People sometimes ask, “Is the universe a black hole?” Or worse, they claim: “The universe is a black hole!” No, it’s not, and it’s worth getting this one straight.
If there’s any quantitative reasoning behind the question (or claim), it comes from comparing the amount of matter within the observable universe to the radius of the observable universe, and noticing that it looks a lot like the relationship between the mass of a black hole and its Schwarzschild radius. That is: if you imagine taking all the stuff in the universe and putting it into one place, it would make a black hole the size of the universe. Slightly more formally, it looks like the the universe satisfies the hoop conjecture, so shouldn’t it form a black hole?
But a black hole is not “a place where a lot of mass has been squeezed inside its own Schwarzschild radius.” It is, as Wikipedia is happy to tell you, “a region of space from which nothing, including light, can escape.” The implication being that there is a region outside the black hole from which things could at least imagine escaping to. For the universe, there is no such outside region. So at a pretty trivial level, the universe is not a black hole.
You might say that this is picking nits, and the existence of an outside region is beside the point if the inside of our universe resembles a black hole. That’s fine, except: it doesn’t. You may have noticed that the universe is actually expanding, rather than contracting as you might expect the interior of a black hole to be. That’s because, if anything, our universe bears a passing resemblance to a white hole. Our universe (according to conventional general relativity) has a singularity in the past, out of which everything emerged, not a singularity in the future into which everything is crashing. We call that singularity the Big Bang, but it’s very similar to what we would expect from a white hole, which is just a time-reversed version of a black hole.
That insight, plus four dollars or so, will get you a grande latte at Starbucks. The spacetime solution to Einstein’s equation that describes a universe expanding from the Big Bang is very similar to the time-reversal of a black hole, but you don’t really learn much from making that statement, especially because there is no outside; everything you wanted to know was already there in the original cosmological language. Our universe is not going to collapse to a future singularity, even though the mass is enough to allow that to happen, simply because it’s expanding; the singularity you’re anticipating already happened.
Still, some folks will stubbornly insist, there has to be something deep and interesting about the fact that the radius of the observable universe is comparable to the Schwarzschild radius of an equally-sized black hole. And there is! It means the universe is spatially flat.
You can figure this out by looking at the Friedmann equation, which relates the Hubble parameter to the energy density and the spatial curvature of the universe. The radius of our observable universe is basically the Hubble length, which is the speed of light divided by the Hubble parameter. It’s a straightforward exercise to calculate the amount of mass inside a sphere whose radius is the Hubble length (M = 4π c3H-3/3), and then calculate the corresponding Schwarzschild radius (R = 2GM/c2). You will find that the radius equals the Hubble length, if the universe is spatially flat. Voila!
Note that a spatially flat universe remains spatially flat forever, so this isn’t telling us anything about the universe now; it always has been true, and will remain always true. It’s a nice fact, but it doesn’t reveal anything about the universe that we didn’t already know by thinking about cosmology. Who wants to live inside a black hole, anyway?
Thx to Sean for the post and to Sergiy Koshkin for the interesting link. 🙂
@Sean (or any informed one)
“It means the universe is spatially flat.”
Sorry for stubbornly insisting, but are you sure?
Let me explain the question: ok a spatially flat observable universe has the density of a black hole. But is it as obvious that having the density of a black hole means being a flat universe*?
Let’s consider a flat universe in which a comsological constant is introduced**: obviously the horizon will come closer in the long run, so one should expect the density to become lower. However, it seems also logical to interpret the cosmological constant as an extra energy. So maybe the density shoud be higher for what it matters.
Is it possible that the net effect is zero?
* of course one may be a black hole instead of an universe. It’s a free country.
** any resemblance to an universe living or dead is purely coincidental
Sean, do cosmologists believe there is anything outside the observable universe or not? Your previous posts have lead me to think the answer is yes, but this one makes it seem the answer is no.
The fact that our Universe is flat rest on three points. It has the correct energy density to be flat based on surveys and SN1A data, the basic idea that an inflation event launched the expansion of the Universe requires a flat Universe (within our cosmic Horizon) and careful analysis of the Cosmic Microwave Background (CMB) indicate a spatially flat Universe to a high degree of certainty.
Origins of the misconception:
I remember Carl Sagan in Cosmos (Chapter 10) saying that the universe is a black hole quite explicitly. “If you want to know what its like inside a black hole, look around you”
How important is the idea of a singularity at the start of time anyway? I decided a while back that assuming “it started with a singularity” was just messing with my head, and I’m much happier now that I’ve switched to “it was smaller, and that’s all we can know”. (And recently Starts With A Bang has been running a series of posts that basically make the story small universe → inflation → expansion, without even mentioning the s-word – and it all makes sense, for once.)
Big difference when you’re in the hole, I guess.
But if I assume that the arrow of time is meaningless anyway, where does that lead? To just two equally valid ways of looking at time-asymmetric “holes” of a certain kind? If we take a black hole and “run it backwards”, what would that look like from the inside? Hmm.
I’m not a fan of “time-reversed” black holes, unless you’re a time traveler. I’d rather people called them white holes and be done with it. Time marches forward.
Here is a more detailed look at the possibility that the Universe is a Black Hole:
http://www.desy.de/user/projects/Physics/Relativity/BlackHoles/universe.html
And another FAQ on the subject:
http://www.mathpages.com/home/kmath339.htm
This will be slightly off topic, I apologize, but since it relates to postdoctoral researchers at UC, I’m guessing it will be useful information for some of the readers.
The labor union UAW is trying to establish a postdoc union at UC, but some postdocs are not happy because of what they see as fraudulent behavior on the part of union reps in collecting authorization cards. Basically, they say many postdocs were tricked (by lying, etc) into signing cards, without knowing what they are doing. Now a decertification effort is under way at http://ucpostdocs.wordpress.com/ Even if you are supporting the union, it’s worthwhile to check it out.
OK, stupid question time:
What does flat mean?
I have been saying this for a long time. I said it in grade school, but no one listened. I said it in Middle school, ignored. I said it in high school, and I got lectured by Mrs. Murphy. The universe is a black hole, everyone says. Well stop getting your science from Flash Gordon comic books.
I’m glad people are finally willing to listen.
That is all,
Dr. K
http://entropy2.com/madscientist/
Sean sez: …”the de Sitter horizon is different for each observer”.
I thought the CC was a Lorentz invariant quantity ?
Can someone then explain why if ALL observers measure the same CC, & then calculate the horizon radius:
RdS =~ sqrt{3/lambda}
Seems like they should all agree.
Apart from size, how else can one differentiate horizons ?
@Jimbo#35:
The horizon radius is the same for all observers, but the location of the horizon is not. That is what Sean meant. If I am slightly to the left of you, my horizon will be slightly to the left of your horizon.
Craig Overend wrote:
I’m not a fan of “time-reversed” black holes, unless you’re a time traveler. I’d rather people called them white holes and be done with it. Time marches forward.
“Time reversed” doesn’t have anything to do with time travel, system A can be said to be a time-reversed version of system B (and vice versa) if a movie of system A’s behavior evolving forward in time, when played backwards, would look just like a movie of system B’s behavior evolving forward in time. For any system A that obeys time-symmetric laws (such as general relativity), the laws of physics should permit such a system B whose forward evolution looks just like a backwards version of A, though due to the thermodynamic arrow of time system B might be terrifically unlikely.
So the universe is not a black hole?
This insight, plus four dollars or so, will get you a grande latte at Starbucks
This sounds like an argument over definition.
Two things:
1) I give quite a few public talks on cosmology, and the issue of a universe being infinite is a constant source of confusion. I try and explain that if it is infinite now, then it was always infinite, right from birth, and that the divergence is in the density of “stuff”. They counter with the fact that someone told them the universe used to be the size of a beach ball, and then discussions of the difference between the universe and observable universe ensure. I hope that by the end of it all I have showed them what modern cosmology is about, but I might also just leave them with the idea that cosmologists are nutters.
2) Cosmic horizons – Just done some work on this, where someone has tried to claim something important about the “cosmic horizon”. In a shameless attempt of publicity, you can read the paper here;
http://arxiv.org/abs/1001.4795
Through the Looking Glass: Why the “Cosmic Horizon” is not a horizon
>> Time marches forward
Actually – it doesn’t. *You* have the choice to define which direction proper-time increases along a world-line – this is the part students hate.
A black hole has no hair. Some mass outside our Hubble horizon is pulling galactic clusters in its direction, and masses near our horizon are undoubtedly affecting masses beyond it. My horizon is slightly different from yours. A BH has only one or two distinct horizons.
Hi Sean,
What do you think about the following argument. Suppose our universe was a black hole. Then it would have a singularity. This singularity would be a “preferred” region of space and thus violate the Cosmological principal. Therefore our universe is not a black hole.
@ Igor #37
Immediately clear, & thanx !
Can you tell me what SHAPE our universe is?
Lab Lemming wrote:
What does flat mean?
“Spatially flat” means that the geometry of space matches 3D Euclidean geometry: a pair of straight lines can have a constant distance between them if they’re parallel, the angles of the corners of a triangle always add up to 180 degrees, etc. If the universe had positive spatial curvature, it would be the 3D analogue of the curved 2D surface of a sphere (the 2D analogy is helpful since we can’t actually visualize a curved 3D surface), where “straight” paths (in the sense that they are the paths with the shortest possible distance between any two points they cross, i.e. great circles) will always cross at two different points, and the angles of a triangle made out of straight paths will add up to more than 180 degrees (see the diagram of triangles on different surfaces here). Finally, negative spatial curvature would be the 3D analogue of the curved 2D surface of a saddle shape, where “straight” paths that started off parallel would diverge, and the angles of a triangle would add up to less than 180 degrees (again see the previous link). For some 2D diagrams of surfaces with positive, negative, and flat curvature, see this image from the WMAP FAQ along with this one, and maybe the top one from part 3 of Ned Wright’s cosmology tutorial.
It seems clear that the universe is not a black hole, but could there be a black hole a the center of the universe?
It seems clear that the universe is not a black hole, but could there be a black hole a the center of the universe?
The universe isn’t thought to have a center according to the most popular model of an expanding universe. See here for more.
I’m curious what Sean thinks about the possibility of the “Big Crunch.” Does it violate his ideas about increasing entropy? My understanding as an undergraduate was that current evidence points away from the big crunch, but that there was enough uncertainty in the data that it couldn’t be rejected outright.
>That insight, plus four dollars or so, will get you a grande latte at Starbucks.
Excellent! Now I know I should be reading this website more often 🙂
And thanks for that little calculation with Friedmann. For those of us who have not studied general relativity (but who do know what a manifold is..etc) these things are very exciting.
>Note that a spatially flat universe remains spatially flat forever..
So the universe as a whole is spatially flat, but small parts of it are not?