I was watching a video by James Beacham, a particle physicist at CERN who has given a number of interesting talks, including at the famous Royal Institution in London. At one point he spoke about the calculated size of the event horizon of a black hole containing the entire mass of the observable universe, for which we have a rough approximation. That size, he said, is greater than the current size of the observable universe.
To my simple mind this seems hugely counterintuitive, if not downright impossible. I thought I must have misunderstood him but he uses this to suggest the interesting hypothesis that the entire universe may, in fact, be a black hole. Google is failing me, although there are some postings on Quora that seem to confirm Beacham’s size estimate.
NASA states that findings from WMAP provide an estimate of an average mass density for the universe of 9.9 x 10-30 g/cm3, which is equivalent to only 5.9 protons per cubic meter. This makes sense – space is very, very empty. So it totally baffles me that if all its matter (including dark matter and dark energy) was gravitationally collapsed to a singularity, the EH would extend out to approximately the Hubble sphere, or the limits of the observable universe. It’s a fascinating concept but it seems to make no sense. After all, the earth is vastly denser than the average density of space, and its total mass would create a black hole about the size of a marble!
So that’s my question. Not to confuse the central question, but just as an aside, one might ruminate how it is that, if we’re inside a black hole, we’re not inexorably falling toward the singularity. But perhaps we are. One might recall that the Einstein field equations for the spacetime geometry inside a black hole suggest that the time dimension and the radial spatial dimension change places. Inside a black hole, one can hypothesize that there is no escape from the singularity because the singularity is your future, and the direction to “outside” the black hole is no longer a spatial direction. And there is indeed a dimension in our universe that inexorably pulls us in one direction only, and that dimension is time.
I’ve wondered that, too, but there are two things that could prevent the collapse off the top of my head, not being an astrophysicist:
– The components outside the visible universe could be exerting an equal pull on the other side, preventing either side from forming a black hole. As I understand it, this is what prevented the early universe from immediately coalescing into lots of tiny black holes, since its density was even greater than now, yet there was no clear path in which any one black hole should form preferably to any others. But I’ve only read a few internet posts about that so I could be wrong.
– Dark Energy. At the edge of the visible universe, the universe itself is flying away from us at relativistic speeds due to dark energy. This can’t help but increase the force needed create a black hole, especially if it is just barely at the edge of collapsing.
To be clear, I don’t think you’re addressing quite the question I asked. I’m not asking why the universe is expanding rather than gravitationally collapsing. There are many hypotheses about that. My question is basically about what the size of a black hole would be if it encompassed the entire mass of the observable universe. There’s a fairly simple formula for the Schwarzschild radius but I’m not sure I understand all the subtleties involved in the appropriate value for M (or for that matter, the appropriate criteria for the radius of the observable universe).
Using the estimate for the Observable Universe off of Wikipedia and this calculator. I get a Schwarzschild radius that’s a factor of five smaller than the observable universe. That’s just the ordinary matter though and as we all know that’s a small fraction of the total energy, so I expect the physicist is correct.
That’s for one particular definition of the radius of the universe though. If we apply our everyday definition for simultanetiy to the whole universe the “current diameter” of what we can currently observe (a lot of which consists of very old light) is much much larger and I’d guess we’re back out of black hole territory again.
Black holes do not all have the same density. The radius of a black hole is directly proportional to its mass, but its volume is proportional to radius cubed. Thus, the density of a black hole will scale as 1/m^2. For a very, very large mass, a black hole would have a very, very low density.
Now, as for the Universe: Back before we knew about dark energy (or the cosmological constant or whatever you choose to call it), the big debate was whether the Universe would continue expanding forever (a situation comparable to being above escape velocity), or if it would eventually reach a maximum size, and then start contracting, eventually ending in a Big Crunch. We didn’t know the density or expansion rate precisely enough to say. Well, the condition for the Universe to be a Big Crunch universe is precisely the same as the condition for it to all be a black hole. The Big Crunch singularity is the black hole singularity, and we’re all inexorably falling towards it. If we’re dense enough, at least, which wasn’t known.
That’s if there were no dark energy. But there is, and that complicates things, to say the least. We can write the equations for the Schwarzschild-deSitter metric (that describes a black hole in a dark-energy universe), but there’s a lot about it that we don’t understand, especially in the case where the black hole is very large. We probably oughtn’t to even use the term “black hole” for such a situation, because it probably behaves very differently, in some important ways, from what we think of as a black hole (in an otherwise-vacuum universe).
There are a couple of issues here, what with space expanding, but (very naively) if you take the obsverable universe to be a sphere with radius 4.4 × 1026 m and volume 3.6 × 1080 m3, and a rough WMAP density of 9 × 10-27 (for a flat Friedmann universe), then that gives a mass/energy of about 3 × 1054 kg or a Schwarzschild radius of 5 × 1027m, which checks out (at least an order of magnitude) with the radius of the observable universe, (as it should!?) but that does not mean it is a black hole or that there is not plenty of unobservable universe.
Thank you all for the thoughtful replies. Gives me lots to think about.
For anyone interested, this is the James Beacham lecture at the Royal Institute in which this came up. If you fast-forward to around 22:00 he leads up to the aforementioned conclusion in several minutes.
I think the idea that our universe is one black hole is called “black hole cosmology”.
IIRC this theory, or one flavor of it, says that there is an overall universe having 11 orthogonal physical dimensions. Black holes in that universe become simpler island universes inside, having 9 orthogonal physical dimensions. There are successive child universes each having 2 fewer dimensions than the one spawning it. Black holes in our universe are reduced to 1 dimension inside (radius).
Anybody who can correct or enhance this, please do!
The other side of a Black Hole’s Event Horizon is in accessible due to extreme gravity rendering escape velocity impossible, due to the finite speed of light.
The Hubble Sphere is a kind of event horizon due to the expansion of of the universe at or exeeding the speed of light measured relative to us at the “center” of the sphere we perceive as the Universe.
If the Hubble Sphere IS in fact a black hole event horizon…and , thus, the universe is in fact a black hole…what extreme form of cosmological topology allows for universe-scaled spherical concepts of “Inside” and “Outside” to switch?
Are you sure you are remembering this right? Do you have a reference? You would need some 11-dimensional theory of gravity, and some sort of geometry (including black holes) where the number of dimensions changes across an event horizon. Note that in our universe, a hole described by relativistic geometry has 4 dimensions, as usual. Indeed, the classical description has you not locally noticing anything different if you fall into a sufficiently large black hole.
It does in fact seem to be true that a 3+1 dimensional black hole somehow retains all of its information at its 2+1 dimensional horizon, and in fact this was the inspiration for the holographic universe models. Does this mean that a black hole is in some sense “really” one dimension lower than the spacetime it finds itself in? I dunno; define “really”.
There’s definitely a theory or group of theories about our universe, or universes in general, being the interiors of black holes nested in another higher universe or multiverse. There’s a short article about “black hole cosmology” on Wikipedia for a start.
And there’s M theory which involves superstrings and 11 dimensions.
I’m trying to find something about the 11 - 9 - 7 - 5 - 3 - 1 dimensional evolution and not finding it. I had saved articles by Max Tegmark and the like, which I thought had something to do with it, but can’t find where they do.
So, no, not at all sure I’m remembering this right. But there’s definitely some related germ of thought that I didn’t just imagine…
The only model where our universe is embedded into a higher dimensional reality that readily comes to mind is the brane-world scenario, which comes in a couple of different flavors, of which the Randall-Sundrum version is probably the most well-known. But we’re not living in a black hole in any version I’ve heard of.
A black hole does play a distinguished role in the AdS/CFT-correspondence, which features a higher-dimensional dynamic ‘bulk’ spacetime emergent from an ordinary quantum field theory living on the boundary, thus giving a particular window into quantum gravity. But that scenario isn’t directly relevant to our universe, since the resulting spacetime (an Anti-de Sitter space) has a cosmological constant with the opposite value we observe.
The dimensions you cite are somewhat reminiscent of the sequence of dimensions in which a classical supersymmetric theory is possible, 2 + 1, 3 + 1, 5 + 1, and 9 + 1, with the latter case being relevant to string theory.
Finally, there’s Lee Smolin’s ‘cosmological natural selection’, where new universes are birthed from parent universes via black holes. The theory was proposed to explain the apparent fine tuning of physical constants: if there’s such a mechanism, then universes should be most highly prevalent if they produce lots of black holes, thus essentially ‘selecting’ favorable values for physical constants.
