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

We are in a Schwarzschild black hole-T or F?

  1. Dec 3, 2007 #1


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    We are in a Schwarzschild black hole--T or F?

    What I am wondering is WHO HERE THINKS WE ARE IN A SCHWARZSCHILD BLACK HOLE where the black hole event horizon coincides with one of the two well-known cosmology horizons?

    There are a couple of well-known horizon radii that we hear about a bunch:

    the Hubble radius
    this is c/H0 and is the distance at which normal recession speed is c.
    If I remember right, something around 13.5 billion LY, current distance.
    (the radius of the Hubble sphere, as sometimes called)

    the radius of the cosmological event horizon

    In past years we've discussed this at PF quite a lot. I recall reading about it in Lineweaver's excellent 2003 paper, where one of the figures shows it as around 16 billion LY. Events that occur today outside the cosmological event horizon cannot ever affect us.
    We are out of causal contact with current events at that distance---ASSUMING the LCDM model with its constant positive Lambda.

    If Lambda is really zero and the present small positive measured value is an artifact, then the cosmological event horizon would not exist---events that occur today at arbitrarily large distances could eventually affect us, light from them could eventually reach us etc. But the LCDM model has this interesting feature (which Lineweaver 2003 presents in a nice clear treatment.)

    I guess either radius could be called a "cosmic horizon" although this runs a risk of confusion because it wouldn't necessarily be clear which of the two was meant.
    There were a couple of recent papers by Melia where he used that term. My impression was that he means the Hubble radius, but I could be wrong.

    Anyway, I get the impression that some people think the universe inside one of these horizons is a Schwarzschild black hole and that the horizon, whichever one is meant, is the BLACK HOLE EVENT HORIZON of the black hole that we are in. This never occurred to me to imagine, and it simply does not make sense to me. But because similar WORDS are used I guess people can get the idea. Or maybe there is more to it, that I don't understand!

    So here's the poll. Are we in a black hole?
  2. jcsd
  3. Dec 3, 2007 #2


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Well Chris for goodness sake please register a "No" on the poll:smile:
    We actually have people here at PF who think Yes, and the poll got a yes vote within 1 minute of being posted.
  4. Dec 3, 2007 #3


    User Avatar

    I voted yes! .... Even though it goes against what I have learned.
    The theoretical work implies it. The observations of some papers are inclined to say yes.
    In fact http://arxiv.org/abs/0711.4810
    Dark Energy in Light of the Cosmic Horizon
    Authors: Fulvio Melia
    (Submitted on 29 Nov 2007)
    Says that we cannot tell. YET....
    It must be worth while not to reject the model.
    Sooooo, how is an amateur able to decide?
  5. Dec 3, 2007 #4


    User Avatar

    If I would not be studying the papers, I would not be able to answer.
  6. Dec 3, 2007 #5
    Hi all. I wrote to Prof. Melia over the weekend about this very issue, and this is what he wrote:

    Hi Patti:

    thanks for writing. I don't mind at all. I may not always be able to
    answer right away, but I try to get to all of my e-mail, so do feel
    free to write whenever...

    Yes, the Cosmic Horizon is not very difficult to understand, nor why
    it arises in the first place. It helps if you know electrodynamics,
    because this effect is similar (though not the same!) as what one
    encounters there.

    If you have a uniform, infinite (or effectively infinite) medium,
    then if you cut out a spherical cavity in that medium, there is
    absolutely no gravitational field/acceleration within the cavity.
    The symmetry provides a perfect cancellation of the field created
    by all of the sources outside of the cavity.

    Thus, if you now place a mass, say an apple, at the center of that
    cavity, then the gravitational field (or curvature, if you prefer
    to think in those terms) produced by that apple inside the cavity
    is as if there were nothing else outside---as if the apple were
    the only source in the whole universe.

    Now imagine gradually filling the cavity while you move out
    to larger radii. Eventually, you reach the radius at which the
    enclosed mass produces a Schwarzschild surface there.

    Using the term "black hole" is not appropriate here because a
    black hole, as we define it, is an object surrounded by vacuum.
    But what is true is that light signals reaching us at the origin
    of our coordinates from that radius, let's now call it the cosmic
    horizon, are infinitely redshifted.

    Does that mean there's nothing "on the other side"? No, of course
    not. The universe is probably infinite. But any light that would
    be approaching us from beyond the Cosmic Horizon is infinitely
    redshifted, and therefore carries no signal or information.

    Please note that this does not mean we live inside a black hole.
    It's important to get that straight, because that term has come
    to mean something else. But it does mean that our Cosmic Horizon
    is as far as we can ever get information from events occuring
    in our realizable universe. Whatever happens outside is not
    communicable to us.

    Also, please note that this Cosmic Horizon is not necessarily
    static. It is only fixed for all time in a so-called de Sitter
    universe, because in such a universe the density does not change,
    so the horizon radius itself does not change. In a more realistic
    universe, containing matter and radiation, as well as possibly
    a vacuum energy density, this radius changes. In fact, it increases
    with time. So as the universe ages, we get to see more and more
    of it.

    But this too must end, if the universe contains a cosmological
    constant. In that case, eventually matter and radiation wither
    away to zero, while the vacuum energy stays constant forever.
    So our future would then be in a de Sitter universe, and the
    Cosmic Horizon would then approach the de Sitter limit and
    stay fixed at that value forever thereafter.

    Best wishes,

    Newly Released: "The Galactic Supermassive Black Hole"

    Fulvio Melia
    The University of Arizona
    Department of Physics & Steward Observatory
    Rm 447, PAS #81 (520) 621-9651
  7. Dec 3, 2007 #6


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Thanks Patty, everything he says agrees with my understanding as well.
    I think what he calls the "cosmic horizon" is the sphere at Hubble radius and he says it is a "Schwarzschild surface" which does not mean there is a black hole but simply that we don't get info from outside that surface.

    so if something is ANALOGOUS to a black hole, it is what is OUTSIDE that spherical surface (our part of the universe is not the analog, it is all the rest that is the analog---and the analogy is very weak)
  8. Dec 3, 2007 #7


    User Avatar
    Science Advisor

    I must admit I'm not entirely sure what he means by his use of the term "Schwarzschild surface" in this context? His apple-in-a-cavity thought experiment implies that there is a requirement for some critical amount of enclosed matter for the horizon to appear, but it is not clear how that critical requirement relates to anything from the Schwarzchild Solution. Well, it's not clear to me at this point anyway.

    I must admit I still haven't read the first Melia paper in the recent pair that came out, and I'm sure it is explained in more detail in there.
  9. Dec 3, 2007 #8


    User Avatar

    I was thought that Schwarzchild radius implied that nothing could get out... it's a "brick wall"... as a result .... anything inside can only bounce around.
    It a good way to get conservation of energy... nothing can escape.
  10. Dec 3, 2007 #9

    George Jones

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Can any stuff here and now escape to future null infinity (scri +)?
  11. Dec 3, 2007 #10


    User Avatar
    Staff Emeritus
    Science Advisor

    That depends on the metric that we're taking, doesn't it?

    I don't understand the idea proposed by people modelling the universe as schwarzschild: How can there be a global schwarzschild geometry when there is an assortment of matter; i.e. the matter is not confined to one specific location (or centre)?
  12. Dec 3, 2007 #11

    George Jones

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Do this even make sense?

    What does the conformal diagram for our [itex]\Lambda[/itex]CDM universe look like?
  13. Dec 3, 2007 #12


    User Avatar
    Science Advisor

    Check the figures from Davis & Lineweaver. They have a very clear figure or two of the conformal representation of several cosmologies, including LCDM.
  14. Dec 3, 2007 #13
    Prof Melia's description of a spherical cavity sounds like the result Peebles describes from Birkhoff's theorem. Which is why it's possible to consider any reasonably sized spherical subset of an expanding universe without regard to all of the mass/energy outside the sphere.

    Also, as I understand it, the typical density of a black hole is about equal to the density of water. Obviously, our universe currently is far less dense. At some early time it was that dense, but I don't think there's any explanation how a black hole could ever get as un-dense as our observable universe.
  15. Dec 4, 2007 #14
    Too many horizons

    There seem to be a lot of candidates for what can be called a cosmic horizon, and it's important not to get them mixed up. (see http://www.chronon.org/articles/cosmichorzns.html)

    1) Hubble Sphere: This has no physical significance whatsoever.

    2) Particle horizon: This is the limit of what can have had any effect on us since the big bang. It occurs in most models of the universe which have gravitating matter.

    3)Cosmological Event horizon: This occurs when the expansion of the universe is accelerating. It has some similarities to the event horizon of a black hole (see http://www.chronon.org/articles/Cosmological_Event_Horizon.html)

    4) Now Melia seems to have invented another horizon, which is the radius at which the matter around us would form a black hole. I'm very suspicious about this, since if you go back in time, this horizon encompasses a smaller and smaller part of the universe, and yet we have somehow got beyond the 'black hole' we were in then. Its interesting to look for what the problem is with Melia's horizon. I would guess that as long as his horizon lies outside the particle horizon, the matter won't be able to form a black hole. If we lived in a closed universe which was destined to recollapse to a singularity then it might be reasonable to say that we were in a black hole.

    Hopefully Chris Hillman will be along in a short while to set us straight on this matter.
  16. Dec 4, 2007 #15

    George Jones

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I have somehow misplaced :grumpy: my hardcopy of this article, but looking online last night, I didn't see what I was looking for. Now, I have roughly the same spacetime coordinates as my books, so I have looked in Hawking and Ellis, which has a conformal diagram for de Sitter spacetime, and, in the future, our [itex]\Lambda[/itex]CDM spacetime looks like this, i.e., future null infinity is spacelike. In the past, however, [itex]\Lambda[/itex]CDM has a Big Bang singularity.

    Here's what I was hinting at.

    For a spacetime M, define a black hole to be the region B = M - J^-(scri^+). Here scri^+ is future null infinity and J^- denotes causal past. A particle (photons included) at any event p in B cannot escape to infinity, since p isn't in the past of infinity.

    For our [itex]\Lambda[/itex]CDM spacetime, which seems to model observations well, everything is in the past of future null infinity, so B is empty; our [itex]\Lambda[/itex]CDM spacetime is not a black hole spacetime.

    (No, I'm not saying that black holes don't exist in our universe.)

    While there are lots of horizons, including event horizons for particles (since future null infinity is spacelike), none of these is a black hole event horizon, since the boundary of the (causal) past of future null infinity is empty.
    Last edited: Dec 4, 2007
  17. Dec 4, 2007 #16
    I chose no, because the dynamics of a BH horizon and the cosmological event horizon seem to behave very differently.

    But then again I could be wrong.
  18. Dec 4, 2007 #17


    User Avatar

    A review by Ruth Gregory. A theoretical physicist at Durham University, UK

    Paul J. Steinhardt
    Endless Universe: Beyond the Big Bang
    Paul J Steinhardt and Neil Turok

    Inflation was designed to solve some problems which can also be solved by the cyclic universe.
    Steinhardt and Turok the universe is simply a slice (known as a brane) through these extra dimensions, and the Big Bang was a collision of branes — a huge cosmic thunderclap. This model builds on an idea called M-theory, in which the strings live on two walls at the end of an 11D space–time. Applying the usual rules of string theory leads to a general picture in which these walls can move across the canyon separating them, and occasionally (every trillion years or so according to Steinhardt and Turok) slam into each other. It is this slamming together that is responsible for what we see as the Big Bang, although from a higher-dimensional point of view it is a collision rather than a singularity.

    One message the authors communicate clearly is that we should never accept something simply because most people say it is true, but should constantly challenge and look for alternatives to any picture that cannot be rigorously proven.
    present vote …
    yes, … 2
    no, … 12
    hehehe :rolleyes: :rofl:
  19. Dec 5, 2007 #18


    User Avatar

    If you have been reading the papers then it is obvious that The Cosmic Horizon
    by Fulvio Melia has got as much observational info for it to be considered a serious candidate as any other model.
    If you support colliding branes then they would create a Cosmic Horizon. There is no reason to assume that our universe was the only one created by colliding branes. Therefore, the logic would be to assume that the “bulk” or “cosmos" is populated with 10^500 universes each having their own Cosmic Horizon. All would be irrelevant … until … they meet and mearged.
  20. Dec 6, 2007 #19


    User Avatar
    Staff Emeritus
    Science Advisor

    While the majority of the posters have already gotten the right answer (no), some of the answers were quite technical.

    I would like to point out that this question is addressed in less technical terms in the sci.physics.faq Is the Big Bang a black hole?

    The short version is that the big bang is definitely not a black hole. The question "Is the big bang a white hole" is more interesting, and the FAQ talks about this in more depth than the section I quoted above, but while this is IMO a more interesting question, it is not what was asked and I don't want to derail the thread.
  21. Dec 7, 2007 #20


    User Avatar
    Science Advisor

    Of course the FRW solution is not the Schwarzschild solution. Prof. Baez answer seems to me like 'both solutions are not the same because they are two different solutions'. To my eyes the interesting question is rather how could the experimental data fit to such a proposal.

    The best agreement with all the cosmological experimental data is provided by the standard model of cosmology. However, it could be a pedagogic exercise to try to figure out how to explain some basic facts assuming a Schwarzschild geometry. For example, is it possible to have redshift, time dilation and variations of brightness according to data in a Schwarzschild solution? If yes, with what constraints or conditions? What then about other cosmological tests such as the CMB or the ratios of light elements?
    Last edited: Dec 7, 2007
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook