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Black hole vs big bang 
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#1
Feb2713, 07:56 PM

P: 71

If the universe was at or near a singularity in the past, why is it not a black hole now? How can part of the universe become a black hole, and not the whole universe?



#2
Feb2713, 08:16 PM

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Your questions don't seem to make sense. It sounds like you are imagining something as real, that according to mainstream cosmology is not realbut what is it exactly? Why would you suppose that the whole universe would become a black hole? As far as we know, from the standard model of cosmology, it is on track to continue expanding. Why should it collapse? You could be more explicit in describing what you are imagining. Here's a good place to get started understanding cosmology: http://www.mso.anu.edu.au/~charley/p...DavisSciAm.pdf The first page is blank, so scroll down. It still uses words like "singularity" a bit confusingly, but it's good for starters. I guess a friendly word of advice would be to be cautious about believing TV and mass market popularization of cosmology. 


#3
Feb2713, 08:28 PM

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#4
Feb2713, 10:32 PM

P: 1,857

Black hole vs big bang
The BB didn't happen as a singularity as per a blackhole.
Rather its a rapid expansion of spacetime. The BB model only describes from 10 to the 44 seconds forward. (sorry for the longhand typing from phone) This era is the planch epoch. I can provides listing of epochs for you if you wish. During the planch epoch, matter did not exist. Instead its best to describe this epoch in terms of planch length, planch time and units of planch time. There are a couple of models that represent how this works. The one with the best fit to data is lamdaCDM. However there is a few models that are valid as to how this all works from before Planch era. One is through what is the false vacuum method described by A. Guth which can easily described as false vaccuum. This model describes everything starting from nothing. Lawrence R Kraus supports a later variation of this model. I'll let others explain Loop quantum gravity. Hope that helps 


#5
Feb2713, 10:47 PM

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I didn't see Mordred already replied. Had to do a chore and got sidetracked half way thru. I'll leave this as it is anyway.
In the standard picture of the early stage of expansion all space was filled with a uniform high density. There was no empty space and there was no one favored location where stuff was concentrated. We don't know 'very small' but we do think "very dense". High density does not by itself cause collapse to a black hole. There is a kind of "tugofwar" contest between the expansion rate and the density. In popularizations they don't tell you about that. They only tell you that IN NONEXPANDING SPACE a certain density concentrated at some location will result in formation of a black hole at that location. They don't discuss other cases. Suppose the expansion rate overwhelms the density. We have no proof that the universe as a whole was "very small" at the start of expansion. According to the mainstream expansion cosmology model it could have been infinite volume, or a large finite volume at the start. Measurements are not yet good enough to put a number on the current volume of the universe as a whole. We only know the size of the currently observable part of it. The main thing is that it was very DENSE. So the currently observable portion would have been concentrated in a very small volume. What we can currently see, out of the limits of observation, was very small at the start. What started the expansion is so far not known. There are various theories. 


#6
Feb2713, 10:56 PM

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#7
Feb2713, 11:06 PM

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#8
Feb2713, 11:10 PM

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We don't know the physics involved under conditions approaching those of the BB model. We only know they are different.



#9
Feb2813, 01:10 AM

P: 71

It sounds a little funny that at the beginning, expansion was super fast. Then during most of forever, expansion was close to linear; and just now expansion is speeding up again. I know that’s what the models say, but it seems like a pibtac



#10
Feb2813, 08:26 AM

P: 1,857

Here is a list of some of the earlier stages. http://www.uni.edu/morgans/astro/course/Notes/section3/bigbang.html In the early universe radiation was dominated so inflation was rapid. Later matter formed after the temperature dropped below 3000 kelvin. Big bang nucleosynthesis best describes this sequence. that matter slowed expansion so its referred to as the matter dominated era. However dark energy (cosmological constant/vacuum energy) continued growing. Eventually in non gravitational bound regions. Ie the voids between galaxy clusters. The energy driving expansion is currently explained as vacuum energy. This energy became strong enough to start accelerating expansion. Leading to what we see today in the lambda dominated universe 


#11
Feb2813, 11:24 AM

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P: 1,676

gregtomko, the models are what they are because that's what the data tells us. As others have said, we don't have a physical model for what, if anything, occurred at t=0. The "big bang" is the name given to the model describing the evolution of the early universe as it expanded from a hot, dense initial phase to an older universe. We can apply this model all the way back to very early times, as Mordred points out, we just can't apply it beyond the Planck scale. What was there? Nobody knows. The singularity that arises from GR does not indicate anything physical, it merely signals that the theory is being applied outside its bounds. So we'd be remiss to take the singularity seriously. In any event, it's certainly true that densities were very high back then. So does this imply that a black hole should have formed? Black holes arise when a spherically symmetric mass distribution becomes sufficiently dense. Was the mass density of the early universe spherically symmetric? No! (at least not from the time that the big bang is a descriptive model of the universe) The early universe was homogeneous and isotropic, and the gravity of such a mass distribution causes the universe to expand (quickly or otherwise, it can even contract!). But it does not form a black hole  it doesn't have the correct symmetry. It turns out that gravitation cares very much about the nature of the mass/energy doing the gravitating. 


#12
Feb2813, 11:42 AM

P: 115

The problem I think is in what we call usually a "singularity"... a Black Hole singularity is not necessarily the same thing as the Big Bang singularity.
Black Hole singularity is a local spacetime singularity and is covered by an event horizon. In the case of Big Bang instead the singularity is nonlocal, is extended to the whole spacetime; moreover the only horizon emerging from this picture is a particle horizon, not an event horizon. The inevitability of singularities was proved in two really important theorems by Roger Penrose (a collapsing star with a mass greater than a certain limit will necessarily end up in a singularity) and by Stephen Hawking (this one indeed regards the Big Bang singularity, even if I do not know in detail what aspects of this singularity). The way used to prove these theorems is to study geodesic incompleteness, meaning that if we show that a geodesic of spacetime cannot be extended to ##+\infty## and ##\infty## in the affine parameter space, then it will end up in a singularity (futureincompleteness for BHs, pastincompleteness for BB). These results anyway are summarized in the book by Hawking & Ellis "The large scale structure of spacetime" (Cambridge university press) but it is really technical and difficult to read, so I would advise to read only having a good knowledge of differential geometry and General Relativity. 


#13
Mar113, 03:15 AM

P: 18

OK so, initially the universe was very dense, and not neccessarily a singularity. But doesn't the common origin of particles/spacetime imply some sort of singularity in the beginning?



#14
Mar113, 03:42 AM

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#15
Mar113, 05:11 AM

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#16
Mar113, 05:20 AM

P: 18

Well, theories aside, I think it still makes sense to talk about a common origin, considering the *unchallenged* uniformity of nature: an electron is just like any other electron, anywhere, as far as anyone can tell, so we know the universe had to have a single/singular origin, we just don't have any conceivable idea how it happened beyond the planck epoch. And, as I understand from the comments above, it needed not to be infinitesimally small at that point. So just how small would have been, say, the currently observable part of the universe in the planck epoch? Or the universe as a whole?



#17
Mar113, 05:31 AM

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#18
Mar113, 06:14 AM

P: 115

Anyway as for the question about the size of the universe, it all depends on inflation (again a theory) http://en.wikipedia.org/wiki/Inflation_(cosmology). And I do not think there is a way (at least by now) to answer your question, as we can not observe anything beyond the Hubble horizon (you can visualize it as a sphere around us with the radius given by the time passed from Big Bang till today). And again, aside knowing that the Big Bang is the most probable "beginning" of the Universe (also here there are theories which avoid the Big Bang, but they are not in agreement with observations most of times), what is actually Big Bang is unknown, and will probably be unknown for many years... giving a size to the Universe is then clearly impossible, at least to my knowledge. 


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