Does the UNIVERSE have a boundary or OUTER LIMIT?

Ok, but why would the BH lose mass in the first place?

It has no mechanism for losing mass, right?

The mechanism is Hawking radiation. (According to Hawking obviously.)

I have no doubt Hawking has fully developed and thoroughly undestands hir HR hypothesis. I really sorta suspect nobody else on the planet has much of a handle on it yet, though. Seems it takes a while for mere humans to wrap their minds around any of Hawking's ideas.

Ros: Let's try this analogy...

One of my former tasks as a Design Engineer, was the design of Jet engine Propulsion Systems.

Firstly, we accept that the event horizon of a Black Hole is the entrance, from which no light can escape (this is similar to the ram-air inlet cowl of a jet engine).
Secondly, once the matter is attracted to the innards of the Black Hole it is accreted with the overall mass/density of existing material through a process of gravitational compression (this is similar to the turbine and compressor phases of a jet engine).
Finally, and perhaps what Hawking is saying, there is an ejection or exhaust phase were matter is able to escape from the, as yet unseen/unknown, backend of the Black Hole, where the effects of an event horizon are not applicable (this is similar to the high speed, afterburner exhaust of a jet engine).
The escape velocity is always greater than the attracting inlet velocity, and perhaps in Hawkings Black Hole cosmology, there is an exit force which is greater than the initial gravitational attraction force at the mouth of the Black Hole.

I may co-author his next book :wink:

You must be right, but I thought that Hawking Radiation pertained to the imbalanced virtual particles, where one of the two particles escaped the event horizon due to coming into existance outside the horizon, which would still leave one particle inside the BH. It still seems like it's balanced... I guess I'm missing something (obviously)

Ros: I've just been in touch with Professor Hawkings office, and they confirm that my jet engine anaolgy is pretty close.

Oh yes, there was a blue moon at the time :wink:

NOVA KNOWS
According to NOVA, there is NO edge to the universe. Do't try to imagine because you can't.

Re: NOVA KNOWS


What authority does NOVA have to make such an emphatic claim?

Finite/Infinite universe
Bib,
perhaps you will want to define what you mean by the universe. Thinking theoretically,...
As far as space is concerned, if there is a boundary out there somewhere....what is on the otherside of the boundary? This argument implies an unlimited spacial universe. The extent of the physical universe is limited. If you take all the positions of all the particles/photons/gravity waves, etc and take the average, that would be the "center" of the physical universe. Some particle(s) would be the furthest from this center. That would represent a sphere centered at our center and encompassing all matter. This sphere could be thought of as expanding radially at the speed of light. yet it is still finite.

House vs Planet
Just put it this way, right now everyone owns a hose on the same plant. Some where in the future its going to be so advanced that every person is going to own a PLANET! How about that for some privacy?

Just imagine the address:

Universe Plato
Hoyto Galaxie
Planet #1,934,752

The simple answer to the original question is our physics isn't advanced enough to give a definite answer.

We can't say if spacetime is closed or not, and what if anything exists beyond it if it has a boundary, nor what this beyond the boundary layer if it exists would be governed by (e.g. quantum gravity) nor whether M-theory is correct and you have to consider things 11 dimensionally!

Sorry - honest answer is we don't know!

and will probably never find out

i am really interested in the boundary of the universe
i have read all these posts and i am still interested in what is beyond. i believe that everything that has a beginning also must have an end, and the universe included, everything has to be somewhere, so where then is the universe, even with the blackhole theory it still doesnt explain where the blackhole is. if the carp only know their pond and nothing outside that, we know where they are, so does it not make sense that we to are in a pond and there is a whole world beyond ours. somehow i find that easier to come to terms with. we are just an adam inside a giants ball and he is in a world that is an adam inside yet a bigger ball.can anyone else see this. just a thought thats easy to grasp

Maybe the edges are asymptotes that can never be quite reached...

I'm speculating that the universe's expansion might not be in the same terms as expanding like a person running from one point to the next, but rather an expansion of the space within itself.....

Yes. The speed of light *is* the edge.

the universe...
I believe concepts like "boundary" and "limit",come from the human existance and experience. Our knowledge is based on our experience.
Everything in our life has a start and a end, but just because that is the case doesnt mean the universe does.Maybe it has something else that we cant comprehend yet. After all we are still making discoveries.
Peace Out...

Re: the universe...


I agree. Welcome to the system Mr.Biggles.

Actually the speed of light may not well define the edge, because lightspeed only governs matter and energy moving relative to an observer within the domains of a frame of reference in spacetime...

Spacetime itself (as a dimension) can apparently unfold faster than lightspeed without invalidating general relativity. This is not a recession velocity between two distant galaxies - saying galaxy A and B are moving apart faster than c. This is instead saying spacetime is still unfolding carrying A and B with it. Spacetime itself isn't limited by GR.

Stunning but that is the way the theory of GR unfolds if you study it carefully. Something that was news to me only 3 weeks ago BTW!

The implication is the Universe could be far bigger than a sphere defined as radius c * age of universe since big bang. It could be 2, 3, 40, 200 times bigger or infintely bigger - ugh!

i'd like to think that the simple answer is that there is an edge....but not of the white picket fence variety....
more likely, it will be the edge at which forces cease to act from the centre point of the bang....
at this point, if we were to look ahead, we would see nought but if we looked back, we would see the fast approaching light from our expanding universe....
though this does not mean that an infintie room doesnt exist out there for expansion....otherwise we wouldnt be expanding for start...

and if we were to sit at that edge, and the content of the universe remained static, then this light would gradually grow dim as the contents continued to spread over a greater region....

however, somehow i dont think this would be the case....nothing doesnt appear to be wholly nothing and seems to consist of something...
so maybe, just maybe, as the universe expands, it may give birth to more matter/energy out of nothing under the influence of stretching forces....the same stretching forces responsible for making it expand....