Expanding Spaces

[FreakBoy]

If I recall correctly, Brian Greene's "The Fabric of the Cosmos" he suggests that space which is lacking in large concentrations of matter would expand at a greater rate than space which is "inhabited" by matter, possibly eventually with the rate of expansion reaching the speed of light.

My father and I were throwing this idea around the other night and we're wondering what the thoughts are here on the following:

I was under the impression that current theories state that there is no empty space, simply a constant process of virtual particles coming in and out of existence with their anti-partcles and anihilating near immediately. Wouldn't this negate the possibility of space expanding at such a high rate of speed?


Ignoring the above...

A galaxy with a large enough zone of "empty" space surrounding it, if Greene is correct, could become enveloped in a region of space expanding at the speed of light and become completely isolated from the universe that had been beyond this boundary previously.

Just for fun, as long as this thought experiment isn't WAY off.... wouldn't this encapsulated galaxy then become the equivalent of a non-singularity black hole? If the boundery region of space is expanding at the speed of light, then no information would be able to traverse this boundary, or get out, including gravitational effects, so would it.... simply cease to exist/matter (no pun intended)? Or as my father calls it, a "Massless Black Hole"

Simply curious to hear thoughts.


Thanks!

 

[Badly Shaved Monkey]

I think I'm right in saying that the idea of a "Black Hole" predates the Einsteinian relativistic one that involves gravitational collapse and a singularity. I think the idea centred on a star that was so big its gravity would restrain its light rendering it invisible. I seem to recall that if the calculations of star mass were extrapolated to vast sizes this was found to be an inevitable outcome. I think it's probably incompatible with our current understanding, but can anyone fill in the details, and explain if that idea could really produce a "Black Hole" or whether, given that gravitational collapse occurs, any real-world object that could work as a black hole as originally envisaged would have to follow Einstein's rules and end up as a singularity-based black hole?

Separate zones of observable area in an inflated universe are not black holes in the sense that mass-energy falling into them cannot escape because the mass-energy can't 'fall' from one region to another if they are separating by an expansion that requires greater than light-speed to connect them.

I think I am also right in saying that the notion of our observable universe as an island in an expanding sea of receding emptiness is the wrong picture. If you were at the 'edge' of our observable universe your observable universe would extend further than we can see from our position, so there may be an infinity of contiguous universes defined by the observability from different spot but mutually unobservable. In other words, the observable Universe centred on Alpha Centaur is slight frameshifted relative to ours but by a negligible amount on universal scales, but the universe observable from a distant quasar would include areas beyond it that are not observable to us...I think

 

[FreakBoy]

...Separate zones of observable area in an inflated universe are not black holes in the sense that mass-energy falling into them cannot escape because the mass-energy can't 'fall' from one region to another if they are separating by an expansion that requires greater than light-speed to connect them.

This I understand.

Was I correct in my understanding that there could be areas of space which could conceivable reach a rate of expansion that could increase the traversable distance between two areas at the speed of light?

For example point A and point B are seperated by extremely large distances. These distances are small enough so that travel at the speed of light would eventually lead from point A to B (even at enormous lengths of time), but as neccessary for the experiment, nothing ever does. Could an enormous stretch of space start to expand fast enough to keep these two points from ever being able to commnicate or observe one another ?

I think I am also right in saying that the notion of our observable universe as an island in an expanding sea of receding emptiness is the wrong picture. If you were at the 'edge' of our observable universe your observable universe would extend further than we can see from our position, so there may be an infinity of contiguous universes defined by the observability from different spot but mutually unobservable. In other words, the observable Universe centred on Alpha Centaur is slight frameshifted relative to ours but by a negligible amount on universal scales, but the universe observable from a distant quasar would include areas beyond it that are not observable to us...I think

Agreed, I'm proposing an hypothetical situation to see if it is allowed by our current understanding of physics.

Thanks!

 

[Soapy Sam]

I'm confused by the definition of a black hole as " a volume of space time from which information cannot escape".
Surely this is a matter of parochial observer bias?

If I am in a galaxy so isolated from the rest of the universe (by inflated spacetime) that I can't get info to or from the RotU, how would I even know?

For that matter , doesn't the above description pretty well fit the universe itself?

 

[Badly Shaved Monkey]

Bearing in mind that this poster is a vet and only just capable of wiring a plug, but until a real physicist turns up...

Was I correct in my understanding that there could be areas of space which could conceivable reach a rate of expansion that could increase the traversable distance between two areas at the speed of light?

Yes

For example point A and point B are seperated by extremely large distances. These distances are small enough so that travel at the speed of light would eventually lead from point A to B (even at enormous lengths of time), but as neccessary for the experiment, nothing ever does. Could an enormous stretch of space start to expand fast enough to keep these two points from ever being able to commnicate or observe one another ?

The answer, I think, would have been no ince we are living in a post-inflation epoch, but now cosmic acceleration has been discovered I suspect that widely separated regions already receding at close to c could be rendered mutually inaccessible by that effect.

Agreed, I'm proposing an hypothetical situation to see if it is allowed by our current understanding of physics.

Thanks!

OK, given the above, if what you are suggesting is a Hard SF scenario you could invent a state in which cosmic acceleration turns out to be patchy and patches of it could suddenly blow one galaxy out of sight from another or one planet out away from its sun. Since we don't know what drives the acceleration you can invent scenarios like this that are vanishingly improbable but not actually impossible. Like...you have created a Black Hole factory to create wormholes for communication over vast distances, but there is the risk of punching through and seeding formation of a new Universe in an adjacent dimension, but by some Star Trek-style feedback process the inflation of the daughter Universe inflates the space of the factory. Like...inflationarily isolated Universes are but particles in an organised UberUniverse of which they effectively the atoms. I shall create a PayPal account to receive royalties from the book deals spawned by these ideas.

 

[Badly Shaved Monkey]

I'm confused by the definition of a black hole as " a volume of space time from which information cannot escape".
Surely this is a matter of parochial observer bias?

If I am in a galaxy so isolated from the rest of the universe (by inflated spacetime) that I can't get info to or from the RotU, how would I even know?

What you might call the Krikkit problem.

If you didn't have the physics, you wouldn't know but you might wonder about it e.g.

"in one corner of the Eastern Galactic Arm lies the large forest planet Oglaroon, the entire ‘’intelligent'’ population of which lives permanently in one fairly small and crowded nut tree. In which tree they are born, live, fall in love, carve tiny speculative articles in the bark on the meaning of life, the futility of death and the importance of birth control, fight a few extremely minor wars, and eventually die strapped to the underside of some of the less accessible outer branches.

In fact the only Oglaroonians who ever leave their tree are those who are hurled out of it for the heinous crime of wondering whether any of the other trees might be capable of supporting life at all, or indeed whether the other trees are anything other than illusions brought on by eating too many Oglanuts."


We have the physics to speculate upon these areas in more concrete terms.

For that matter , doesn't the above description pretty well fit the universe itself?

Notwithstanding the isolated observable patches of our own Universe you could extend to some idea of realms "beyond" even our inflated Universe, but there the separation would not be the real spacetime of which our Universe is made, but I suppose additional dimensions completely unnconnected with our 10, 11, 26 or whatever could exist without ever impinging on us at all, but if they are utterly excluded from ever interacting with our Universe then what can we say about them...?

 

[69dodge]

I'm confused by the definition of a black hole as " a volume of space time from which information cannot escape".
Surely this is a matter of parochial observer bias?

What do you mean?

Nothing inside a black hole's event horizon can get outside it, according to any observer.

 

[FreakBoy]

What do you mean?

Nothing inside a black hole's event horizon can get outside it, according to any observer.

From what I've read, the recent understandings regarding the standard relativity based black hole is that they DO radiate, just at extremely low tempuratures.

Could be wrong, wouldn't be my first time,

 

[Badly Shaved Monkey]

What do you mean?

Nothing inside a black hole's event horizon can get outside it, according to any observer.

I think this is a semantic not a physics problem. If you say Black Hole, capital B, capital H, it suggests an object in our space with a singularity at its heart and an event horizon, which is observer-independent. If you say that the key property of "black holes" is mutual inaccessibility even at light speed then I think that could apply to large regions without singularities that are contiguous and each observer would be centred in his own region overlapping with other observers.

Still waiting for a physicist...

 

[Badly Shaved Monkey]

From what I've read, the recent understandings regarding the standard relativity based black hole is that they DO radiate, just at extremely low tempuratures.

Yes, they give off "Hawking radiation", but the deeper problem turns out to be information loss.

http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/info_loss.html

but this may have been solved by Hawking again;

http://www.newscientist.com/article.ns?id=dn6151

 

[69dodge]

Hawking radiation has something to do with quantum mechanics. In classical relativity, black holes are totally black.

 

[Badly Shaved Monkey]

Hawking radiation has something to do with quantum mechanics. In classical relativity, black holes are totally black.

Yes, in classical relativity the event horizon is infinitely accurately located and you are either one side or the other of it. Quantum mechanics imposes uncertainty in that and creates a constant bubbling into existence of pairs of particles that appear then mutually annihilate and disappear. But some pair members disappear over the event horizon leaving the partner free in the outside universe to wander off. To an outside observer this appears to be radiation emitted by the black hole. By the miracles of quantum mechanics that mass is deducted from the black hole- explaining this seems to require actual mathematics and that's beyond me.

 

[Badly Shaved Monkey]

Have all of our physicists gone off on holiday together? I think a rota should be worked out so someone is on duty at all times so we are not left swinging in the breeze like this with vital questions being answered badly by a vet!

 

[Dr. Fendetestas]

OK, a physicist is still on duty...

If I recall correctly, Brian Greene's "The Fabric of the Cosmos" he suggests that space which is lacking in large concentrations of matter would expand at a greater rate than space which is "inhabited" by matter, possibly eventually with the rate of expansion reaching the speed of light.

The concept of rate of expansion is a complicated one, it depends on your definition of distance and time (of which there are several, different from the one in SR, so you can actually have a distance increasing at a rate greater than c. Check Ned Wright's Cosmology Tutorial. More on the rate of expansion later.

My father and I were throwing this idea around the other night and we're wondering what the thoughts are here on the following:

I was under the impression that current theories state that there is no empty space, simply a constant process of virtual particles coming in and out of existence with their anti-partcles and anihilating near immediately. Wouldn't this negate the possibility of space expanding at such a high rate of speed?

Regarding the non empty space: Yes, classical vacuum doesn't exist within the framework of quatum mechanics, because of the time-energy uncertainty relation. But keep in mind our understanding of gravitation is still not too well conciliated with quantum mechanics.

Regarding the rate of expansion with no matter: In the equations of GR, the quantity of matter determines the curvature of the Universe and its Cosmology. A Universe with a lot of matter (density) would recollapse, a matter with low density would expand forever, but slowly. But, we have very recently found that the mass content of the Universe is not the only factor in this: there's also a thing called Cosmological Constant (or dark energy). This is a very strange thing with negative pressure and repuslive gravitation. A Universe with no matter at all (boring) but with a repulsive cosmological constant would expand at an exponential rate, the so called scale factor would increase very quickly. This is called a de Sitter Universe. It's very important because our Universe will eventually turn into one of these. Why? Because it's expanding and the amount of matter is constant, so eventually the density will reach a negligible value. The expansion of our Universe is asympotically exponential. The vacuum fluctuations are so small that they won't stop this.

Ignoring the above...

A galaxy with a large enough zone of "empty" space surrounding it, if Greene is correct, could become enveloped in a region of space expanding at the speed of light and become completely isolated from the universe that had been beyond this boundary previously.

Just for fun, as long as this thought experiment isn't WAY off.... wouldn't this encapsulated galaxy then become the equivalent of a non-singularity black hole? If the boundery region of space is expanding at the speed of light, then no information would be able to traverse this boundary, or get out, including gravitational effects, so would it.... simply cease to exist/matter (no pun intended)? Or as my father calls it, a "Massless Black Hole"

Hmmm... This is quite different from a Black Hole, but it's an interesting question.

Let us suppose our Universe to be infinite. Then at a given time we cannot see all of it, because light from very distant regions may have not reached us yet. This leads to the concept of visible Universe. The size of this visible Universe changes (increases) with time, but it never encompasses the entire Universe. And obviously, different regions of space have different visible Universes. What's more, some regions of the Universe will never be inside our visible Universe. This is, I think, something similar to what you are suggesting. But there's a difference. The visible universes form a continuum, in the sense that they overlap one another. So these islands do not really exist.

I realise that what you mean is slightly different. You say (I believe): if the distance between A and B grows at a rate greater than c, light from one would not reach the other. But this is wrong, because the distance we are using here is not the same distance used in SR (that would be impossible). The distance grows, if you want, behind the light ray, so it can still reach B. By the way, we have observed objects whose distances from us grow at superluminal rates, for this reason.

This answer has been very general, if you have more precise questions, don't hesitate to ask.

 

[Dr. Fendetestas]

I think I'm right in saying that the idea of a "Black Hole" predates the Einsteinian relativistic one that involves gravitational collapse and a singularity. I think the idea centred on a star that was so big its gravity would restrain its light rendering it invisible. I seem to recall that if the calculations of star mass were extrapolated to vast sizes this was found to be an inevitable outcome. I think it's probably incompatible with our current understanding, but can anyone fill in the details, and explain if that idea could really produce a "Black Hole" or whether, given that gravitational collapse occurs, any real-world object that could work as a black hole as originally envisaged would have to follow Einstein's rules and end up as a singularity-based black hole?

Not really a star extrapolated to vast masses. It's more like this: when a star dies, several outcomes are possible.

• Small star (like our Sun): enters a phase of red giant and eventually turns into a white dwarf.
• Big star: it explodes (supernova) and a supernova remnant forms either a neutron star or a black hole (very big is a black hole, big is a neutron star).

A black hole is a concentration of matter, so dense that space is so curved around it that nothing can get past an event horizon. Inside a black hole there is a singularity, we don't really know what this is, but rest assured is not very comfortable. For instance, near the horizon, the gravity changes so rapidly that tidal forces would rip you apart quite easily.

[...]
I think I am also right in saying that the notion of our observable universe as an island in an expanding sea of receding emptiness is the wrong picture. If you were at the 'edge' of our observable universe your observable universe would extend further than we can see from our position, so there may be an infinity of contiguous universes defined by the observability from different spot but mutually unobservable. In other words, the observable Universe centred on Alpha Centaur is slight frameshifted relative to ours but by a negligible amount on universal scales, but the universe observable from a distant quasar would include areas beyond it that are not observable to us...I think

This is correct.

Yes, in classical relativity the event horizon is infinitely accurately located and you are either one side or the other of it. Quantum mechanics imposes uncertainty in that and creates a constant bubbling into existence of pairs of particles that appear then mutually annihilate and disappear. But some pair members disappear over the event horizon leaving the partner free in the outside universe to wander off. To an outside observer this appears to be radiation emitted by the black hole. By the miracles of quantum mechanics that mass is deducted from the black hole- explaining this seems to require actual mathematics and that's beyond me.

This is actually more complicated, but similar to this. Black Holes are evaporating (but the time it will take for them to disappear is lots of orders of magnitude greater than the age of the universe).

 

[Badly Shaved Monkey]

Not really a star extrapolated to vast masses. It's more like this: when a star dies, several outcomes are possible.

No. I'm sure I'm correct in remembering that pre-Einstein someone suggested the idea of a mass so big it would absorb its own light. Pre-relativity and pre-QM their basic idea could not work, but it was suggested, I just wish I could remember by whom.

 

[Badly Shaved Monkey]

Here 'tis;

http://en.wikipedia.org/wiki/Black_hole

I was thinking of Laplace, but it was John Michell who first did the calculation in 1783.

 

[luchog]

Inside a black hole there is a singularity, we don't really know whThis is actually more complicated, but similar to this. Black Holes are evaporating (but the time it will take for them to disappear is lots of orders of magnitude greater than the age of the universe).

There is speculation that micro-black holes created during the initial expansion stages of the universe (<1s post Big Bang) may have evaporated already, leaving naked singularites. It's supposedly mathematically possible, but don't ask me to try and replicate the formulae, since that's not really my strong point. There was a good bit on naked singularities in Gribbin's Black Holes

 

[Dr. Fendetestas]

Here 'tis;

http://en.wikipedia.org/wiki/Black_hole

I was thinking of Laplace, but it was John Michell who first did the calculation in 1783.

Also Laplace (a bit later). But a big mass is not enough

If the semi-diameter of a sphere of the same density as the Sun were to exceed that of the Sun in the proportion of 500 to 1[...]

Note the phrase same density as the Sun. What matters is the mass:radius relationship. The escape velocity is

v^2 = 2GM / R

if M/R is very big, v could get indefinitely large, eventually reaching the value of the speed of light, so that it couldn't escape. This was, more or less, what they thought. Nowadays the quantity

R = 2GM / c^2

is called Schwarzschild radius. An object whose mass is inside its Schwarzschild radius is a black hole. A typical black hole may be a few solar masses and the size of a city.

Anyway, I misunderstood your post, I didn't realise you were talking about pre-Relativity ideas.

 

[Dr. Fendetestas]

There is speculation that micro-black holes created during the initial expansion stages of the universe (<1s post Big Bang) may have evaporated already, leaving naked singularites. It's supposedly mathematically possible, but don't ask me to try and replicate the formulae, since that's not really my strong point. There was a good bit on naked singularities in Gribbin's Black Holes

Yes. Micro black holes might even be produced on Earth, in particle accelerators or detected in cosmic rays (much more energetic). Some people have even speculated that the electron itself is one of them.

A micro black hole would be one in which quantum mechanics plays a big role. There is work invested on them because if they existed, a lot of light would be shed on quantum gravity. But they are no more than somewhat vaue ideas yet.