Big Bang singularity vs Black hole singularity

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One speaks of the Big Bang initial condition as a singularity possessing an extremely low entropy (to explain the growth of entropy throughout time to our present stage of the universe). If that singularity truly possesses infinite density, that would make perfect sense, since what would become sub-atomic particles later can have no meaningful individuality within the BB singularity when the density state is infinite. To my way of thinking the infinite density would demand some kind of amorphous 'blob' of matter/energy in a continuous rather than discrete form. To speak of micro states in this condition would have no meaning, since the blob can only exist in one micro/macro state. So far, so good (for me).

Assuming black holes also exist as a singularity at its 'heart,' the same condition found in the BB singularity, namely infinite density, would also require the same kind of 'one micro/macro state for the black hole's singularity. I say this not knowing if the BB singularity is identical to the black hole's singularity. If 'you've seen one singularity, you've seen 'em all' is true, why then do we believe that the entropy is in its highest form for the black hole, just the opposite of the BB's singularity? Since I do not know for sure if a singularity has true infinite density I may be wrong in that assumption.
 
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  • #2
tom.stoer
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Geometrically these singularities are different.
 
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bcrowell
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Geometrically these singularities are different.
To amplify on this a little, you can classify singularities according to whether they can exist in the past and/or future light cones of an observer. A black hole singularity can only be in an observer's future light cone. The big bang singularity can only in in someone's past light cone. A naked singularity can be in both.

[EDIT] In addition, a black hole singularity has an event horizon, and the way an event horizon is normally defined doesn't really make sense if you try to apply it to a cosmological spacetime.

The following FAQ is relevant: https://www.physicsforums.com/showthread.php?t=506992 [Broken]
 
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  • #4
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quant posts:
I say this not knowing if the BB singularity is identical to the black hole's singularity.
they are NOT the same as already posted....

One big difference is that a BH singularity is collapsed mass-energy while the BB singularity is collapsed everything, even space-time. The BB never became a black hole due to gravitational attraction; apparently it did the opposite due to some repulsive phenomenon.

Here are some descriptions I have saved trying to understand different viewpoints:


From THE NATURE OF SPACE AND TIME, Hawking,Penrose:

Penrose observes that (in my possibly inaccurate paraphrase):

(1) The Big Bang was not a generic state. A generic Big Bang state would have had a large Weyl curvature, but the universe we see looks nothing like the one that would have resulted from such an initial state. Our Big Bang appears to have had a small or even vanishing Weyl curvature.

(2) The evolution of our universe has led to a state with nonvanishing Weyl curvature. (At black hole singularities, we even have diverging Weyl curvature.)



from Roger Penrose THE ROAD TO REALITY....PG 766:

Let us now think of a universe evolving so that an initially uniform distribution of material [with some density fluctuations] gradually clumps gravitionally, so that eventually parts of it collapse into black holes. The initial uniformity corresponds to a mainly Ricci-curvature [matter] distribution, but as more and more material collects gravitationally, we get increasing amounts of Weyl curvature....The Weyl curvature finally diverges to infinity as the black-hole singularities are reached. If we think of the material as having been originally spewed out from the Big Bang in an almost completely uniform way, then we start with a Weyl curvature that is....[essentially] zero. Indeed, a feature of the FLRW models is that the Weyl curvature vanishes completely. ...For a universe to start out closely FLRW we expect the Weyl curvature to be extremely small, as compared with the Ricci curvature, the latter actually diverging at the Big Bang. This picture strongly suggests what the geometrical difference is between the initial Big Bang singularity- of exceedingly low entropy- and the generic black hole singularities, of very high entropy.



A Quantum Gravity Extension of the Inflationary Scenario
Ivan Agullo, Abhay Ashtekar, William Nelson
(Submitted on 7 Sep 2012)
http://arxiv.org/abs/1209.1609

This strong repulsive gravitational force due to quantum geometry is a pre inflationary dynamic applicable from a Planck scale big bounce to the onset of slow roll inflation.
anda from a related paper by the same authors:

This paper …. provides a detailed extension of the cosmological perturbation theory to the Planck regime. ..., the FLRW space-times of interest are invariably incomplete in the past due to the big bang singularity where matter fields and space-time curvature diverge.
 
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bcrowell
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One big difference is that a BH singularity is collapsed mass-energy while the BB singularity is collapsed everything, even space-time.
I don't think this is really a correct distinction. Both are collapsed everything.

The BB never became a black hole due to gravitational attraction; apparently it did the opposite due to some repulsive phenomenon.
You do not need repulsion (e.g., a cosmological constant or violation of an energy condition) to get a big bang cosmology. The first BB cosmologies were developed with [itex]\Lambda=0[/itex] and normal forms of matter.

(1) The Big Bang was not a generic state. A generic Big Bang state would have had a large Weyl curvature, but the universe we see looks nothing like the one that would have resulted from such an initial state. Our Big Bang appears to have had a small or even vanishing Weyl curvature.
This has more to do with distinguishing our BB from hypothetical high-entropy big bangs. It's not that closely related to distinguishing a big bang from a black hole (although I suppose it's true that no-hair theorems apply to black holes but not to big bangs).
 
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  • #6
DrDu
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Isn't this related to Glenn Miller compactification :-)
 
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  • #7
Thank you all who replied to my initial post. Your observations, comments and citings were as fascinating and insightful as one would expect from such talented minds!
 
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Quote by Naty1
One big difference is that a BH singularity is collapsed mass-energy while the BB singularity is collapsed everything, even space-time.

bcrowell: I don't think this is really a correct distinction. Both are collapsed everything.
[well, you LOOK 'uncollapsed!!] [LOL]


Note to Marcus: Look what YOU did....got me in trouble with crowell!! [LOL]


The BB never became a black hole due to gravitational attraction; apparently it did the opposite due to some repulsive phenomenon.
bcrowell: You do not need repulsion (e.g., a cosmological constant or violation of an energy condition) to get a big bang cosmology. The first BB cosmologies were developed with Λ=0 and normal forms of matter.

I was referring only to the initial bang....not the subsequent inflation.....such an initial repulsive force seems an aspect of quantum cosmology....maybe even keeps collapse finite....
 
  • #9
Bill_K
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In addition to which, a black hole singularity and the big bang singularity have different symmetries. Near a black hole singularity, spacetime is cylindrically symmetric, collapsing in the radial directions and expanding along the axis.
 
  • #10
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Quote by Naty1

One big difference is that a BH singularity is collapsed mass-energy while the BB singularity is collapsed everything, even space-time.
bcrowell:
I don't think this is really a correct distinction. Both are collapsed everything.
I had the misfortune of thinking about this after I signed off and would like to explore
this issue a bit more. Of course maybe it's just terminology.

Marcus actually made such a post [I saved it] and I liked it because it matched my understanding that when an inertial observer free falls towards a black hole, time and space are normal as the observer passes the Schwarzschild radius, r= 2M.....the event horizon.....so it seems space and time exist inside the horizon and only as the observer reaches near the singularity do space and time and everything else appear to be squished into a 'quantum foam'...an apparent 'singularity' if you wish.

So where am I going wrong here?? Is there a more refined description?
 

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