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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Kenneth <kdw### [at] earthlinknet> wrote:
> Something I've always wondered about (and which I've never been able to find an
> answer to, in any layman's science book) is this: If space expands (not just the
> space *between* objects, but space itself), then at what scale, what microscopic
> level, does this process cease? If at all?

  IIRC the Universe only expands (ie. new space is formed) at intergalactic
space, not inside galaxies. The gravity of galaxies is high enough to stop
(or at least greatly diminish) expansion from happening inside them.

> If it does, then, since essentially *everything* expands equally

  Everything does *not* expand equally. Gravity works against the expansion,
slowing it down (if not even stopping it completely), AFAIK.

-- 
                                                          - Warp

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Warp <war### [at] tagpovrayorg> wrote:

>   IIRC the Universe only expands (ie. new space is formed) at intergalactic
> space, not inside galaxies. The gravity of galaxies is high enough to stop
> (or at least greatly diminish) expansion from happening inside them.
> [clip]
> Everything does *not* expand equally. Gravity works against the expansion,
> slowing it down (if not even stopping it completely), AFAIK.
>

So in other words, local gravity effects actually keep local *space* from
expanding? If so, that's a new concept for me; I thought space itself--whatever
it actually *is* ;-) --was sort of 'independent' of gravity, expanding at its
own independent pace, as its own 'entity.' Although, now that I think of it,
"gravity warps space," so what you say isn't so weird after all. Geez, that's
always been a difficult concept to grasp. But is 'new space' actually formed? Or
is it just 'stretched' (whatever *that* concept means)?

Ken

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Warp <war### [at] tagpovrayorg> wrote:

> > 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.)

Yeah; AFAIU it, 'dark matter' and 'dark energy' are somewhat ad-hoc
postulates--there was a 'hole' in the current cosmological model for awhile,
where certain observations about the universe just didn't match the model; hence
*something* must be missing from it (if it's to better match the observations.)
DM and DA would fill that hole; but whether they truly exist or not is another
matter. The observed effects could be something else entirely, something even
stranger (IMO).

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On 11/18/2010 12:13 PM, Darren New wrote:

> (OK, I see that gravity fell out very early on...)

Gravity, having fallen out in early childhood from its siblings Strong, 
Weak, and the twins Electro and Magneto, remains estranged to this day.

Regards,
John

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Warp wrote:
>   I have hard time believing that. 

It's just what I've heard. Maybe you wouldn't notice the tides if you didn't 
look out the window, is what they meant. (I think I heard it from Sagan or 
Asimov or something like that.)

Your argument is good, tho. Thinking on it, I can't imagine how you wouldn't 
notice. :-)

>   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.

I was talking about overall.  (You may be taking me too seriously. :-)

>> 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.)

For "found" read "found the affects attributed to..."

-- 
Darren New, San Diego CA, USA (PST)
   Serving Suggestion:
     "Don't serve this any more. It's awful."

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Kenneth wrote:
> DM and DA would fill that hole; but whether they truly exist or not is another
> matter. The observed effects could be something else entirely, something even
> stranger (IMO).


Well, there's really convincing evidence for DM, at least. They found two 
galaxies that had crashed together, and the stars were all jumbled up in the 
middle, but the Dark Matter kept going (as indicated by gravitational 
lensing from the DM).

Oh, there it is.  http://www.abc.net.au/science/news/stories/s1720848.htm


-- 
Darren New, San Diego CA, USA (PST)
   Serving Suggestion:
     "Don't serve this any more. It's awful."

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Warp wrote:
>   IIRC the Universe only expands (ie. new space is formed) at intergalactic
> space, not inside galaxies. The gravity of galaxies is high enough to stop
> (or at least greatly diminish) expansion from happening inside them.

FWIW, that's not what I've heard. However, I'm not sure anyone actually 
knows the answer there. :-)

-- 
Darren New, San Diego CA, USA (PST)
   Serving Suggestion:
     "Don't serve this any more. It's awful."

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Darren New <dne### [at] sanrrcom> wrote:
> Warp wrote:
> >   IIRC the Universe only expands (ie. new space is formed) at intergalactic
> > space, not inside galaxies. The gravity of galaxies is high enough to stop
> > (or at least greatly diminish) expansion from happening inside them.

> FWIW, that's not what I've heard. However, I'm not sure anyone actually 
> knows the answer there. :-)

  Thinking about it, I can think of one argument of why that would be so:
If space was expanding evenly everywhere, it would mean that there should
be an extra "force" pulling planets away from the Sun, and an even "stronger
force" pulling stars away from thir galaxy's center, and this extra "pulling"
factor would have to be taken into account in orbit equations. But is it?

  (OTOH maybe the expansion inside our solar system is so minuscule that
it basically unnoticeable and doesn't affect orbits in practice...)

  Wikipedia (the always trustworthy source) seems to confirm this idea.
For example at http://en.wikipedia.org/wiki/Metric_expansion_of_space it
says:

"Metric expansion is a key feature of Big Bang cosmology and is
modeled mathematically with the FLRW metric. This model is valid in
the present era only at relatively large scales (roughly the scale of
galactic superclusters and above). At smaller scales matter has
clumped together under the influence of gravitational attraction and
these clumps do not individually expand, though they continue to
recede from one another."

"In addition to slowing the overall expansion, gravity causes local
clumping of matter into stars and galaxies. These stars and galaxies
do not subsequently expand, there being no force compelling them to do
so."

"However the only locally visible effect of the accelerating expansion
is the disappearance (by runaway redshift) of distant galaxies;
gravitationally bound objects like the Milky Way do not expand."

  The page http://en.wikipedia.org/wiki/Hubble%27s_law could perhaps have
more on this, but I can't easily find anything to corroborate either claim,
except perhaps for:

"In systems that are gravitationally bound, such as galaxies or our
planetary system, the expansion of space is a much weaker effect than
the attractive force of gravity."

-- 
                                                          - Warp

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nemesis <nam### [at] gmailcom> wrote:
> Try sci.astro and have old wolves devour your idea.

  I'm reading some of the latest posts there. Over half of the posts are
spam (*sigh*, I still remember the time in the mid-90's when there was
virtually *no* spam in usenet, as incredible as that might sound today)
and most of the rest seems to consist of pseudoscientists writing miles-long
essays on their newest fads and denigrating established theories and known
scientists.

  I'm guessing that if I posted there, nobody would actually answer my
question and instead I would get a few totally useless "the Big Bang
theory is BS" replies.

-- 
                                                          - Warp

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