Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Black holes bounce theories

  1. Aug 28, 2007 #1

    wolram

    User Avatar
    Gold Member

    If theories of cosmology produce a (bounce) what happens to Black holes, it is hard to imagine that these posited things being riped apart.
     
  2. jcsd
  3. Aug 28, 2007 #2

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    black holes can MERGE to form increasingly massive black holes

    I think one way to picture a cosmological bounce is to start by imagining the collapse that leads to it, and
    that collapse is not unlike the collapse that forms an ordinary black hole, but on a grander scale.
    As I picture it the collapse could easily involve lots of black holes, as well as other stuff, all merging into one.

    At the Planck density (when gravity becomes repellant due to quantum corrections in some models)
    all that detailed structure is lost. black holes can no longer exist when gravity becomes repulsive
    so all that detail would melt together, as i picture it, and turn into a big bang.

    this is certainly not a precise picture. So far the people who do computer modeling of big bounce, in various cases, have not tried to simulate anything that complicated. So far the different bounces that have been studied, either analyti cally or with computer modeling have been comparatively simple. They keep making them more complex step by step---less uniformity, more irregular structure. Lot more will have to be done along those lines before a question like yours can be answered except in a token way (as I have attempted).

    Lots of unanswered questions here.
     
    Last edited: Aug 28, 2007
  4. Aug 28, 2007 #3

    wolram

    User Avatar
    Gold Member

    If BHs merge the result will be a cul de sac or one way valve, how can a bounce occur
    from a super massive BH, so far repulsive gravity is just a conjecture? without it bounce
    can not happen.
     
    Last edited: Aug 28, 2007
  5. Aug 28, 2007 #4

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    repulsive gravity is something that comes out of the math that was not intentionally put in.
    it comes out of LQC----an approach to making a quantum version of classical cosmology.

    LQC has to be tested, by, for example, deriving predictions about the CMB temperature map from it and then comparing with the real map.

    if LQC can pass some tests then it may gain some credibility, and then one will be more inclined to trust the OTHER things derived from it, like gravity becoming repulsive at around 80 percent of the planck density.

    for now, the whole thing is hypothetical---just a possible model. so things derived from it are conjectural. there should, however, be testable predictions coming out of LQC which refer to things we can see and measure. Bojowald talks about some testable predictions in a recent paper (the one with "patchwork universe" in the title---I dont recall the full title just now.)
     
  6. Aug 28, 2007 #5

    wolram

    User Avatar
    Gold Member

    Is it even possible in physics for a (force) gravity being outside the accepted definition of force, to reverse sign?
     
  7. Aug 29, 2007 #6

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    Yes it is. The basic equation in cosmology is called after Alex Friedmann and it relates the expansion rate to the density.
    in quantum cosmology that equation acquires quantum corrections
    these can have the net effect of changing the sign on the righthand side of the equation.

    so under ordinary conditions the equation says that the change in the expansion rate is proportional to MINUS density (the concentration of energy)
    and the more concentration of energy the more it slows down expansion or accelerates collapse

    but at very high density the quantum correction terms become dominant and then the change in the expansion rate is proportional to PLUS the density.. It becomes directly proportional, without the minus sign.
    so then the more concentration of energy you have, the more it speeds up expansion, or slows down collapse

    and this effect can actually reverse collapse and cause a brief period of inflation (exponentially fast expansion)
    ==================

    they TALK about it by saying "gravity is reversed". What it amounts to is changing sign on the righthandside of the Friedmann equation, or more generally the Einstein equation of GR (of which the Friedmann is just a simplified version for easy calculation.)

    Probably that is as good a way to talk as any other. I can't think of a better way to put it.
     
  8. Aug 29, 2007 #7
    Is there a minimum amount of mass required to produce this density?

    Say, for example, could a supercluster have enough mass to eventually reach planck density and "bounce"? Or does it require the mass of the entire universe to reach that density?
     
  9. Aug 29, 2007 #8

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    Boomboom, remember we are talking about a MODEL that needs to be tested.
    Within that model (to answer your question) there is no minimum mass required at the level you are talking.
    There is a Bojowald paper where they derived a minimum mass for a black hole to form but it was tiny. Less than a milligram IIRC, and even that limit not sure.

    So you might conclude that if LQC is correct that you could get a bounce from pretty much any collapse---even from a single dead star forming a black hole.

    But that isn't right either! Because so far the collapse-bounce-expansion pictures they have studied are fairly regular and symmetric. Like cosmologists always picture the universe: approximately uniform, balanced in all directions, not lopsided.

    Only recently have the people doing this kind of research begun working with less and less regular shapes of what collapses. They want to make the result more robust so they can say that a bounce happens in many or all situations---instead of just in special circumstances. This work is in progress.

    So the problem isn't that you might not have enough mass. It is the other things you have to have right. We can't know what the outcome of the research will be.

    It might turn out that ordinary astrophysical black holes don't bounce! Maybe only universes do! Some research (Leonardo Modesto) indicates black holes bounce but he assumes a fairly symmetric picture. And maybe his conclusions are wrong. So we have to wait.:frown:

    My own suspicion, though, from the way I see the research going, is that even stellar-size black holes bounce and produce a new region of spacetime out the back door:approve:
     
    Last edited: Aug 29, 2007
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?