First of all, I apologize to any properly educated physicists, in case my post is horribly naive(which it probably is). I just have an interest on physics, but not much more. I'm just laying down some thoughts and questions. So bear with me :) Anyway, I had a strange idea today, I don't remember if I had read something about it long ago, but I may have since there seems to be such a 'theory'. Basically, I was thinking, is it possible that our Universe is a huge black hole created from another 'parent' universe? It seems strange at first, but the fact is that something has to take place inside a black hole, and I find it hard to believe that black holes are just point objects and all the energy/mass just stays stationary. In my naive understanding, all the particles that are collapsing to form a black hole must have some huge velocity when they reach the center, so it's logical that they will come out 'on the other side' with that velocity. Also, could someone with some proper physics understanding, unlike me, explain in simple terms how the Universe is not a black hole? According to Big Bang, there was a tremendous amound of matter/energy in a single point, which does sound like a black hole. Furthermore, the timespace is expanding, as we know, faster than the speed of light, so, like black holes, nothing(not even light) can escape or reach the boundaries of the universe(event horizon?). Can it be expanding because the Big Bang was not a momentary event, but it's part of a continous flow of matter between the 'parent' and out universe, which means that matter keeps coming to our universe, warping spacetime more and more, causing it to 'expand'? Any opinions or links to material about this would be welcome :)

Do you realise the universe could also be a sub-atomic particle inside Homer's brain? No gifted scientist would be able to prove such an assumption to be wrong.

Theorically, a black hole is just another kind of massive object. It's expected to have such a strong gravity field that it would be able to bend space-time to no end. Physically it wouldn't be that much different from a neutron star or another kind of massive body.

I wouldn't make assumptions about black-holes before we have direct proof that they actually exist.

Quote: Original post by owl
Do you realise the universe could also be a sub-atomic particle inside Homer's brain? No gifted scientist would be able to prove such an assumption to be wrong.

Theorically, a black hole is just another kind of massive object. It's expected to have such a strong gravity field that it would be able to bend space-time to no end. Physically it wouldn't be that much different from a neutron star or another kind of massive body.

I wouldn't make assumptions about black-holes before we have direct proof that they actually exist.

What do you mean 'theoretically' and 'physically'? Is there something that prevents the theory from beging what actually happens? Btw, you said 'don't make assumptions about black holes', but in the very previous sentence you made exactly that, with 'physically it wouldn't be that much different from a neutron star or another kind of massive body', which btw I don't think is actually correct, a black hole, if it exists, is certainly something entirely different that 'just another massive body' :)

It seems unlikely to me that the universe is a black hole. Although I'm not a physicist, many of them completely agree on this point. The matter forming a black hole has maximal entropy (e.g., entropy proportional to mass squared), where the matter forming non-black holes (e.g., you, the Sun, a cow) does not have maximal entropy (e.g., entropy roughly proportional to mass). If the entire universe were one giant black hole, there would be no cow "stuff", only black hole "stuff" (I'm being serious).

A quick word on owl's idea about the universe as a sub-atomic particle in Homer's brain: The holographic principle leads us to believe that one cannot fit an arbitrarily large amount of entropy into an arbitrarily small volume of space. That is, you cannot fit a universe worth of information inside the space taken up by an atom. The most compact thing you could fit would be a black hole -- a black hole of radius equal to the Bohr radius of an atom would have very little mass even compared to a cow, let alone a galaxy, or an entire universe.

As for point-like black holes, there are many models nowadays which instead describe the black hole as an extended object (e.g., Mathur's fuzzballs). You're not alone in your conviction that point-like black holes are undesirable.

The universe does not have a boundary in space like a black hole does. Classically, the universe is a four dimensional object, and it is the 3D surface of this object upon which we live. Like an ant crawling along the 2D surface of a 3D beach ball, if we were to scoot far enough along the 3D surface of our 4D universe, we could end up arriving exactly at the same spot where we started. To imply that the universe is a black hole implies that there is something outside of it (e.g., an exterior region) -- this is not the case.

The notion of continual matter creation is called the "steady state" theory. It has major problems, and was pretty much put to rest when we confirmed that the cosmic microwave background radiation exists.

Most of this stuff can be read about on wikipedia. If you need clarifications, I can try to help. Not a lot of this will make sense when you first read it. :)

Thanks taby, pretty good explanations. What I don't get still is how the universe, close to the big bang, was not a black hole, since it pretty much fills the definition, doesn't it? Enormous mass in a very small space. What's the reason it's not regarded as a black hole?

Check this out: Our world may be a giant hologram (15 January 2009)

Quote:
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For many months, the GEO600 team-members had been scratching their heads over inexplicable noise that is plaguing their giant detector. Then, out of the blue, a researcher approached them with an explanation. In fact, he had even predicted the noise before he knew they were detecting it. According to Craig Hogan, a physicist at the Fermilab particle physics lab in Batavia, Illinois, GEO600 has stumbled upon the fundamental limit of space-time - the point where space-time stops behaving like the smooth continuum Einstein described and instead dissolves into "grains", just as a newspaper photograph dissolves into dots as you zoom in. "It looks like GEO600 is being buffeted by the microscopic quantum convulsions of space-time," says Hogan.

If this doesn't blow your socks off, then Hogan, who has just been appointed director of Fermilab's Center for Particle Astrophysics, has an even bigger shock in store: "If the GEO600 result is what I suspect it is, then we are all living in a giant cosmic hologram."

The idea that we live in a hologram probably sounds absurd, but it is a natural extension of our best understanding of black holes, and something with a pretty firm theoretical footing. It has also been surprisingly helpful for physicists wrestling with theories of how the universe works at its most fundamental level.

The holograms you find on credit cards and banknotes are etched on two-dimensional plastic films. When light bounces off them, it recreates the appearance of a 3D image. In the 1990s physicists Leonard Susskind and Nobel prizewinner Gerard 't Hooft suggested that the same principle might apply to the universe as a whole. Our everyday experience might itself be a holographic projection of physical processes that take place on a distant, 2D surface.

The "holographic principle" challenges our sensibilities. It seems hard to believe that you woke up, brushed your teeth and are reading this article because of something happening on the boundary of the universe. No one knows what it would mean for us if we really do live in a hologram, yet theorists have good reasons to believe that many aspects of the holographic principle are true.

Susskind and 't Hooft's remarkable idea was motivated by ground-breaking work on black holes by Jacob Bekenstein of the Hebrew University of Jerusalem in Israel and Stephen Hawking at the University of Cambridge. In the mid-1970s, Hawking showed that black holes are in fact not entirely "black" but instead slowly emit radiation, which causes them to evaporate and eventually disappear. This poses a puzzle, because Hawking radiation does not convey any information about the interior of a black hole. When the black hole has gone, all the information about the star that collapsed to form the black hole has vanished, which contradicts the widely affirmed principle that information cannot be destroyed. This is known as the black hole information paradox.

Bekenstein's work provided an important clue in resolving the paradox. He discovered that a black hole's entropy - which is synonymous with its information content - is proportional to the surface area of its event horizon. This is the theoretical surface that cloaks the black hole and marks the point of no return for infalling matter or light. Theorists have since shown that microscopic quantum ripples at the event horizon can encode the information inside the black hole, so there is no mysterious information loss as the black hole evaporates.

Crucially, this provides a deep physical insight: the 3D information about a precursor star can be completely encoded in the 2D horizon of the subsequent black hole - not unlike the 3D image of an object being encoded in a 2D hologram. Susskind and 't Hooft extended the insight to the universe as a whole on the basis that the cosmos has a horizon too - the boundary from beyond which light has not had time to reach us in the 13.7-billion-year lifespan of the universe. What's more, work by several string theorists, most notably Juan Maldacena at the Institute for Advanced Study in Princeton, has confirmed that the idea is on the right track. He showed that the physics inside a hypothetical universe with five dimensions and shaped like a Pringle is the same as the physics taking place on the four-dimensional boundary.
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So would they be able to detect a holographic projection of grainy space-time? Of the five gravitational wave detectors around the world, Hogan realised that the Anglo-German GEO600 experiment ought to be the most sensitive to what he had in mind. He predicted that if the experiment's beam splitter is buffeted by the quantum convulsions of space-time, this will show up in its measurements (Physical Review D, vol 77, p 104031). "This random jitter would cause noise in the laser light signal," says Hogan.

In June he sent his prediction to the GEO600 team. "Incredibly, I discovered that the experiment was picking up unexpected noise," says Hogan. GEO600's principal investigator Karsten Danzmann of the Max Planck Institute for Gravitational Physics in Potsdam, Germany, and also the University of Hanover, admits that the excess noise, with frequencies of between 300 and 1500 hertz, had been bothering the team for a long time. He replied to Hogan and sent him a plot of the noise. "It looked exactly the same as my prediction," says Hogan. "It was as if the beam splitter had an extra sideways jitter."
Incredibly, the experiment was picking up unexpected noise - as if quantum convulsions were causing an extra sideways jitter

No one - including Hogan - is yet claiming that GEO600 has found evidence that we live in a holographic universe. It is far too soon to say. "There could still be a mundane source of the noise," Hogan admits.
...

Quote: Original post by mikeman
Thanks taby, pretty good explanations. What I don't get still is how the universe, close to the big bang, was not a black hole, since it pretty much fills the definition, doesn't it? Enormous mass in a very small space. What's the reason it's not regarded as a black hole?

The common line of reasoning is that entropy generally increases or remains the same over time (second law of thermodynamics).

If the universe had started out as a black hole with maximal entropy, then at some point it would have had to undergo some kind of transition that drastically reduced its entropy (in order to get to the state it's in now, where cow "stuff" exists). A lot of people seem to figure that this kind of thing would entirely fail at maintaining the second law of thermodynamics (entropy should NOT generally decrease like this), and so it is assumed by these people that the entropy of the universe was very low at the beginning of time. This goes against the possibility that the universe started out as a black hole (no maximal entropy equals no black hole). I agree with them.

Quote: The common line of reasoning is that entropy generally increases or remains the same over time (second law of thermodynamics).

This only applies to isolated systems though, which don't exist, except for the assumption that the universe as a whole is an isolated system.

So if the universe started out that way (maximal entropy), something *outside* of the universe would've had to muck with it (which goes against the definition of universe).


That said though, if you put all of the mass/energy of the entire universe into a single point, how come it creates a big bang, and doesn't just collapse into a black hole? Is it because it was just energy and didn't have mass yet, at that point?

[Edited by - Hodgman on February 13, 2010 6:49:39 PM]

Quote: Original post by Hodgman

Quote: The common line of reasoning is that entropy generally increases or remains the same over time (second law of thermodynamics).

This only applies to isolated systems though, which don't exist, except for the assumption that the universe as a whole is an isolated system.

So if the universe started out that way (maximal entropy), something *outside* of the universe would've had to muck with it (which goes against the definition of universe).


That said though, if you put all of the mass/energy of the entire universe into a single point, how come it creates a big bang, and doesn't just collapse into a black hole? Is it because it was just energy and didn't have mass yet, at that point?

You're right about the multiverse thing. I don't know if you've read Sean Carroll's new book, but it might be interesting to you since you are familiar with these notions.

That's a really good question, about why the high mass density didn't form a black hole. Here's an article on it: http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/universe.html

Quote: Original post by mikeman

Quote: Original post by owl
Do you realise the universe could also be a sub-atomic particle inside Homer's brain? No gifted scientist would be able to prove such an assumption to be wrong.

Theorically, a black hole is just another kind of massive object. It's expected to have such a strong gravity field that it would be able to bend space-time to no end. Physically it wouldn't be that much different from a neutron star or another kind of massive body.

I wouldn't make assumptions about black-holes before we have direct proof that they actually exist.

What do you mean 'theoretically' and 'physically'? Is there something that prevents the theory from beging what actually happens? Btw, you said 'don't make assumptions about black holes', but in the very previous sentence you made exactly that, with 'physically it wouldn't be that much different from a neutron star or another kind of massive body', which btw I don't think is actually correct, a black hole, if it exists, is certainly something entirely different that 'just another massive body' :)

theoretically as in how theories talk about them, and I said physically in the context of such theories.

You just got it all wrong, I never said "don't make assumptions" I said: "I woudln't". What I said before weren't *my* assumptions.

Learn to read. Then, learn to understand. Then, learn to be polite.