Darren New <dne### [at] sanrrcom> wrote:
> Warp wrote:
> >   1) Spacetime inside an event horizon is pretty weird, and the spacetime
> > we currently reside in isn't (relatively speaking, at least).

> It's only weird if the event horizon is small. A galaxy-sized black hole, 
> you could fall into and not notice.

  I have hard time believing that. Space is extremely warped close to the
event horizon of a black hole. For example, at an altitude (from the central
singularity) of 3/2 of the Schwarzschild radius there exists the so-called
photon sphere: This is an area around the black hole where a photon could
orbit the black hole if it was traversing perfectly parallel to the "surface"
of the event horizon. If you were at that distance from the black hole,
it would look like the event horizon is a plane that occupies half of the
Universe. That's how warped the space is close to the event horizon.

  If you get closer to the event horizon than that, the event horizon will
look like it's curling up, engulfing you, and the rest of the Universe
would look like it would get compressed into an ever-shrinking circle
directly above. (When the circle shrinks to zero size, it means you have
touched the event horizon and are forever doomed. From this point forward
every direction points to the singularity. It's like the singularity would
surround you from all sides.)

  And this is completely independently of how big the black hole is.

  If the black hole is rotating (as most/all of them are assume to be),
it gets even weirder. There are now (AFAIR) two photons spheres of
different shapes, and additionally there's a space outside the event
horizon called an ergosphere which has weird physical properties. If
you get inside this ergosphere you will be dragged at a velocity larger
than c (relative to the rest of the Universe) around the black hole, so
I'm assuming it would look really, really weird.

  So I really don't buy the whole "you wouldn't even notice crossing the
event horizon". Maybe if you are completely blinded, you wouldn't (I really
don't know if there are any physical consequences of being close to the
event horizon of a very large black hole).

>  And it's not like you can get outside 
> the event horizon of the universe.

  Since the Universe is expanding faster than c, there's a so-called
cosmological horizon which limits the size of the observable Universe
(observable to us, of course; the observable Universe would naturally
be different in an alien planet on the other side of the Universe).

  However, this horizon is caused by the expansion of the Universe, not
by gravity, and it's always relative to where the observer is to begin
with.

> >   2) All spacetime geodesics inside an even horizon point towards the
> > singularity. (Ok, in a rotating black hole it's more complicated than
> > that, but in principle I suppose it's the same.) We are not moving towards
> > a singularity; we are expanding, hence moving *away* from any possible
> > "central point" of the Universe. That kind of contradicts the idea.

> Unless there'a big crunch? :-)

  Current observations seem to indicate that there won't be.

> >   Of course I am no physicist, and I have zero knowledge of the GR
> > equations, so I could be completely off track with this. My point is,
> > however, that I just don't understand how that hypothesis could be even
> > worthy of consideration.

> I think it was more popular before people found dark energy and thought we 
> were heading for a "big crunch" or something?

  I think the word "found" is wrong here. It hasn't been measured; it's a
hypothesis. (It could perfectly well be the *correct* hypothesis, but until
it's measured it's just that.)

-- 
                                                          - Warp

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