the side of the hole which would eventually bring the remains to rest at the bottom.
  14. Sep 10, 2011 #13
    Assuming, of course, that you did not enter a tunnel along the axis of rotation.
  15. Sep 10, 2011 #14
    Not against either outward jet of mass in opposite directions when those tunnels open!
  16. Sep 17, 2011 #15
    Your reply "Unless the hole is on the axis of rotation of the Earth", is right on the mark.
  17. Sep 17, 2011 #16


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    (my bold) :biggrin:
  18. Sep 17, 2011 #17
    Because maybe the big bang came from a ginormous black hole, a singularity that became so unstable that it burped out the entire universe :)
    Last edited: Sep 17, 2011
  19. Sep 20, 2011 #18
    From what ive heard,my understanding of the early universe (big bang) is that matter and anti-matter was popping in to existance then instantly anihilating producing energy.But once in a bllion times they didn't anihalate and a particle of matter was left over.thus there is enough energy to force the matter to expand away from the singularity,creating space and time.question was all the matter produced in an instant or did an expanding shock wave tear matter and anti-matter apart as it expanded?
    Is my understanding wrong?
  20. Oct 12, 2011 #19
    Something like this?

    2M + 2A --> E + M

    M = Matter
    A = Anti-matter
    E = Energy
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