Is there a possibility of us being in a black hole?

[culov]

What exactly tells us that we arent in a black hole? No one has ever seen the milky way “galaxy” from outside. Light is allowed to enter, but are we SURE that light can escape? Would the pull from outside the black hole have an effect on us (assuming earth is near the center of the black hole) that would not allow our existence? Im pretty sure that my “theory” is wrong, but i couldnt prove myself wrong. Someone want to help out?
Thanks,

culov

[jbird3000]

If our light isn’t escaping, where is it going?

[culov]

Well where does the light go when it goes past the event horizon?

[Exapno_Mapcase]

In fact, there have been many calculations (and you can find them here on the SDMB if you search) that if you look for the size of a black hole with the mass of the known universe, you get the size of the known universe.

Does this mean we are inside a black hole? Probably not. It just indicates how little we understand about what would lie inside the event horizon of a black hole that we can’t rule it out completely.

[culov]

Would you happen to know any of the threads that the calculations were done in? I searched for 10+ strings and found nothing.
thanks again,
culov

[Chronos]

The Universe is a black hole iff it will eventually recollapse in a Big Crunch, in which case the Crunch singularity is the center of the black hole. But this does not currently appear to be the case: First of all, the Universe is currently thought to be flat, which means that with no other effects, it would just barely expand forever. Secondly, we have a fairly significant cosmological constant, which is causing the Universe to not only expand, but to expand faster and faster as time passes.

The last time we discussed this was in Comments on Cecil’s Columns, to my knowledge.

[CurtC]

Scientific American’s Ask The Experts recently handled a similar question, and the fourth expert answer more directly addresses the OP. It says:

A black hole is a local region from which light cannot escape. It has a boundary called the event horizon. Inside the event horizon, light cannot escape to infinity, whereas outside the event horizon, light can escape to infinity if it is traveling in the right direction. Even outside the event horizon, however, light that travels straight in toward the black hole will not escape.

In contrast, the universe is thought to be homogeneous and isotropic. Isotropic means that all directions appear the same; this property of the universe is well established by observations that show the effective temperature of the cosmic microwave background is identical in all directions to one part in 100,000. Homogeneous means that any place in the universe is equivalent to any other place. We can observe the universe from only one position, of course, but it does appear to be homogeneous on very large scales, after smoothing over the stars, galaxies, clusters of galaxies and superclusters.

A homogeneous space cannot have a boundary, so there can be no event horizon. And the future behavior of light rays cannot depend on their directions in an isotropic space. Thus, a homogeneous and isotropic universe is not a black hole. The universe does have one similarity to a black hole: light cannot escape from it. But this is true for any place in the universe and for light traveling in any direction, unlike the case for a black hole.

[Chicago_Faucet]

I consider it a possibility, since no one can explain as of yet why 99.9 percent of the universe is made up of Black Matter, but we have yet to observe any. Maybe they are actually observing the Event Horizon from the inside?

[Bippy_the_Beardless]

Inside a black hole’s event horizon, is it possible to move away from the singularity and towards the event horizon? Or does all such movement require infinite energy?
If all movement away from the singularity and towards the event horizon requires infinite energy, then wouldn’t there exist a ‘direction’ in which those inside a black hole cannot go?

[MC_Master_of_Ceremonies]

*Originally posted by Bippy the Beardless *
**Inside a black hole’s event horizon, is it possible to move away from the singularity and towards the event horizon? Or does all such movement require infinite energy?
If all movement away from the singularity and towards the event horizon requires infinite energy, then wouldn’t there exist a ‘direction’ in which those inside a black hole cannot go? **

Ignoring Kerr-Newman blackholes as they are more complex and have two event horizons (IIRC in these it is possible to orbit within the evnt horizon) in the Scwarzchild solution you must always be moving towards the singularity once you are in the event horizon, which you will meet within a finite period of time (the last stable orbit is the photonsphere at 1.5 * the Scwarzchild radius).

[bryanmcc]

Also note, however, that within a blackhole, time becomes a spacelike dimension – that is, the singularity lies inexorably in ones future. So, if the Universe were a black hole, the Big Crunch at the end would be the singularity in the inescapable time direction – we must move forward in time towards the singularity, although we’re still free to move in space. (This is just what I recollect off the top of my head. Let one of these General Relativity people expound (or correct) this.)

On a related subject, I remember that Chronos once remarked that the way to avoid the singularity the longest was to fall straight in. Now, intuitively, it seems that the way to remain outside the singularity for the longest period of time is to thrust engines with maximum power directly away from the singularity. However, I will be the first to admit that General Relativity is nothing if not subtle and complicated. So would anyone care to expound on why falling straight in takes longer than trying to escape?

[msmith537]

*Originally posted by bryanmcc *
On a related subject, I remember that Chronos once remarked that the way to avoid the singularity the longest was to fall straight in. Now, intuitively, it seems that the way to remain outside the singularity for the longest period of time is to thrust engines with maximum power directly away from the singularity. However, I will be the first to admit that General Relativity is nothing if not subtle and complicated. So would anyone care to expound on why falling straight in takes longer than trying to escape? **

I don’t know but tidal forces would rip you apart long before you could find out.

[Omphaloskeptic]

*Originally posted by bryanmcc *
On a related subject, I remember that Chronos once remarked that the way to avoid the singularity the longest was to fall straight in. Now, intuitively, it seems that the way to remain outside the singularity for the longest period of time is to thrust engines with maximum power directly away from the singularity. However, I will be the first to admit that General Relativity is nothing if not subtle and complicated. So would anyone care to expound on why falling straight in takes longer than trying to escape?

The problem is that you can’t point your engine “directly away from the singularity” once you pass the horizon. The singularity is in the future; you’d have to point your engine in the direction of the future, to push you toward the past. For an analogy, which direction would you point your rocket in if you wanted to avoid next week?

The reason any attempts at escape only quicken the approach to the singularity is that the proper time along a stationary trajectory is longer than the proper time for any moving trajectory between the same two points, because of time dilation (recall the Twins Paradox).

[Achernar]

*Originally posted by msmith537 *
I don’t know but tidal forces would rip you apart long before you could find out.

Not for a black hole the size of the galaxy they wouldn’t. However, I think the answer to the OP is no, and there are no such black holes.

Now, there is a possibility that the Schwarzschild radius of the universe is close to its actual radius, so in that sense the universe could be a black hole. However, usually when we talk about the properties of black holes, we’re using the assumption that it’s a singularity embedded in an asymptotically flat external medium. But ha, what medium is the universe embedded in? So you can’t just take what you know about stellar black holes and apply it to the universe.