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View Full Version : Is space expanding near and into a black hole?



tommac
2008-Apr-14, 06:17 PM
I was thinking a little bit about the expanding universe. At one point there is belief that the universe is expanding faster than the speed of light into the not visible universe.

I thought of what else in our universe reacts similarly. A black hole also receedes space-time in a seemingly similar fashion.

So then I streched my very limited understanding of such things a bit to back holes are also expanding the universe. In fact that as we get close to black holes space gets larger ( as space-time compresses ) and going into the black hole it is expanding at such a rate that we cant see it expanding.


Hmmm ... then as I think this through again I am confused a bit more ...
As my ruler compresses the rest of space looks larger and expands away from me

The closer I get to the event horizon the less and less space I can visibly see. this effect would be from the fact of "universal expansion" as parts of the universe receed at near the speed of light and now I am receeding the visible universe will change for me. It will still appear the same distance from me but as I move in distant galaxies will go out of range and other closer galaxies will move towards the horizon, close objects will appear larger but farther. Peculiarly things in the direction of the BH will start to become visible??? As my time is starting to receed closer to the speed of light as to balance of its recession of space time.

When I pass through the event horizon ( will I notice?? ) things on the inside of the BH will become visible, small particles will seem large, I dont think that I would notice passing through the event horizon and in fact I think I would just continue to see an event horizon as I get smaller and smaller and my space-time receeds at rates similar to parts of the black hole.

By this I would think that there is nothing special about the event horizon of a black hole other than the fact that it is the point that space-time is receeding relative to us with the speed of light. A distant black hole ( in a quickly receeding galaxy ) would seem to have an enormous event horizon.


Well I think this became a random bable session. Sorry :confused:

Steve Limpus
2008-Apr-14, 08:30 PM
I don't know if this helps but you could consider that space is observed to be expanding only on the largest scales, between large groups of galaxies. Near a black hole it's probably quite safe to completely ignore the effects of expansion because they are overwhelmed by the other forces such as gravity. The same thing is happening here in our Solar system where the other forces (gravity, nuclear etc) hold everything together, the sun, the planets, us.

In fact, if you read some of the other threads around here, not all of which I can follow that well either, very knowledgeable people will suggest that 'expanding space' is just one way to think about what we observe - they'll refer to things like co-ordinate systems and relativity.

Having said that, using the popular mainstream descriptions seems to suffice quite well for most discussions. It's all fascinating if you ask me. Googling and Wiki-ing help too.

Stick at it. :)

tommac
2008-Apr-14, 09:36 PM
The point is that the expansion of the universe is just like a black hole. Except one is in a point ( singularity ) and one is the opposite of a point ( maybe ??? ) of being all over.

Maybe the universe is like a funnel that wraps around on itself:
Expanding
~\ /~
| \ / |
| \ / |
| | | |
----- -----
Contracting

Maybe it is a black hole spewing out regurgitated matter ( really a white hole / black hole combo special ) Where as the matter gets spews out or sucked in either of the openings at the speed of light and thrown into universal orbit and either sucked back into the black hole or emmited out into eternity.

Jeff Root
2008-Apr-15, 12:28 AM
tommac,

Some of the questions you have asked seem to be inspired by comments I
have made in posts here. (Whether they actually were or not doesn't matter.)

My inexpert impression is that as a star collapses to form a black hole, the
spacetime that the collapsing matter occupies stretches in the time direction.
The black hole gets "deeper" and "deeper" as the matter collapses further
and further, and that collapse has no end -- although it very rapidly reaches
a size which is effectively a point in space and endless in time. Anything
falling into a black hole -- including the matter which forms the hole in the
first place -- is stretched out in the radial direction and squeezed in the
circumfrential direction: It's spaghettified. The atoms or subatomic particles
are pulled apart from from each other, resulting in a line of particles falling
toward the singularity at the center of the black hole, with the particles
getting farther and farther apart, even as the density of the matter at the
center increases without limit! Whether individual subatomic particles can
also be torn apart in this way, I can't guess.

So I think the space is stretching at the singularity, though not elsewhere.

Ken G or publius may disagree with my impression. If so, their opinions are
the safer bet.

-- Jeff, in Minneapolis

Steve Limpus
2008-Apr-15, 01:02 AM
I would say the interesting link between big bang cosmology and black holes is the singularity and event horizon. I came across this page (http://abyss.uoregon.edu/~js/ast123/lectures/lec17.html) where they invoke the idea that nature constructs event horizons to prevent the observation of singularities:


"Cosmic Singularity:

One thing is clear in our framing of questions such as `How did the Universe get started?' is that the Universe was self-creating. This is not a statement on a `cause' behind the origin of the Universe, nor is it a statement on a lack of purpose or destiny. It is simply a statement that the Universe was emergent, that the actual of the Universe probably derived from a indeterminate sea of potentiality that we call the quantum vacuum, whose properties may always remain beyond our current understanding.

Extrapolation from the present to the moment of Creation implies an origin of infinite density and infinite temperature (all the Universe's mass and energy pushed to a point of zero volume). Such a point is called the cosmic singularity.

Infinites are unacceptable as physical descriptions, but our hypothetical observers back at the beginning of time are protected by the principle of cosmic censorship. What this means is that singularities exists only mathematically and not as a physical reality that we can observe or measure. Nature's solution to this problem are things like the event horizon around black holes. Barriers built by relativity to prevent observation of a singularity."

Profound I thought...

And I suppose you could say the Big Bang has two event horizons, one at the singularity and one at the current particle horizon. Or would you same it's the same event horizon at different times?

mugaliens
2008-Apr-15, 03:45 AM
Excellent question! From what I understand of space-time, it all gets back to Planck lenth (http://en.wikipedia.org/wiki/Planck_length)and Planck time (http://en.wikipedia.org/wiki/Planck_time).

In both, the speed of light, c, is a factor (in the denominator). For length, it's the square root of c^3, for time, it's the square root of c^5.

The deeper you get into a gravity well, time slows down, as does the speed of light. Thus, since it's in the denominator, as the speed of light slows down, Planck length and Planck time increase.

Planck length, by the way, is defined as the distance it takes light to travel in one unit of Planck time.

Since Planck length's c denominator is only raised to the third power under a square root, and Planck time's c denonminator is raised to the fifth power under a square root, the further you get into a gravity well, as c is reduced, it increases Planck time faster than it increases Planck length.

Thuse, space-time is warped, but the time increases faster than the length, and as an object falls towards a gravity well, it accelerates and time slows down.

Occams Ghost
2008-Apr-20, 10:50 PM
The laws of physics are obsolete when considering any outside to the universe. Why? Because there isn't any spacetime outside the universe, so everything that counts remains in spacetime.

Hopefully this is starting to catch on...

tommac
2008-Apr-21, 03:34 AM
Does this effect also effect gravity? since gravity has both a time and space component? So does gravity effect the observed gravity relative to something being pulled by it? Can something appear to be being pulled by gravity more strongly than it appears to be being pulled?

I would think that as time slows down this would be true.


Excellent question! From what I understand of space-time, it all gets back to Planck lenth (http://en.wikipedia.org/wiki/Planck_length)and Planck time (http://en.wikipedia.org/wiki/Planck_time).

In both, the speed of light, c, is a factor (in the denominator). For length, it's the square root of c^3, for time, it's the square root of c^5.

The deeper you get into a gravity well, time slows down, as does the speed of light. Thus, since it's in the denominator, as the speed of light slows down, Planck length and Planck time increase.

Planck length, by the way, is defined as the distance it takes light to travel in one unit of Planck time.

Since Planck length's c denominator is only raised to the third power under a square root, and Planck time's c denonminator is raised to the fifth power under a square root, the further you get into a gravity well, as c is reduced, it increases Planck time faster than it increases Planck length.

Thuse, space-time is warped, but the time increases faster than the length, and as an object falls towards a gravity well, it accelerates and time slows down.