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PeterR280
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Reged: 05/27/13

A Very Large Black Hole
      #6194777 - 11/14/13 03:09 PM

There have been some discussions on GR and galaxies, which prompted this question.

Mass increases with the cube of the radius of a spherical object while the Schwartzschild radius is proportional to the mass of the object. So for very large black holes the density can be quite low. The universe is estimated to have a Schwarzschild radius of about 10 billion light years by some.

Based on a quick calculation, (please correct me if I made an arithmetic error) a large spherical "galaxy" with a density of one solar mass per cubic light year would lie within its Schwartzschild radius if it were about 200 million light years across. It can't be made stable and would eventually collapse but it would take a while.

The question is, if the solar system were one of the stars, and an object from the outside fell in, What would you see? How would it appear? Bear in mind, even though you would see a boundary from the outside, there would not be a visible boundary from the inside.


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GregLee1
professor emeritus


Reged: 07/21/13

Loc: Waimanalo, HI
Re: A Very Large Black Hole new [Re: PeterR280]
      #6194952 - 11/14/13 04:39 PM

I happened across this NASA Q&A thing, which seems relevant to your question. Jonathan Keohane is answering a question about why the early universe didn't form a black hole and cease to exist. He gives an easy answer and a hard answer. The easy answer, if I've got it straight, is that the early universe and the present universe did form a black hole, and we're in it right now:

Quote:

Why then was the early Universe not a black hole? Well, lets figure out its Schwarzschild radius to get a basic rule of thumb idea of what is going on.

Rsch = 3km x Mass of the whole Universe in solar masses
= about 10 to 100 billion light years
= about the current size of the whole Universe

So, in the basic definition of a black hole I used above (where the size of the object is smaller than the Schwarzschild radius) the whole Universe is one big black hole with us on the inside.




The hard answer he gives, however, seems not to answer the question.


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PeterR280
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Re: A Very Large Black Hole new [Re: GregLee1]
      #6194956 - 11/14/13 04:42 PM

We are probably in a black hole. The question would be the same. If there was something outside the universe that fell in, what would you see?

We just don't know if there is anything outside the universe.


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maugi88
Carpal Tunnel


Reged: 08/25/13

Loc: SE MN
Re: A Very Large Black Hole new [Re: PeterR280]
      #6195295 - 11/14/13 07:25 PM

Quote:


The question is, if the solar system were one of the stars, and an object from the outside fell in, What would you see? How would it appear? Bear in mind, even though you would see a boundary from the outside, there would not be a visible boundary from the inside.




If its true, we are inside a universal black hole. Maybe we have already seen it in the deep field. Maybe the smeared galaxies that are attributed to gravity lensing of dark matter is actually a galaxy as it looks after just entering our black hole. Maybe, just a thought.


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llanitedave
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Re: A Very Large Black Hole new [Re: PeterR280]
      #6195702 - 11/14/13 11:17 PM

Quote:

We are probably in a black hole. The question would be the same. If there was something outside the universe that fell in, what would you see?

We just don't know if there is anything outside the universe.




I expect that anything that fell through the event horizon would be approaching the speed of light at that moment. It would also be smeared by any lateral component of its motion or by the black hole's rotation.

So, I would guess that if we are in a black hole, and there are objects falling though the event horizon, that we would perceive them only as a diffuse glow of electromagnetic radiation.

Of course, if we are inside a black hole, I don't see how we can avoid being sucked into the singularity ourselves, so I think the whole question is moot.


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PeterR280
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Re: A Very Large Black Hole new [Re: llanitedave]
      #6195714 - 11/14/13 11:23 PM

there would be no singularity for a while. The density of the matter inside the large black hole would be diffuse. The universe doesn't have a singularity. (Actually there are many singularities in the universe.) I also think the matter falling in would become diffused.

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llanitedave
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Re: A Very Large Black Hole new [Re: PeterR280]
      #6195764 - 11/15/13 12:02 AM

Good point. So, maybe the perceived acceleration of cosmic expansion is really just us accelerating towards a singularity doom?

But anyway, even if the internal density of a black hole is low, I don't know of any theoretical treatment that allows light to travel freely in all directions inside it. And that's what appears to be happening in our neighborhood.


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PeterR280
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Re: A Very Large Black Hole new [Re: llanitedave]
      #6195774 - 11/15/13 12:09 AM

I think it's due to the size of the universe. It expanded faster than light so the visible universe doesn't include all the light. I think it's a matter of time for the doom.

When I was in school the professor was explaining how the sun would become a red giant and engulf the earth in about 4 billion years. One of the students who was half asleep asked "how long?" The professor answered 4 billion years. The student said " Thank god, I thought you said 4 million years."


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maugi88
Carpal Tunnel


Reged: 08/25/13

Loc: SE MN
Re: A Very Large Black Hole new [Re: PeterR280]
      #6195990 - 11/15/13 06:22 AM

Quote:

I think it's due to the size of the universe. It expanded faster than light so the visible universe doesn't include all the light. I think it's a matter of time for the doom.

When I was in school the professor was explaining how the sun would become a red giant and engulf the earth in about 4 billion years. One of the students who was half asleep asked "how long?" The professor answered 4 billion years. The student said " Thank god, I thought you said 4 million years."




That's Hilarious


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deSitter
Still in Old School


Reged: 12/09/04

Re: A Very Large Black Hole new [Re: maugi88]
      #6196192 - 11/15/13 09:48 AM

If you want to think this way - I remember doing this simple calculation as a kid - since by definition you cannot escape the universe (if you exist, you are in the universe), the whole thing *must* be a black hole, with the redshift horizon as its Schwarzschild radius. (Everything beyond the redshift horizon is not in causal contact with us.) This lets you *calculate* the required density. It turns out to be right on the money

I was once told you couldn't think this way. But that never stopped me!

-drl


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deSitter
Still in Old School


Reged: 12/09/04

Re: A Very Large Black Hole new [Re: PeterR280]
      #6197935 - 11/16/13 10:48 AM

Quote:

We are probably in a black hole. The question would be the same. If there was something outside the universe that fell in, what would you see?





Things that are beyond the redshift horizon are really beyond it. Nothing will be appearing over it. No matter how you interpret it, as (really ill-defined) spatial expansion, or as the effect of (precisely defined) conformal geometry, the redshift horizon is an absolute limit that defines everything that can possibly be causally connected to us (a fancy way of saying visible).

-drl


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Andy Taylor
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Re: A Very Large Black Hole new [Re: deSitter]
      #6198313 - 11/16/13 03:05 PM

Quote:

If you want to think this way - I remember doing this simple calculation as a kid - since by definition you cannot escape the universe (if you exist, you are in the universe), the whole thing *must* be a black hole, with the redshift horizon as its Schwarzschild radius. (Everything beyond the redshift horizon is not in causal contact with us.) This lets you *calculate* the required density. It turns out to be right on the money

I was once told you couldn't think this way. But that never stopped me!

-drl




No further questions required...

That just about sums it up for me.


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PeterR280
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Re: A Very Large Black Hole new [Re: Andy Taylor]
      #6198347 - 11/16/13 03:27 PM

The universe doesn't have an edge so if something fell in it could appear anywhere if in fact it survived the entry and remained in one piece.

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choran
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Reged: 12/28/12

Re: A Very Large Black Hole new [Re: PeterR280]
      #6198388 - 11/16/13 03:56 PM

Confusing to me, but what else is new? If there's no edge, where/what is the boundary between "out' and "in" when something falls "in".

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llanitedave
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Reged: 09/26/05

Loc: Amargosa Valley, NV, USA
Re: A Very Large Black Hole new [Re: choran]
      #6198410 - 11/16/13 04:07 PM

If things can appear anywhere, that would imply they can disappear from anywhere too.

That would explain a couple of missing keys, at least.


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PeterR280
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Re: A Very Large Black Hole new [Re: llanitedave]
      #6198412 - 11/16/13 04:10 PM

It can't disappear. Nothing can escape a black hole.

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PeterR280
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Re: A Very Large Black Hole new [Re: PeterR280]
      #6198434 - 11/16/13 04:24 PM

I am not sure how you would map the surface of a black hole with the inside. There is a discontinuity in spacetime. If something were falling in you would see it slow down and eventually stop at the surface. Time would stop. I don't know what would happen from the falling bodies' perspective. You can also never get any information on anything going on beyond the event horizon.

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llanitedave
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Reged: 09/26/05

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Re: A Very Large Black Hole new [Re: PeterR280]
      #6198465 - 11/16/13 04:46 PM

Quote:

It can't disappear. Nothing can escape a black hole.




Tell that to my keys.


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PeterR280
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Re: A Very Large Black Hole new [Re: llanitedave]
      #6198486 - 11/16/13 04:57 PM

it could also explain the socks that disappear in my dryer.

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deSitter
Still in Old School


Reged: 12/09/04

Re: A Very Large Black Hole new [Re: PeterR280]
      #6200251 - 11/17/13 04:25 PM

Quote:

I am not sure how you would map the surface of a black hole with the inside. There is a discontinuity in spacetime. If something were falling in you would see it slow down and eventually stop at the surface. Time would stop. I don't know what would happen from the falling bodies' perspective. You can also never get any information on anything going on beyond the event horizon.




No discontinuity. The light cones are tipped over, but a horizon is a horizon, and it's all smooth.

-drl


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