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Space Science

Why the Universe Didn't Become a Black Hole 109

StartsWithABang writes: With some 10^90 particles in the observable Universe, even stretched across 92 billion light-years today, the Universe is precariously close to recollapsing. How, then, is it possible that back in the early stages after the Big Bang, when all this matter-and-energy was concentrated within a region of space no bigger than our current Solar System, the Universe didn't collapse down to a black hole? Not only do we have the explanation, but we learn that even if the Universe did recollapse, we wouldn't get a black hole at all!
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Why the Universe Didn't Become a Black Hole

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  • I mean the universe collapses in the beginning ... then it is a 'black hole' ...
    But you can only see form the outside that there is a black hole! As there is mo outside of the universe you can not observe it, hence you don't notice the black hole, hence it is not there.

    Oh! That was simple!

    • The Hitchhiker's Guide to the Galaxy, "Universe":

      1. Area: Infinite. Bigger than the biggest thing ever and then some. Much bigger than that in fact, really amazingly immense, a totally stunning size, real "wow, that's big," time. Infinity is just so big that, by comparison, bigness itself looks really titchy. Gigantic multiplied by colossal multiplied by staggeringly huge is the sort of concept we're trying to get across here.

      2. Imports: None. It is impossible to import things into an infinite area, there b

    • Is it a case of "Honey, I Blew Up the Universe"? The total mass/energy would remain constant but space between particles would just grow larger. The energy which created, and is now expanding the universe, would likely be external to the universe itself.
      • The universe seems to be expanding faster and faster. Dark Energy is coming from nowhere to do that work.

        • by tnk1 ( 899206 )

          Dark "energy" is probably a false description, or a misleading one. Dark energy is thought by many to simply be a property of space-time itself. It isn't being added, it is just what happens to space-time in this configuration or density.

          Much like a rubber band will be elastic up to a certain point, and then full stop, the universe could also hit such a point with no warning (because that point has never been reached before) and then change its acceleration again, or even suddenly boomerang back. Until w

  • by penguinoid ( 724646 ) on Saturday August 16, 2014 @10:49AM (#47684423) Homepage Journal

    You can't use the Schwarzschild radius calculation for expanding space. The only kind of new part was the bit about not becoming a black hole if it should re-collapse.

    • by Rockoon ( 1252108 ) on Saturday August 16, 2014 @11:11AM (#47684497)
      Even if we use the Schwarzschild radius, the radius is so large that we cannot rule out actually being in a black hole anyways.
      • by Tablizer ( 95088 )

        A survey of what's on TV will confirm that.

      • Which is really interesting, as being within a SR could be equated to being within the 'universe' as there is no know way to escape either via simple acceleration...
        • I find it more interesting that if you graph the density of black holes of varying mass/radius, that you see that the required mass density to form a black hole drops as the mass/radius increases. The radius of the event horizon of a mass equal to the estimated mass of the visible universe (just the ordinary matter) is about 15 billion light years and has a density of only 8.703E-27 kg per cubic meter (about 29 orders of magnitude less than the density of ordinary water at sea level.)
    • The only kind of new part was the bit about not becoming a black hole if it should re-collapse.

      Which he stated, but did not attempt to explain.

      I always wondered why collapsing matter would drag space with it. After reading the link, I still do.

      • As far as I understood the physics, it was because "space" doesn't have a set of well-defined borders with customs etc. but is a curved area, where the curve is defined by the energy available inside the universe (energy == mass). Once the whole thing collapses it should curve the area much steeper, "contracting" the universe.

        But I may be totally off base here, I'm not a physicist.

    • Yes you can use the Schwarzschild argument. Expanding space is only a handwaving rationalization, a coordinate-dependent way of thinking that is not compatible with the principle of general covariance.

      If the gravitational source density was ever more than zero, then it follows that the contents of the universe were less massive in the past. In an inertial set of coordinates, not the screwy Freidmann coordinates, it can be understood that the shards of the Big Bang, flying apart at next to lightspeed, still

      • Yes you can use the Schwarzschild argument. Expanding space is only a handwaving rationalization, a coordinate-dependent way of thinking that is not compatible with the principle of general covariance.

        I've never heard anything about that, care to explain? It also seems contrary to what I know, see below.

        A less-known fact about black holes is that the bigger they are, the lower the density. If you use the Schwarzschild radius, for an arbitrarily large mass, the radius gets arbitrarily large, and the density gets arbitrarily close to zero. If this applied to expanding space, then it wouldn't be an open question as to whether the universe was infinite or not, as if it were we would be guaranteed to be in a

        • by mburns ( 246458 )

          Actually, because of the principle of general covariance, it is necessary for academics to show that their cosmological calculations work in different coordinates. Thinking that is bound to special coordinates such as the Freidmann coordinates will only be confused by the fictitious forces exhibited therein. The standard mathematical precautions taken against this sort of problem have failed in this case.

          Relativity itself disallows the necessity of thinking that space inherently expands or contracts. The pr

  • by demachina ( 71715 ) on Saturday August 16, 2014 @10:49AM (#47684425)

    So does ./ have some kind of promotional relationship with startswithabang? If so you should disclose it.

    The blog does have interesting material, and its appropriate for /., so its not like its bad that every article on there is making the /. front page. But its kind of odd that every article on there is making the ./ front page.

  • So to extrapolate from the TFA: The laws of physics do not exist in a vacuum...

    Someone give this guy an xkcd award, then punch him in the face.

  • Hadn't they proved (mathematically) that just after the big bang there was a time (inflation) when it was expanding faster than light. If you are going faster than light then you can escape from a black hole.

    • Re:Early universe (Score:4, Interesting)

      by BitZtream ( 692029 ) on Saturday August 16, 2014 @11:52AM (#47684619)

      If you are going faster than light then you can escape from a black hole.

      There is no part of physics that says speed has anything to do with escaping a black hole. If you could produce enough thrust to travel at just one meter per billion years, you could escape a black hole ... assuming you could keep that speed while inside the event horizon of the black hole. Unfortunately, from a mathematical perspective this appears to be impossible.

      After a certain point (the event horizon) light simply bends two quickly back on itself to escape a black hole and stays inside the radius of the event horizon. It doesn't slow down, it changes directions, because space is all sorts of fubar inside the event horizon of a black hole.

      What they've proved mathematically as that at the event horizon of a black hole the math fails. It falls apart and no longer makes any sense because the numbers get too large on one side of the equation.

      In reality, this doesn't mean 'nothing can escape a black hole'. It means 'nothing we've observed can escape a black hole'. Well, except it can. Hawking radiation escapes a black hole as it evaporates, but all the explanations for why are just silly as they are pretty arbitrary compared to 'light' not escaping.

      Another obvious but often overlooked theory is that our universe IS a black hole inside a larger universe. It explains a great many aspects that don't make sense ... but then it also introduces a whole bunch of aspects that don't make sense without making a bunch of assumptions about what is outside our universe, and these assumptions are so absurd from our view point that we just assume they are false.

      The truth of the matter is ... science knows a lot less than they claim to about black holes, the big bang, and the nature of the universe. Many scientist treat theories with holes the size of the planet in them as obvious fact when they are no such thing. They have no fucking clue why the universe exists in the state it exists today, but many of them refuse to acknowledge that FACT to anyone. The good ones do. Einstein as an example, had no problem admitting his theories were nothing more than theories and that they were often wrong because they were simply based on the little bits of the universe we can observe.

      • Re:Early universe (Score:5, Interesting)

        by boristhespider ( 1678416 ) on Saturday August 16, 2014 @12:49PM (#47684775)

        "What they've proved mathematically as that at the event horizon of a black hole the math fails. It falls apart and no longer makes any sense because the numbers get too large on one side of the equation."

        Not so. The maths dies at the singularity at the centre of the hole, but it doesn't at the event horizon except in a badly-chosen coordinate system. Alas, the usual coordinate system we'd present the Schwarzschild solution in is indeed badly-chosen and has an apparent singularity at the horizon, but this is not an actual singularity, as can be seen quickly by calculating a scalar curvature invariant - the Ricci scalar is the immediate choice, it's basically a 4d generalisation of the more-familiar Gaussian curvature - and seeing that it's entirely well-behaved except at the centre of the hole. So we look for a coordinate system well-behaved at the horizon and quickly come across Painleve-Gullstrand coordinates, in which spacetime is locally flat and perfectly behaved at the horizon. The implication is the poor sod wouldn't be able to tell that he'd got to the horizon, except through tidal forces (which depend on the size of the hole), and then he'd struggle to navigate before slamming into a singularity.

        Even more confusingly, for a *realistic* hole, the insides are rather different. A Schwarzschild hole has a singularity inevitably in the future - all future-directed paths one can travel on, or light can travel on, end at the singularity. That's a bit of a bummer if you happen to be in a Schwarzschild hole. But a Schwarzschild hole is not physical; it is a non-rotating, uncharged hole, and that's not a realistic setup. In a charged (Reisser-Noerdstrom) or a rotating (Kerr) or, come to that, a charged rotating (Kerr-Newman) hole the singularity is "spacelike" -- there exist paths on which we could, in principle, travel, that avoid the singularity. In the case of a Kerr(-Newman) hole it's even smeared out into the edge of a disc. In reality, good luck navigating in there, but the singularity is not inevitably in the future in there.

        A bit closer to the point, you're right that speed doesn't really have anything to do with it. Instead it's the type of path you can travel on, and where *they* go. An event horizon can be defined as the surface on which "null" geodesics, on which light travels, remain equidistant from the hole. If you travel, as massive particles do, on a "timelike" geodesic then you're fucked; you're never going to be able to accelerate enough that you even travel on a null geodesic, let alone a "spacelike" geodesic along which you can basically access anywhere. On a spacelike geodesic you could get out of a hole no problem. You could also travel in time, and you could break causality fifteen times before breakfast. I'd like to travel on a spacelike geodesic - it would be fun. Though managing to get back to a timelike geodesic might be significantly less so.

        "Another obvious but often overlooked theory is that our universe IS a black hole inside a larger universe."

        That's an extraordinarily strong statement. Our universe might be indistinguishable from a black hole from the outside, yes, but there's a big "might" in there, and an "outside" that doesn't necessarily make much sense either. It all depends on the setup you're assuming. Sure, we could end up finding that the universe is "inside" a black hole for a given definition of "universe", "inside" and "black hole", or we might find that that statement does not make any extent. I wouldn't want to say anything stronger than that, frankly, not least as I'm aware of models of cosmology that are observationally indistinguishable from a standard, infinitely-extended, flat universe, which are also flat, but which have finite extent. One way to do so is to simply put the universe into a toroidal topology. Since GR is a local theory it says nothing about topology, and it would be hard to argue that a universe extended on a torus would look like a black hole from the "outside", since that would be the entire extent of spacetime.

  • Short version (Score:5, Informative)

    by Livius ( 318358 ) on Saturday August 16, 2014 @11:31AM (#47684553)

    If at some point in the past the mass of the universe was in a volume wholly contained within its own Schwarzschild radius, why did the universe not become (or, more accurately, remain) a black hole?

    "...Schwarzschild’s solution is a static one, meaning that the metric of space does not evolve as time progresses. But there are plenty of other solutions—de Sitter space, for one, and the Friedmann-Lemaître-Robertson-Walker metric, for another—that describe spacetimes that either expand or contract."

    Literally everything else in the article was off-topic, and I can't help but feel this highly evasive 'answer' might have been "Ask Ethan" admitting he just didn't know.

    Which is a pity because it is a fascinating question.

    • ...then how about this one? []

      One mystery which has not been solved as of 2009 is the absence of red dwarfs with no metals. (In astronomy, a metal is any element heavier than hydrogen or helium.) The Big Bang model predicts the first generation of stars should have only hydrogen, helium, and trace amounts of lithium. If such stars included red dwarfs, they should still be observable today, but none have yet been identified. The preferred explanation is that without heavy elements only large and not yet observ

  • by StripedCow ( 776465 ) on Saturday August 16, 2014 @12:24PM (#47684689)

    Wasn't there some conjecture some time ago that entropy decreased inside a black hole, and that our universe corresponded to a time-reversed version of a star collapsing into a black hole? Which of course would be interesting because the "arrow of time" would point two opposite ways in the "meta-universe".

    • because the "arrow of time" would point two opposite ways in the "meta-universe".

      This is a seriously silly question, but has any serious physicist conjectured that the "arrow of time" could have more than two directions? I mean, like going sideways or something? And what would be the bizarre implications of that?

  • terrible article (Score:4, Informative)

    by Charliemopps ( 1157495 ) on Saturday August 16, 2014 @12:24PM (#47684691)

    Stop posting links to the worst Science site I've ever seen short of timecube... wait, actually timecubes at least entertaining.
    All of their articles boil down to:
    Subject "Could *insert some inane scifi topic* really be??"
    10 pages of images scraped from geocities homepages, font type and spacing worthy of a freshman English paper and then...
    No, not really, but thanks for reading!

    You want real science news? Here you go: []

  • Because I'm not. But, where does the 92 billion light year thing come from? I would think what, 28 billion across if it's 14 billion years old?

  • Perhaps the answer doesn't lie in the 3rd dimension.

    One of the possible consequences of the curvature of 4th-dimensional space-time is that our universe may be a 3-dimensional surface of a 4th-dimensional hypersphere []. And if the 4-dimensional universe is expanding, the 3-dimensional universe would expand too.

    This model of the universe was also used in a famous sci-fi novel [].

  • TFA proposes a kinda-stable universe if it contains an exact amount of stuff, not one neutrino more or less. My question is: "If the universe has net angular momentum, can't it be stable for a large range of stuff?" much like a solar system?
    • It would certainly change the stability analysis, yes, but the universe does not have a significant angular momentum. That would leave a characteristic signature on the CMB, a preferred direction, and it's been hunted, with each new and improved dataset, and we still don't have it. The hunt has turned up other interesting anomalies such as the appallingly-named "axis of evil", but those signatures are also (probably) not due to an intrinsic net angular momentum. That doesn't say there isn't one, just that i

  • > the Universe is precariously close to recollapsing. Hah what?
  • And we are inside of its event horizon. Anything that might exist beyond it, if the laws of physics are the same as they are here (which they may not be), would not be able to see anything that happens inside. Similarly, we cannot see outside of it because every straight line (how light travels) in our space does not go beyond its event horizon, just like in a black hole.
    • Hmm, in a black hole a theoretical photon trying to escape is sucked back in because of gravity. But what will happen when a photon hits the hypothetical edge of the universe? Bounce back? I haven't heard of anyone theorising that.

      • by mark-t ( 151149 )
        Photons trying to escape are don't actually escape not because they are sucked back in by gravity, but because space is so curved by gravity that straight lines which are inside of the event horizon never leave it.
  • One part in 10^24 is why having a Creator makes a difference. One part in 10^24 is no accident. And some people still don't what to admit what created the Big Bang but there is no other answer. How much faith does it take to believe all this was an accident?

    What about the "some 92 billion light-years worth of space contained in a volume of space no bigger than our own Solar System"? That was a miracle. Accidents don't "give rise to all the wondrous diversity of nuclear, atomic, molecular, cellular, geologic

  • I wonder why nobody pointed out the real possibility of our 3D universe being inside a black hole.
    I could even imagine temporal displacement causing us to see the bigbang as a instant event, while on the outside its just near ethernity.

    I've often wondered about this, only to end up with the question how many black holes deep we are..

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