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

New universe predictions by GR and LQG

  1. Nov 16, 2004 #1
    The question is whether I am naive to suppose that either GR or LQG predicts new universes. I have also posted essentialy the same question (and post) in the Steller Astrophysics forum.

    Martin Bojowald has removed the singularities from GR using LQG.
    See http://xxx.lanl.gov/abs/gr-qc/0312045. According to Coule, Bojo's model leads to baby universes. (That seems to be his nickname)

    Being non-expert in GR, I am intrigued by a recent series of three papers on black holes in which new universes are seemingly predicted on the basis of GR:

    Title: The river model of black holes
    Authors: Andrew J. S. Hamilton, Jason P. Lisle (JILA, U. Colorado)

    Title: Inside charged black holes I. Baryons
    Authors: Andrew J. S. Hamilton, Scott E. Pollack (JILA, U. Colorado)

    Title: Inside charged black holes II. Baryons plus dark matter
    Authors: Andrew J. S. Hamilton, Scott E. Pollack (JILA, U. Colorado)

    In the first, space is flowing into the spherical black hole. At the event horizon, space inflow is at the speed of light so light cannot escape. If the black hole does not rotate, the River Flow model predicts, incorrectly as the first paper claims, that space drops through the Cauchy horizon and expands into a new universe. The reader is referred to the second paper for a correct prediction.

    In paper two with charged baryonic matter inflow, along with space, the charged baryons either plunge directly into the central singularity if the black hole is uncharged, or if charged they are repelled by the charge on the singularity, become outgoing and pass through the outgoing inner horizon, namely the Cauchy horizon. No mention of a new universe on the other side in this paper. Also no mass inflation so this solution is also presumably incorrect.

    The correct solution is found in paper three where the outgoing, repelled baryons encounter an incoming flow of dark matter which leads to exponential mass inflation. The claim is that after mass inflation ceases, the outgoing baryons become ingoing and plunge to the spacelike singularity at zero radius, except when the baryons completely absorb the inflowing dark matter. In that case the baryons pass through the Cauchy horizon (again no mention of a new universe).

    Now what intriques me in all this is the prospect of predicting a new universe from GR. Is it correct to assume that whenever the Cauchy horizon is passed through, that a new universe is entered??? And that this may happen whenever counterstreaming ceases?

    Now what is even more intriquing is the relationship of these solutions to Bojowald's treatment of the spacelike singularity at zero radius using Loop Quantum Gravity (LQG).

    Counterstreaming certainly ceases in the singularity. Perhaps the physics captured by the LQG approximate model is essentially what Hamilton has also found outside the singularity, where GR is applicable.
  2. jcsd
  3. Nov 16, 2004 #2


    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    Maybe not completely, or entirely by himself. Martin's paper on the black hole singularity ("Non-Singular Quantum Geometry of the Schwarzschild Black Hole Interior") has not appeared yet. Martin dealt with the Bang singularity in 2001, and since then there have been many papers by him and a bunch of other people. But the Bang singularity is just one of several important failures of GR.

    The Hole singularity has so far been dealt with in two papers by Modesto and IIRC one paper by Viqar Husain and Oliver Winkler. Modesto is at Marseille and the other two are at New Brunswick. I guess one can say that Martin Bojowald and others have dealt with the classical singularities in GR----but the job is still in progress!

    Since Bojowald is a widely cited authority, and he and Ashtekar are known to be working on loop-quantizing black holes, probably the wise thing to do would be not to try to guess, but simply wait until Ashtekar and Bojowald's paper appears.

    One good source for a foretaste is the talk that Ashtekar gave at Penn State in September 2004-----it's about recent LQG black hole research and it refers to two papers that he and Bojowald are co-authoring---the audio and slides are available for download at

    Ashtekar's talk focuses on conditions around where the black hole's former (classical) singularity was supposed to exist----and also on black hole evaporation. It is fascinating, there has been a lot of new work done just in the past few months.

    Since I have only Ashtekar's talk to go by----the promised two papers by Ashtekar and Bojowald are not yet out----it is risky for me to project what I think they are saying. But I will say that, judging by this very interesting talk they do NOT talk about a bounce in the hole, the way Modesto does.

    So there is a difference between Modesto's two papers, which derive a bounce (which could continue forming a future universe), and A/B's work which apparently does not have a bounce, or else that part has not been reported.

    Since Ashtekar is one of the three people who founded LQG, and Bojowald is the main initiator of its application to cosmology (LQC), I would urge you to download Ashtekar's talk. go straight to authoritative sources, in other words.

    If you find it too much trouble to download the talk, Ashtekar says something about this in his recent paper Gravity and the Quantum
    Last edited: Nov 17, 2004
  4. Nov 16, 2004 #3


    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    yanniru, my other point (in preceding post) is more of a quibble.
    It does not detract from your main question.

    I just wanted to make clear that AFAIK Bojowald's word on the Black Hole singularity is not out. Maybe someone else has more recent info, but I am thinking i should wait and see because the details may be unexpected.

    the main question is a compelling one.
    It is difficult to reply to.
    Does LQG predict continuation (into an expanding spacetime) at the pit of a black hole----where the classical singularity used to be located?

    I wish i could see how to reply but right now, without the necessary papers having appeared, I dont see how

    BTW the two papers I am talking about might be available if one writes to Ashtekar. he cites them as "preprint" in his recent review paper
    they are his references [49] and [50] in that paper:
    ---quote from page 37 of "Gravity and the Quantum"---
    [49] Ashtekar A and Bojowald M 2004 Non-Singular Quantum Geometry of the Schwarzschild Black Hole Interior Preprint
    [50] Ashtekar A and Bojowald M 2004 Black hole evaporation: A paradigm Preprint
    --end quote--
    Last edited: Nov 16, 2004
  5. Nov 16, 2004 #4


    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    I cant answer this with a lot of confidence, but I will try:

    I think the answer is YES you are naive if you suppose this. Or at least it is very premature to suppose this.

    there is a logical alternative that makes more sense.
    If one assumes that the black hole singularity is replaced by a bounce (so that there is a continuation into an expanding region)
    then one can predict certain observable measurable things

    that is the essence of Smolin's CNS proposal. this is something which Smolin does not claim is true----he claims it is empirically falsifiable and that it has not yet been reliably ruled out.

    Indeed there is some indication that quantum gravitists will decide that in a quantized version of GR there is a bounce (we have Modesto, V. Husain, Winkler papers but I can't believe these are the final word). And so what?
    So far this does not predict any observable effect.

    Smolin's CNS is more interesting right now, I believe, because it can be falsified---or made extremely improbable---by confirmed observation of neutron stars exceeding a certain mass. Smolin's CNS does not "predict" black hole bounce (what you call a new universe inside the black hole)
    but rather it ASSUMES black hole bounce, and from this assumption (which I must say seems reasonable enough since roughly speaking bounce is a kind of generic result of a classic crunch) it infers some testable conclusions.

    I really like the aspect of disprovability. CNS is especially interesting as a possibility because one can shoot it down (if one finds, say, extra massive neutron stars).

    Other multiverse scenarios, in my experience, dont make committed predictions like that---so its hard to say how one could refute them, and so tell what they mean
    Last edited: Nov 17, 2004
  6. Nov 17, 2004 #5

    Thank you for such detailed attention. But I would like to emphasize one more aspect of my post, that particles passing through the Cauchy horizon enter into a new universe. I was wondering if that is a known and accpted result in the GR treatment of black holes.

    As you say, we should just wait for the A/B papers to appear. But if GR predicts new universes under certain circumstances, then I expect that LQG could make similar predictions.

  7. Nov 17, 2004 #6


    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    maybe this is a lazy answer-----as regards the part of your question just having to do with classical General Relativity.

    I think Andy Hamilton's picture of a black hole as a kind of drain that space is flowing down at the speed of light is a non-standard picture aimed at conveying intuition.

    I think in classical GR the singularity acts as a boundary.

    It is where the theory blows up.

    Infinities of curvature and density cannot exist in nature and do not mean anything computationally or physically. therefore the region occupied by the singularity does not exist.

    The theory stops computing at that point, or surface.

    Therefore the singularity is an impenetrable wall (for classical GR)
    and there can be nothing beyond it.

    So no particle can pass thru it.

    I am being stodgy and conventional, I believe, as contrasted with Andy Hamilton who is wonderfully imaginative. Andy has a great website where he shows movies of what it looks like to fall into a black hole.
    And he has these movies of strange wheeling spiders, and what it looks like when you travel near the speed of light etc.

    He is also a qualified college teacher---a professor at U. Colorado IIRC---and a very talented explainer.

    But I balk at passing thru singularities. I want to first make sure the theory is properly quantized so it fixes the singularities and then I will consider going thru. But not before!
  8. Nov 18, 2004 #7
    Hamilton does stop at the singularity. But he passes into a new universe when he goes through the Cauchy horizon, which is outside the singularity.

    But thank you for refering tom his website. Perhaps I can get his opinion.
  9. Nov 18, 2004 #8
    Well besides learning how naive I am regarding Cauchy horizons, I learned something about google. A google search on 'cuachy horizon' yielded my Physics Forums Stellar Astrophysics post ' What is the significance of the Cauchy Horizon' as the second hit (ha,ha). Search for 'Cauchy horizon' on arXive yields 614 hits, almost all relevant. It's time to kill this thread. Thank you for your attention.
  10. Nov 18, 2004 #9


    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    You are most courteous, yanniru, and you are certainly not the naive one here as regards classical GR black holes and the concept of Cauchy horizon.

    Your thread prompted me to make the arXiv search you suggested and I found, for instance, this

    Signaling, Entanglement, and Quantum Evolution Beyond Cauchy Horizons
    Ulvi Yurtsever, George Hockney

    "Consider a bipartite entangled system half of which falls through the event horizon of an evaporating black hole, while the other half remains coherently accessible to experiments in the exterior region. Beyond complete evaporation, the evolution of the quantum state past the Cauchy horizon cannot remain unitary, raising the questions: How can this evolution be described as a quantum map, and how is causality preserved?..."

    In their Figure 1, they draw a penrose diagram of the formation of a black hole followed by its complete evaporation. and they draw the Cauchy Horizon in red
    (for them it is the future null cone of the "point", or rather the singularity, of complete evaporation). I will quote part of the caption under Figure 1:

    "...The red line illustrates the Cauchy horizon H^+(Sigma) for Sigma or Sigma_0. It is the future null cone of the “point” (really a singularity) of complete evaporation. Because evaporation is largely thermal, quantum evolution through H^+(Sigma) from the time slice Sigma_0 to the slice Sigma_late cannot be described as a unitary map."

    For me several of the ideas in this thread are unfamiliar and/or seemingly paradoxical (like the river model with "counterstreaming" caused by the the charge on the singularity---even the idea of a naked singularity---to mention only a few). So if anyone is naive, i am afraid it is me.

    I have a suspicion that the source of much that is puzzling here is simply the singularity itself, which has a questionable existence.
    The definitions of things like Cauchy Horizon seem to go back to---and depend on---the singularity.
    If the singularity goes away-----so my naive suspicion---then some of the puzzles may prove artifacts of it and will dematerialize as well.

    So I am reminded of Ashtekar's 10 September 2004 seminar talk at Penn,
    which was on Black Hole Evaporation. It did not have any singularity, and he seemed able to assure unitary evolution and resolve the information paradox. He drew a number of Penrose diagrams which seemed to me, if not entirely comprehensible, at least simpler to underestand than this one here called Figure 1.

    You might want to download the slides and audio from

    downloading audio is a nuisance and takes 20 minutes or so, but it can run in the background while you do other things.

    Anyway, there is at least a hope that a quantum model of the black hole, getting rid of the classical singularity, will make things simpler and
    resolve some apparent contradictions.

    still in the dark (at least as much as you if not more)
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: New universe predictions by GR and LQG
  1. LQG makes predictions? (Replies: 6)