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More black-hole questions

  1. Nov 9, 2006 #1
    I know a lot of black hole questions have been posted, but these ones probably haven't arose before. I'd be gratefull for any responses.

    1. Is it possible to have a black-hole 'inside a black-hole'?
    I ask this because there is mathematical evidence to suggest the Universe is a supermassive black-hole, in which case the black-holes we observe would have to exist 'inside' it.

    2. If an observer was inside a black-hole, the Universe could not exist, from that observer. Correct?
    Thus on similar logic we can say a universe exists outside our own, but we would be unable to prove it.

    Finally, with regard to the accelerating Universe.
    Can we posit the Universe to undergo accelerating and decelerating expansion consecutively, so that one cancels out the other?
    This would make it so the Universe would eventually contract, fitting in with the idea of a closed black-hole.
  2. jcsd
  3. Nov 9, 2006 #2


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    Can you cite this evidence? Considering the universe is expanding, it makes it very difficult to imagine it's also a black hole
  4. Nov 9, 2006 #3
    In order to view the universe as a "Black Hole" or to be a bit liberal with the characterization to loosely describe it as “Black Hole Like” I don’t think we could accept the view we have a currently “increasing” rate of expansion. What ever the expansion is; it would already need to have some acceleration slowing it down.
    The “Black Hole Like’ view would require that the Big Bang inflation established a very high speed expansion by the end of that inflation. with some negative acceleration already being applied to slow it down and stop it at some horizon limit to prevent escape from the “Black Hole”.
    Then with the same negative acceleration contracting the universe back into a Singularity Like event to possibly repeat the Big Bang (or Big Crunch as many like to call it).

    Sounds good except the current reports from the Cosmologists, is reporting that they believe there is a positive acceleration (increasing speed) to the inflation, not a negative one.
    To support the Black Hole Like idea this accelerating would need to be refuted and replaced with an inflation that was already slowing down although still inflating due to some small negative (deceleration) already in place.

    I understand the preference for a “closed” universe with repeating Big Bangs. But just finding where many have that preference, even speculating on what it may mean and how it might work doesn’t really stand as evidence to suggest it is true.
    If there is evidence that opposes the Cosmologists view I’ve not seen it.
  5. Nov 10, 2006 #4
    The following thread seems to be somewhat similar so you may be interested:

    "https://www.physicsforums.com/showthread.php?t=140921" [Broken]

    Wai Wong
    Last edited by a moderator: Apr 22, 2017 at 1:57 PM
  6. Nov 10, 2006 #5
    I think it is possible. Consider for instance a mini-black hole falling into a large black-hole. In this case, there would be two singularities within the event horizon of the large black hole.
  7. Nov 10, 2006 #6
    The density required to create a black hole can be given by p = 3M/4piR^3
    From Schwarzschild equation the radius of a black hole in relation to its mass is given by R = 2GM/c^2.
    Substituting this into the previous equation, we obtain p = 3c^6/32piG^3M^2

    Therefore - p is inversely proportional to the mass squared of the black-hole. The greater the mass, the less the required density.

    Consider the following parameters:
    the age of the Universe can be estimated as 1/H0, where H0 is the Hubble constant. The size of the Universe can be given as c/H0 = 20bl years (although current estimates put it between 14-18)
    Using this distance as the R of a black hole we can obtain the density required for the Universe to be a black hole.

    M = Rc^2/2GM --> p = 3c^2/8piGR^2
    by replacing c/H0 for R we get p = 3H0^2/8piG
    Using a H0 of 50km/s per Mparsec we find p = 3H0^2/8piG

    this gives us a density of 4.5*10-27 kg/m^3 - almost the exactly estimate of the Universes density.

    Thanks for the link to https://www.physicsforums.com/showthread.php?t=140921 . It seems the only suggestion to solving the 'horizon within a horizon' problem is that 'somehow the 'event horizon' of the Universe, or expanding sphere of spacetime, coincides with the event horizon of all black-holes' (aether). Then how can we observe a black hole B1 within the Universe? we see it effecting matter, distorting spacetime, and thus it must be seperate.
    But how can a black hole exist within a black hole?

    http://www.sciencedaily.com/releases/2003/09/030917072015.htm raises the possibility that the universe was 'born inside a black hole'. But then we get the problem of infinite universes - one inside the other.
  8. Nov 11, 2006 #7
    "If there is evidence that opposes the Cosmologists view I’ve not seen it."

    I accept your analyses as adequate evidence to pursue the idea that the universe is at least “Black Hole Like”.
    However I don’t understand why after assuming the universe is a black hole you still have the question “how can a black hole exist within a black hole?” when you immediately follow with the argument:
    The flaw in your follow up is in declaring “thus a black hole must be separate from the universe”.
    By what evidence do you assume this?
    Rather when you assume the universe is a black hole, logical you must conclude that the black holes we see (and their mass) is a part of that larger black hole/universe, and not separate from it.
    Meaning your interpretation must deny the popular idea of “white holes” and black holes as providing some path to a place “away” from our local reality or universe.
    Including that would make your argument more complete.

    Which is correct, inside black holes is a place that is place still a part of the universe, or a place separate from our universe?
    That will take more information to better define exactly WHAT a black hole is.
  9. Nov 11, 2006 #8
    The reason for saying this is because we observe black-holes with distinct properties; charge, mass and spin. If a black hole is a 'universe of itself', implied if it is a region of closed spacetime, then how can these properties arise?

    I've made two postulates about black-holes that may be helpfull:
    a) the event horizon is the singularity (nothing exists 'below' the EV; as was indicated, all information about the black hole exists on the event horizon)
    b) the black-hole exists on the closed spacetime of the universe, but is locally closed itself - a bit like a 2-d sphere on the larger 4-d sphere of the universe

    Regarding "white holes": these are hypothetical entities which require the existence of another universe, whereby matter enters a black hole and is ejected at the end of a 'spacetime tunnel' from the white hole. But where are these entities? Unless time reverses, black holes will continue being black.
  10. Nov 11, 2006 #9


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    I don't see how this is helpful. The event horizon is not the singularity by the standard meaning of the word. Are you perhaps thinking of the holographic principle? This is a speculation in quauntum gravity, but it does not say that the event horizon is the singularity, any more than it says that the walls of a room are the room. (The walls of the room may contain the information needed to model the room if the conjecture is true, but the walls of the room and the room are still two different entities).

    In any event, viewing this as a question about classical GR, the answer should not depend on the correctness of the holographic principle conjecture - quantum gravity should not be relevant.

    As far as the original question goes - I'm not sure what the answer is, unfortunately.
  11. Nov 11, 2006 #10
    As I said, following from your argument that the universe IS a black hole, you must conclude that the black holes we see are a part of that universe along with us and NOT a separate 'universe of itself' or a ‘closed space-time’ as some views claim. Your view cannot make that claim and therefore should have no problem with how those properties arise.

    The point is your view does not agree with the idea that such problem or paradox even exists. It just needs a better description of what a black hole is. A description that does not exclude what is behind the event horizon from the rest of our universe.
    Sure that is more easily said than done; but that is the task, if you want to be convincing about the universe “being a black hole with black holes inside”, at least IMO.
  12. Nov 19, 2006 #11

    Chris Hillman

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    Uh oh--- I suspect you have been misled by a bad analogy sometimes seen in popular books. It is not true that gtr suggests that "the universe is a supermassive black hole", not even close. To understand why, however, I think you will need to know the definition of "black hole" used in classical gravitation, which is unfortunately rather subtle. Try Hawking and Ellis, The Large Scale Structure of Space-Time (see http://www.math.ucr.edu/home/baez/RelWWW/reading.html [Broken] for some more suggestions).

    I don't think that even makes sense, although I appreciate that it is probably quite difficult (if not impossible) to express what you mean in nonmathematical language. It might help to observe that according to gtr, an observer who has fallen under the event horizon of a black hole can still see the outside universe, he just can't communicate with it. See for example http://casa.colorado.edu/~ajsh/schw.shtml.

    I don't really understand why you might think that, but perhaps this is moot, since gtr does not in fact imply this.

    I am not sure I understand what you have in mind, but D'Inverno, Introducing Einstein's Relativity, offers a good discussion of FRW models WITH Lambda (rather fortuituously, since this book came out BEFORE the recent discovery of the so-called "acceleration").

    Chris Hillman
    Last edited by a moderator: Apr 22, 2017 at 2:07 PM
  13. Dec 29, 2006 #12
    Apologies for the late reply to this post.

    With regard to my second point, 'that the universe would not exist from the perspective of a black-hole': the reason for this is because of Lorentz contraction - as the escape velocity of the star matches the speed of light. To observers outside the black-hole, time 'stops' which means space must contract to zero legnth. But to an observer within the black-hole, the effects of time dilation and legnth contraction are applied to the Universe.

    At the black-hole singularity spacetime would be 'infinitely curved' so it would be impossible to infer the existence of an outside spacetime with its own curvature.

    Unless i'm missing something, this can only mean that from the perspective of the black-hole there is no other Universe - but clearly the Universe (external spacetime) effects properties like mass, charge and spin.

    Theres plenty of evidence to suggest the Universe as a supermassive black-hole; most textbooks include a brief discussion of the possibility (i.e Kauffmans 'Universe' and Foundations of Cosmology)
  14. Dec 29, 2006 #13
    Actually from the perspective of an observer inside the black hole the outside universe is speeding up not slowing down!
  15. Dec 29, 2006 #14
    But from the "perspective" of an observer inside the black hole there ain't the "outside universe".
  16. Dec 29, 2006 #15
    Note that event horizons are like one-way membranes; nothing can return once passed but it does not block anything that is coming in. Hence, the inside is causally disconnected from the outside but the outside is not causally disconnected from the inside.
  17. Dec 29, 2006 #16
    Jennifer,from the perspective of the observer inside it appears just like an inflation of his own universe (there are undetrmined outside horizonts)
    Last edited: Dec 29, 2006
  18. Dec 29, 2006 #17
    Sorry but I cannot follow what you say, what do you mean his universe there is only one universe.
  19. Dec 29, 2006 #18
    Not sure if you have interpreted my post correctly; i'm looking at it from the perspective of an observer approaching the event horizon and an outside observer uneffected by the black-holes curvature.
  20. Dec 29, 2006 #19
    Sorry but I can't follow you.
  21. Dec 29, 2006 #20


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    If mass went into the event horizon, it would speed up to over the speed of light right?

    Following the realtivity factor: From observers outside the horizon would observe his time to be completely still. Its length would be 0, and it's mass would be infinite. From his perspective things outside would go with a time of infinity. Wouldn't that mean that his trip from the beginning of the event horizon to the surface of the black hole be 0seconds? That means his speed had to be infinite.

    So, does this mean that the speed of mass inside an event horizon is (X) m\s > 299 792 458 m\s or infinite?
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