# Big Bang singularity vs Black hole singularity

1. Feb 3, 2013

### quantafyre

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.

Last edited by a moderator: Feb 4, 2013
2. Feb 3, 2013

### tom.stoer

Geometrically these singularities are different.

Last edited by a moderator: Feb 4, 2013
3. Feb 4, 2013

### bcrowell

Staff Emeritus
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]

Last edited by a moderator: May 6, 2017
4. Feb 4, 2013

### Naty1

quant posts:
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:

Last edited by a moderator: Feb 4, 2013
5. Feb 4, 2013

### bcrowell

Staff Emeritus
I don't think this is really a correct distinction. Both are collapsed everything.

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 $\Lambda=0$ and normal forms of matter.

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).

Last edited by a moderator: Feb 4, 2013
6. Feb 4, 2013

### DrDu

Isn't this related to Glenn Miller compactification :-)

Last edited by a moderator: Feb 4, 2013
7. Feb 4, 2013

### quantafyre

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!

8. Feb 4, 2013

9. Feb 5, 2013

### Bill_K

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. Feb 5, 2013

### Naty1

Quote by Naty1

bcrowell: