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This spawned a question for me that seems like it must be nonsensical but I don’t know the answer. Why did the Big Bang not just produce a Big Hole? IOW, if al

by sulam 8y ago

This spawned a question for me that seems like it must be nonsensical but I don’t know the answer. Why did the Big Bang not just produce a Big Hole? IOW, if all the current mass of the universe was in a small volume wouldn’t it have promptly created a black hole? I’m assuming the answer has something to do with mass not actually existing in its current form until the universe expanded enough. However I don’t have any mental framework to understand why that would be true. If someone reading this knows the answer and can point me at it, I’d be grateful. TIA!

myinnerbanjo 8y ago
I think inflation had something to do with it, the only time in the history of the universe something faster than the speed of light was possible--but only because space and time itself was expanding so quickly. Just stumbled upon this: https://www.reddit.com/r/askscience/comments/8p7hnn/where_any_black_holes_created_immediately_after/ https://www.reddit.com/r/askscience/comments/8p7hnn/where_an...
DougWebb 8y ago
I'm absolutely not an expert, but I believe there wasn't any mass at first; it was all energy. It had to slow down to become mass, at which point it was spread out enough. Or something along those lines. Another possibility is that black holes are places where lots of mass accumulates in the same place in space, while the big bang was a rapid expansion of space itself. Even if there was enough mass in one spot to be what we'd consider a black hole, the space it occupied expanded, dragged the mass along with it, and ripped the black hole apart.
- twiceaday 8y ago
  Doesnt E=mc^2 form an equivalence? I would expect energy to warp space-time just as mass does.
  - waterhouse 8y ago
    I believe it does. If you have a bottle full of chemicals with potential energy, and then you make them react and get the energy from them (via, I dunno, heat), while all the particles remain in the bottle, then I think the bottle weighs less afterwards—though probably by an undetectably small amount. I think I read that the change in mass is detectable after, say, nuclear fission.
  - pdonis 8y ago
    > I would expect energy to warp space-time just as mass does. It does. So do pressure and stresses. The full "source" of gravity in General Relativity is the stress-energy tensor, which includes all of these things.
- mygo 8y ago
  > it was all energy. It had to slow down to become mass If there was no mass then what made the energy slow down? For example a photon will travel at the speed of light in a vacuum indefinitely. In what scenario does energy just slow down without interacting with matter?
- InclinedPlane 8y ago
  "Mass" is just another name for "rest-energy". A single photon cannot have mass because it has zero energy in the reference frame where it is at rest (zero net momentum). But a pair of non-parallel photons can have rest-mass, and indeed the mass of the photons inside a star is considerable, for example.
  - pdonis 8y ago
    > "Mass" is just another name for "rest-energy". Not if you're talking about what the source of gravity (spacetime curvature) is. Massless particles still have energy and still cause spacetime curvature.
    - InclinedPlane 8y ago
      The "source" of gravity is the stress-energy tensor. "Mass" (rest-energy) is a simplification that is useful for approximations and conceptualizations.
      - pdonis 8y ago
        > "Mass" (rest-energy) is a simplification that is useful for approximations and conceptualizations. Yes, but not in a context where radiation is present, since radiation can have energy but has no rest mass (rest energy).
  - pdonis 8y ago
    > A single photon cannot have mass because it has zero energy in the reference frame where it is at rest (zero net momentum) I didn't catch this before, but it is not correct. There is no such thing as a frame in which a photon is at rest.
- flukus 8y ago
  You can make black holes with massless particles: https://en.wikipedia.org/wiki/Kugelblitz_%28astrophysics%29 https://en.wikipedia.org/wiki/Kugelblitz_%28astrophysics%29
- pdonis 8y ago
  > I believe there wasn't any mass at first; it was all energy. It had to slow down to become mass This is partly true. During inflation, the universe was vacuum; all of the energy was in the "false vacuum" state of whatever field caused inflation (it's usually called the "inflaton" field, which IMO is a very confusing name). When inflation ended, all of that energy got transferred to the Standard Model fields, forming matter and radiation at very high temperature and expanding very rapidly. So the "all energy at first" part is true; but the "had to slow down to become mass" part is not. The matter and radiation was there as soon as inflation ended; it didn't have to slow down first.
  - jacobush 8y ago
    But did it end?
    - pdonis 8y ago
      You mean, did inflation end? Yes, it did.
      - jacobush 8y ago
        So the inflation is different from the current expansion.
        pdonis 8y ago
        Yes.
twiceaday 8y ago
Neither space nor time existed prior to the big bang and energy levels were so high that a different set of laws of physics dominated the early evolution.
bpchaps 8y ago
My naive (and likely wrong) understanding is that cosmological inflation was caused by a "false vacuum" of the higgs field. Think of it as if the energy pressure of a system is higher than a vacuum, but has similar pressure properties of a vacuum. That had the effect of gravitational pressure pushing matter outwardly, instead of pulling it inwardly like we have it today (to answer your question). That caused enormously fast expansion of the universe. Very shortly afterwards, the false vacuum "decayed", and gravity started behaving as you know it. Check out Alan Guth's inflationary universe - a surprisingly accessible book on a deeply complex subject. Someone please correct me where I'm wrong!
- pdonis 8y ago
  Your description of what happened during inflation is basically correct. However, in order to explain why a black hole didn't form as soon as inflation ended and gravity started behaving "as we know it", you have to include the fact that all of the matter that was formed at the end of inflation was expanding very, very rapidly. (The universe was actually vacuum during inflation--at the end of inflation all of the energy in the false vacuum got transferred to the Standard Model fields, creating matter and radiation at extremely high temperature and expanding.)
bglusman 8y ago
I don't know the technical/methematical explanation for this, or if one is really needed (though it seems like a valid question to pose), but it is noteworthy that black holes and the big bang are the original "singularity", and that there are several related theories in which they are directly related, notably Big Bounce[1] and my favorite (which I learned of from Lee Smolin's amazing Life of The Cosmos[3]) Cosmological Natural Selection[2]. There's also a variant of Cosmological Natural Selectionm CNS-I[4], but that one possibles borders on religious, though it's possible it could be falsifiable/supportable some day by modeling. I suppose it's even somewhat plausible that the answer to your question, in combinations with the other theories, are "all true", and that for various reasons or models the first N big bangs did result in immediate/fast collapse back to another big crunch/black hole, and that each subsequent crunch/black hole created another universe, until there was a universe that survived long enough to create several black holes, and so on and so on... [1]https://en.wikipedia.org/wiki/Big_Bounce https://en.wikipedia.org/wiki/Big_Bounce [2]http://evodevouniverse.com/wiki/Cosmological_natural_selection_(fecund_universes) http://evodevouniverse.com/wiki/Cosmological_natural_selecti... [3]https://www.amazon.com/Life-Cosmos-Lee-Smolin-ebook/dp/B004TW1YY6 https://www.amazon.com/Life-Cosmos-Lee-Smolin-ebook/dp/B004T... [4]http://evodevouniverse.com/wiki/Cosmological_natural_selection_(fecund_universes)#CNS_with_Intelligence_.28CNS-I.29 http://evodevouniverse.com/wiki/Cosmological_natural_selecti...
T-A 8y ago
The early universe was ridiculously homogeneous. We can infer this with reasonable certainty from the extreme homogeneity of the CMB and from the gravitational instability of density fluctuations: as you expect, they attract more mass and accrete to form stars and galaxies. So if you start from the universe as it looks now and run time backwards, you get ridiculous homogeneity at the bang. And that's why you didn't get a giant black hole. There was a lot of mass at very high density, and gravity was pulling very hard at it - but it was pulling (almost exactly) equally hard in all directions.
- Filligree 8y ago
  So, are we assuming the universe is infinite? Otherwise there'd be a border to that.
  - FreeFull 8y ago
    The universe can be finite without a border. Think about living on the surface of a sphere or a torus (finite, no edge), as opposed to living on a sheet of paper
    - hndamien 8y ago
      Part of me thinks that black holes are the border, and we might be inside one.
  - kryptiskt 8y ago
    The Earth's surface is finite, but without a border. A finite universe would be similar, just adding a dimension.
  - richardknop 8y ago
    I think almost all options are currently still in play. Finite with border, finite in infinite space, infinite.
  - arithma 8y ago
    If it was finite, does it mean it was definitely not flat?
infradig 8y ago
You assume it didn't. Perhaps the universe as we know it is the inside of a black-hole.
InclinedPlane 8y ago
This is an important question with a slightly surprising answer. First you need to understand that the Big Bang was not an explosion of matter into space, it was an explosion of matter and space (technically space-time). This might be hard to wrap your head around, it might be easier to think about space-time as non-infinite, say curving back on itself like the surface of a balloon (though as far as we can tell it is infinite). Second, during the early Universe just after the Big Bang when the energy density of the Universe was incredibly high the density was nevertheless incredibly uniform. So, let's say you are inside a soup of matter and energy with a density as high or higher than the core of a star, but there is matter in every direction, what way is gravity pulling you? This is the other non-intuitive bit, gravity isn't pulling you in any direction. Pick a volume of matter in space around you and consider how its gravity affects you, for every single volume there is a corresponding volume on the opposite side of you that has exactly the same mass and is exactly as far away from you, cancelling out the gravity from the first chunk. And this is true for every single bit of volume around you, there's a 1:1 mapping of bits of stuff around you with other bits of stuff on the opposite side of you which exactly counterbalances the gravitational force from the first bit. The gravitational field is uniform, so nothing collapses, no black holes form, no stars or planets form either. It's only after the Universe continues to expand and minute variations in local density are gradually amplified (over millions of years) into denser and denser clumps that it becomes possible to form galaxies and galaxy clusters/super-clusters (and thus black holes). Now, we don't know for sure whether or not there were isolated little "super clumps" of extra density in the early Universe which allowed for the formation of black holes prior to the formation of galaxies (so-called primordial black holes). That's one theory for explaining the existence of supermassive and especially ultramassive black holes, but it's unconfirmed. However, now it seems as though we have enough evidence to show that primordial black holes can't be the explanation for the evidence for dark matter.
- perl4ever 8y ago
  "First you need to understand that the Big Bang was not an explosion of matter into space, it was an explosion of matter and space (technically space-time)." This is a type of statement I have read many, many times, but to me, it's the same as saying "wakalixes!". I've never read anything that gave me a hint of why and how there should be a difference between two objects receding from each other and "space itself" expanding.
pdonis 8y ago
> if all the current mass of the universe was in a small volume wouldn’t it have promptly created a black hole? No, because it was expanding too rapidly. The usual intuitions about how much mass in how small a volume it takes to make a black hole assume that the mass is stationary, or nearly so.
- sulam 8y ago
  This matches a description I was able to find online. What that seems to imply though is that expansion was literally faster than light. Yes? No? [edit] and it seems this is true and can be explained by the space-time matrix not being bound by light speed limits.
  - pdonis 8y ago
    > What that seems to imply though is that expansion was literally faster than light. Yes? No? No. No particle of matter outruns a light ray at the same location and moving in the same direction. Or, to put it another way, all pieces of matter are moving within the light cones at their spacetime locations. "Moving within the light cones" is the correct General Relativity version of "nothing goes faster than light". The coordinate speed of pieces of matter can be greater than c, but in a curved spacetime that, in itself, is not a problem.
  - HeadsUpHigh 8y ago
    Yes it was. I think it's called the inflation period and is used as a possible explanation of why there are big voids in the universe.
  - cryptonector 8y ago
    The expansion of space is always faster than the speed of light if you pick any two points far enough apart from each other, at least as long as space is expanding and the universe is infinite. Or if the universe is finite, some time after the bang there will be points which separate faster than the speed of light from each other. These points aren't moving, of course.
- cryptonector 8y ago
dantillberg 8y ago
Maybe the big bang did produce a black hole, and we're living inside it? Back in my undergrad physics days, I postulated that our "universe" could be a big black hole inside some "extraverse". I worked through classical gravitational calculations to determine, from an estimate of the mass density of the universe, what the Schwarzschild radius (aka event horizon radius) of that black hole would be, and it came out to between 5 and 40 billion light years or so (the density of the universe is hotly debated, see other discussion in this thread about dark matter), which is at least the right order of magnitude.
- jobigoud 8y ago
  How is it the right order of magnitude? Did you compute that based on the size of the observable Universe? But that's arbitrary, the Universe is way bigger than that.
  - Simon_says 8y ago
    No. The input into the equation is the density of matter in space, nothing having to do with the size of the observable universe. On large scales in the visible universe, matter is fairly evenly distributed at a density of about 1 atom of hydrogen per cubic meter. For any constant nonzero density form of matter, if you pack enough of it together in a sphere in a flat spacetime, its Schwarzchild radius will eventually exceed its radius. This is because the Schwarzchild radius is proportional to the mass, but the radius of the actual matter is proportional to the cube-root of the mass. It's a fun thought experiment, but not physically meaningful in this universe because our spacetime is expanding, and the original calculations assume there's nothing at the edge pulling 'outward', but in our universe there is.
    - pdonis 8y ago
      > For any constant nonzero density form of matter, if you pack enough of it together in a sphere in a flat spacetime, its Schwarzchild radius will eventually exceed its radius. You can't do this in flat spacetime, because the matter curves spacetime, and the denser you pack it, the more it curves spacetime.
headShrinker 8y ago
There are major challenges/issues with Big Bang theory. The easiest issue to see and understand is temperature in the CMB appears to indicate “communication”... basic descriptions of the problem here https://youtu.be/dbm3M9Bz4RE https://youtu.be/dbm3M9Bz4RE No one really knows anything about the universe before 10^-42 seconds, and there is no physics to describe what likely happened. Photons/light and matter as we known it didn’t exist and wouldn’t for another 400,000 years. There are many very deep issues that are explained pretty well here https://youtu.be/JDmKLXVFJzk https://youtu.be/JDmKLXVFJzk It’s pretty heady so be ready
mikekchar 8y ago
Ob: I know nothing. But the way I look at it is: what is the difference between a point and infinite space? Taken from the perspective of light (travelling at the speed of light), the universe is still a single point. That is, the size of the universe is Lorenz contracted to zero. It also takes zero time to go from one end to the other ;-) From the perspective of light, what is the difference in the universe at the instant of the big bang and some time after it? Well... potentially nothing. The structure of the universe only makes sense from within the context of the universe itself. The space of the universe only makes sense inside the universe. The time of the universe only makes sense inside the universe. It's not the case that the big bang exploded material into empty space. The amount of energy/mass in the universe has never changed from the perspective of the universe. The big bang is the distinction of structure in the universe. What is the difference between a point and an infinite space of uniformity? When an infinite space of uniformity loses absolute uniformity, what happens? Why did the universe not collapse into a singularity? Because it was already a singularity. It would require some kind of structure in space in order for a "collapse" to make sense. As soon as the structure is created, it is no longer uniform, and so it will no longer collapse uniformly - especially if space is expanding at the same time. I suspect there are several black holes at the "centre" of the universe, but I can not think of a way for all the matter to collapse into a singularity.
thecupisblue 8y ago
In my mind: There is Universe n-1 (before ours). Last matter and anti-matter collide. Then, as there is nothing and as there is everything, the cycle begins again since we can't achieve "the balance". A big bang occurs, a universe n is born, with it's own set of laws (the laws of physics, like a new buildconfig). Everything slowly spreads and gets into perfect balance. Then everything dies. Then the last bits of matter stop existing. The Boom. A new universe is born, n+1. It has it's own set of constants. Maybe they aren't balanced properly, so it lives for a short moment and then dies and n+2 is born. And so on and so on.
swamp40 8y ago
Perhaps once a black hole has nothing left to suck in, it explodes?

by sulam 8y ago

myinnerbanjo 8y ago

I think inflation had something to do with it, the only time in the history of the universe something faster than the speed of light was possible--but only because space and time itself was expanding so quickly. Just stumbled upon this: https://www.reddit.com/r/askscience/comments/8p7hnn/where_any_black_holes_created_immediately_after/ https://www.reddit.com/r/askscience/comments/8p7hnn/where_an...

DougWebb 8y ago

I'm absolutely not an expert, but I believe there wasn't any mass at first; it was all energy. It had to slow down to become mass, at which point it was spread out enough. Or something along those lines. Another possibility is that black holes are places where lots of mass accumulates in the same place in space, while the big bang was a rapid expansion of space itself. Even if there was enough mass in one spot to be what we'd consider a black hole, the space it occupied expanded, dragged the mass along with it, and ripped the black hole apart.

twiceaday 8y ago
Doesnt E=mc^2 form an equivalence? I would expect energy to warp space-time just as mass does.
- waterhouse 8y ago
  I believe it does. If you have a bottle full of chemicals with potential energy, and then you make them react and get the energy from them (via, I dunno, heat), while all the particles remain in the bottle, then I think the bottle weighs less afterwards—though probably by an undetectably small amount. I think I read that the change in mass is detectable after, say, nuclear fission.
- pdonis 8y ago
  > I would expect energy to warp space-time just as mass does. It does. So do pressure and stresses. The full "source" of gravity in General Relativity is the stress-energy tensor, which includes all of these things.
mygo 8y ago
> it was all energy. It had to slow down to become mass If there was no mass then what made the energy slow down? For example a photon will travel at the speed of light in a vacuum indefinitely. In what scenario does energy just slow down without interacting with matter?
InclinedPlane 8y ago
"Mass" is just another name for "rest-energy". A single photon cannot have mass because it has zero energy in the reference frame where it is at rest (zero net momentum). But a pair of non-parallel photons can have rest-mass, and indeed the mass of the photons inside a star is considerable, for example.
- pdonis 8y ago
  > "Mass" is just another name for "rest-energy". Not if you're talking about what the source of gravity (spacetime curvature) is. Massless particles still have energy and still cause spacetime curvature.
  - InclinedPlane 8y ago
    The "source" of gravity is the stress-energy tensor. "Mass" (rest-energy) is a simplification that is useful for approximations and conceptualizations.
    - pdonis 8y ago
      > "Mass" (rest-energy) is a simplification that is useful for approximations and conceptualizations. Yes, but not in a context where radiation is present, since radiation can have energy but has no rest mass (rest energy).
- pdonis 8y ago
  > A single photon cannot have mass because it has zero energy in the reference frame where it is at rest (zero net momentum) I didn't catch this before, but it is not correct. There is no such thing as a frame in which a photon is at rest.
flukus 8y ago
You can make black holes with massless particles: https://en.wikipedia.org/wiki/Kugelblitz_%28astrophysics%29 https://en.wikipedia.org/wiki/Kugelblitz_%28astrophysics%29
pdonis 8y ago
> I believe there wasn't any mass at first; it was all energy. It had to slow down to become mass This is partly true. During inflation, the universe was vacuum; all of the energy was in the "false vacuum" state of whatever field caused inflation (it's usually called the "inflaton" field, which IMO is a very confusing name). When inflation ended, all of that energy got transferred to the Standard Model fields, forming matter and radiation at very high temperature and expanding very rapidly. So the "all energy at first" part is true; but the "had to slow down to become mass" part is not. The matter and radiation was there as soon as inflation ended; it didn't have to slow down first.
- jacobush 8y ago
  But did it end?
  - pdonis 8y ago
    You mean, did inflation end? Yes, it did.
    - jacobush 8y ago
      So the inflation is different from the current expansion.
      - pdonis 8y ago
        Yes.

twiceaday 8y ago

Neither space nor time existed prior to the big bang and energy levels were so high that a different set of laws of physics dominated the early evolution.

bpchaps 8y ago

My naive (and likely wrong) understanding is that cosmological inflation was caused by a "false vacuum" of the higgs field. Think of it as if the energy pressure of a system is higher than a vacuum, but has similar pressure properties of a vacuum. That had the effect of gravitational pressure pushing matter outwardly, instead of pulling it inwardly like we have it today (to answer your question). That caused enormously fast expansion of the universe. Very shortly afterwards, the false vacuum "decayed", and gravity started behaving as you know it. Check out Alan Guth's inflationary universe - a surprisingly accessible book on a deeply complex subject. Someone please correct me where I'm wrong!

pdonis 8y ago
Your description of what happened during inflation is basically correct. However, in order to explain why a black hole didn't form as soon as inflation ended and gravity started behaving "as we know it", you have to include the fact that all of the matter that was formed at the end of inflation was expanding very, very rapidly. (The universe was actually vacuum during inflation--at the end of inflation all of the energy in the false vacuum got transferred to the Standard Model fields, creating matter and radiation at extremely high temperature and expanding.)

bglusman 8y ago

I don't know the technical/methematical explanation for this, or if one is really needed (though it seems like a valid question to pose), but it is noteworthy that black holes and the big bang are the original "singularity", and that there are several related theories in which they are directly related, notably Big Bounce[1] and my favorite (which I learned of from Lee Smolin's amazing Life of The Cosmos[3]) Cosmological Natural Selection[2]. There's also a variant of Cosmological Natural Selectionm CNS-I[4], but that one possibles borders on religious, though it's possible it could be falsifiable/supportable some day by modeling. I suppose it's even somewhat plausible that the answer to your question, in combinations with the other theories, are "all true", and that for various reasons or models the first N big bangs did result in immediate/fast collapse back to another big crunch/black hole, and that each subsequent crunch/black hole created another universe, until there was a universe that survived long enough to create several black holes, and so on and so on... [1]https://en.wikipedia.org/wiki/Big_Bounce https://en.wikipedia.org/wiki/Big_Bounce [2]http://evodevouniverse.com/wiki/Cosmological_natural_selection_(fecund_universes) http://evodevouniverse.com/wiki/Cosmological_natural_selecti... [3]https://www.amazon.com/Life-Cosmos-Lee-Smolin-ebook/dp/B004TW1YY6 https://www.amazon.com/Life-Cosmos-Lee-Smolin-ebook/dp/B004T... [4]http://evodevouniverse.com/wiki/Cosmological_natural_selection_(fecund_universes)#CNS_with_Intelligence_.28CNS-I.29 http://evodevouniverse.com/wiki/Cosmological_natural_selecti...

T-A 8y ago

The early universe was ridiculously homogeneous. We can infer this with reasonable certainty from the extreme homogeneity of the CMB and from the gravitational instability of density fluctuations: as you expect, they attract more mass and accrete to form stars and galaxies. So if you start from the universe as it looks now and run time backwards, you get ridiculous homogeneity at the bang. And that's why you didn't get a giant black hole. There was a lot of mass at very high density, and gravity was pulling very hard at it - but it was pulling (almost exactly) equally hard in all directions.

Filligree 8y ago
So, are we assuming the universe is infinite? Otherwise there'd be a border to that.
- FreeFull 8y ago
  The universe can be finite without a border. Think about living on the surface of a sphere or a torus (finite, no edge), as opposed to living on a sheet of paper
  - hndamien 8y ago
    Part of me thinks that black holes are the border, and we might be inside one.
- kryptiskt 8y ago
  The Earth's surface is finite, but without a border. A finite universe would be similar, just adding a dimension.
- richardknop 8y ago
  I think almost all options are currently still in play. Finite with border, finite in infinite space, infinite.
- arithma 8y ago
  If it was finite, does it mean it was definitely not flat?

infradig 8y ago

You assume it didn't. Perhaps the universe as we know it is the inside of a black-hole.

InclinedPlane 8y ago

This is an important question with a slightly surprising answer. First you need to understand that the Big Bang was not an explosion of matter into space, it was an explosion of matter and space (technically space-time). This might be hard to wrap your head around, it might be easier to think about space-time as non-infinite, say curving back on itself like the surface of a balloon (though as far as we can tell it is infinite). Second, during the early Universe just after the Big Bang when the energy density of the Universe was incredibly high the density was nevertheless incredibly uniform. So, let's say you are inside a soup of matter and energy with a density as high or higher than the core of a star, but there is matter in every direction, what way is gravity pulling you? This is the other non-intuitive bit, gravity isn't pulling you in any direction. Pick a volume of matter in space around you and consider how its gravity affects you, for every single volume there is a corresponding volume on the opposite side of you that has exactly the same mass and is exactly as far away from you, cancelling out the gravity from the first chunk. And this is true for every single bit of volume around you, there's a 1:1 mapping of bits of stuff around you with other bits of stuff on the opposite side of you which exactly counterbalances the gravitational force from the first bit. The gravitational field is uniform, so nothing collapses, no black holes form, no stars or planets form either. It's only after the Universe continues to expand and minute variations in local density are gradually amplified (over millions of years) into denser and denser clumps that it becomes possible to form galaxies and galaxy clusters/super-clusters (and thus black holes). Now, we don't know for sure whether or not there were isolated little "super clumps" of extra density in the early Universe which allowed for the formation of black holes prior to the formation of galaxies (so-called primordial black holes). That's one theory for explaining the existence of supermassive and especially ultramassive black holes, but it's unconfirmed. However, now it seems as though we have enough evidence to show that primordial black holes can't be the explanation for the evidence for dark matter.

perl4ever 8y ago
"First you need to understand that the Big Bang was not an explosion of matter into space, it was an explosion of matter and space (technically space-time)." This is a type of statement I have read many, many times, but to me, it's the same as saying "wakalixes!". I've never read anything that gave me a hint of why and how there should be a difference between two objects receding from each other and "space itself" expanding.

pdonis 8y ago

> if all the current mass of the universe was in a small volume wouldn’t it have promptly created a black hole? No, because it was expanding too rapidly. The usual intuitions about how much mass in how small a volume it takes to make a black hole assume that the mass is stationary, or nearly so.

sulam 8y ago
This matches a description I was able to find online. What that seems to imply though is that expansion was literally faster than light. Yes? No? [edit] and it seems this is true and can be explained by the space-time matrix not being bound by light speed limits.
- pdonis 8y ago
  > What that seems to imply though is that expansion was literally faster than light. Yes? No? No. No particle of matter outruns a light ray at the same location and moving in the same direction. Or, to put it another way, all pieces of matter are moving within the light cones at their spacetime locations. "Moving within the light cones" is the correct General Relativity version of "nothing goes faster than light". The coordinate speed of pieces of matter can be greater than c, but in a curved spacetime that, in itself, is not a problem.
- HeadsUpHigh 8y ago
  Yes it was. I think it's called the inflation period and is used as a possible explanation of why there are big voids in the universe.
- cryptonector 8y ago
  The expansion of space is always faster than the speed of light if you pick any two points far enough apart from each other, at least as long as space is expanding and the universe is infinite. Or if the universe is finite, some time after the bang there will be points which separate faster than the speed of light from each other. These points aren't moving, of course.
cryptonector 8y ago

dantillberg 8y ago

Maybe the big bang did produce a black hole, and we're living inside it? Back in my undergrad physics days, I postulated that our "universe" could be a big black hole inside some "extraverse". I worked through classical gravitational calculations to determine, from an estimate of the mass density of the universe, what the Schwarzschild radius (aka event horizon radius) of that black hole would be, and it came out to between 5 and 40 billion light years or so (the density of the universe is hotly debated, see other discussion in this thread about dark matter), which is at least the right order of magnitude.

jobigoud 8y ago
How is it the right order of magnitude? Did you compute that based on the size of the observable Universe? But that's arbitrary, the Universe is way bigger than that.
- Simon_says 8y ago
  No. The input into the equation is the density of matter in space, nothing having to do with the size of the observable universe. On large scales in the visible universe, matter is fairly evenly distributed at a density of about 1 atom of hydrogen per cubic meter. For any constant nonzero density form of matter, if you pack enough of it together in a sphere in a flat spacetime, its Schwarzchild radius will eventually exceed its radius. This is because the Schwarzchild radius is proportional to the mass, but the radius of the actual matter is proportional to the cube-root of the mass. It's a fun thought experiment, but not physically meaningful in this universe because our spacetime is expanding, and the original calculations assume there's nothing at the edge pulling 'outward', but in our universe there is.
  - pdonis 8y ago
    > For any constant nonzero density form of matter, if you pack enough of it together in a sphere in a flat spacetime, its Schwarzchild radius will eventually exceed its radius. You can't do this in flat spacetime, because the matter curves spacetime, and the denser you pack it, the more it curves spacetime.

headShrinker 8y ago

There are major challenges/issues with Big Bang theory. The easiest issue to see and understand is temperature in the CMB appears to indicate “communication”... basic descriptions of the problem here https://youtu.be/dbm3M9Bz4RE https://youtu.be/dbm3M9Bz4RE No one really knows anything about the universe before 10^-42 seconds, and there is no physics to describe what likely happened. Photons/light and matter as we known it didn’t exist and wouldn’t for another 400,000 years. There are many very deep issues that are explained pretty well here https://youtu.be/JDmKLXVFJzk https://youtu.be/JDmKLXVFJzk It’s pretty heady so be ready

mikekchar 8y ago

Ob: I know nothing. But the way I look at it is: what is the difference between a point and infinite space? Taken from the perspective of light (travelling at the speed of light), the universe is still a single point. That is, the size of the universe is Lorenz contracted to zero. It also takes zero time to go from one end to the other ;-) From the perspective of light, what is the difference in the universe at the instant of the big bang and some time after it? Well... potentially nothing. The structure of the universe only makes sense from within the context of the universe itself. The space of the universe only makes sense inside the universe. The time of the universe only makes sense inside the universe. It's not the case that the big bang exploded material into empty space. The amount of energy/mass in the universe has never changed from the perspective of the universe. The big bang is the distinction of structure in the universe. What is the difference between a point and an infinite space of uniformity? When an infinite space of uniformity loses absolute uniformity, what happens? Why did the universe not collapse into a singularity? Because it was already a singularity. It would require some kind of structure in space in order for a "collapse" to make sense. As soon as the structure is created, it is no longer uniform, and so it will no longer collapse uniformly - especially if space is expanding at the same time. I suspect there are several black holes at the "centre" of the universe, but I can not think of a way for all the matter to collapse into a singularity.

thecupisblue 8y ago

In my mind: There is Universe n-1 (before ours). Last matter and anti-matter collide. Then, as there is nothing and as there is everything, the cycle begins again since we can't achieve "the balance". A big bang occurs, a universe n is born, with it's own set of laws (the laws of physics, like a new buildconfig). Everything slowly spreads and gets into perfect balance. Then everything dies. Then the last bits of matter stop existing. The Boom. A new universe is born, n+1. It has it's own set of constants. Maybe they aren't balanced properly, so it lives for a short moment and then dies and n+2 is born. And so on and so on.

swamp40 8y ago

Perhaps once a black hole has nothing left to suck in, it explodes?