# Planck Stars and GR black and white holes

Tags:
1. Mar 25, 2015

### wabbit

Rovelli & Vidotto's Planck Stars describes a possible quantum black hole - white hole transition through a quantum bounce somewhat analogous to the LQC bounce.
In another thread, @marcus pointed out to me that this was not necessarilly considered the most likely scenario for a QG black hole.
Is this not however in a way, qualitatively, an almost unavoidable scenario in quantum gravity?
If I understand it correctly, the GR black hole in Kruskal coordinates already describes a pair (BH, WH) joined by a singularity at r=0, with a metric $$ds^2=\frac{4k}{r}e^{-\frac{r}{k}}(-dT^2+dX^2)+r^2d\Omega^2$$
If quantum gravity (or any modification to GR at high density) resolves the singularity, and assuming an effective spacetime description remains possible near the singularity, as seems to be the case in LQC, one would expect this to be replaced by an effective metric of a form more or less similar to $$ds^2=\frac{4k}{\phi(r)}e^{-\frac{r}{k}}(-dT^2+dX^2)+\phi(r)^2d\Omega^2$$where $\phi(r)\sim r\text{ for }r\gg r_{min}\text{ but }\phi(0)>0$.
But then it would seem that this naturally extends to the whole Kruskal solution and must be describing the type of scenario explored in the above reference?

Is this too simplistic, or are the assumptions above too strong? What am I missing here?

Thanks

Last edited: Mar 25, 2015
2. Mar 25, 2015

### rootone

I like the idea, and I don't remember where I came across it -
That what we see as a 'big bang', is actually the same stuff as what goes in to black holes.
A variation on the cyclic universe idea .

* prepares to be trashed *

3. Mar 26, 2015

### marcus

Thanks for signaling me about this thread. I'm not sure I have anything substantive to contribute. To be explicit for any reader not already familiar with it, in the Planck Star picture the rebound is drastically delayed by gravitational time dilation (from an outsider's perspective) and the explosion occurs in the same neighborhood where the BH formed. We see it, if we're still around when it finally happens.

You are right that there is, so far, no consensus among LQG folk about what happens to black holes. For instance Gambini and Pullin are longtime productive highly-regarded LQG researchers and they have a different BH picture. In their version they find a QG way around the information loss puzzle so the BH can just peacefully evaporate according to the original Hawking evaporation timetable. They aren't the only LQG researchers to reach this conclusion. Alejandro Perez is one of the younger bunch that may have found solutions different from that of Gambini and Pullin, that however also permit non-explosive evaporation.

There are also older papers (by a wide range, not just LQG) where the BH bounces but vents somewhere else, in some other tract of spacetime, perhaps creating another universe. My impression is I haven't seen much if any recent work along those lines. There could be some and I just missed it.

If we leave out the older "baby universe" idea and stick to current LQG BH research, then I think it narrows down to two basic pictures.
A. Planck star delayed rebound explosion, a possible explanation for certain Gamma Ray Bursts (GRB) and unexplained Radio bursts.
B. Peaceful evaporation with a brief high temperature flare at the end (Hawking temp goes up as mass dwindles down to nearly nothing)

Have to admit I'm really intrigued by the former, the Planck star idea. However for that idea to survive it's essential to find a class of observable blasts or flashes that can arguably be explained as end-bursts of primordial black holes. No other BHs have been around long enough to be exploding.

Last edited: Mar 26, 2015
4. Mar 26, 2015

### wabbit

Thanks for your comments. In terms of visibility they (final bursts vs white hole) presumably would be distinguishable in the kind of GRBs or other flare either might produce (I suppose at least much higher absolute magnitude at a given distance for A's since they would be far more massive than B's in their final instant) - assuming some primordial black hole would be kind enough to oblige.

Edit: this paper discusses primordial black hole detection:
Primordial Black Holes, Jane H MacGibbon, Tilan N. Ukwatta, J.T. Linnemann, S.S. Marinelli, D. Stump, K. Tollefson (Submitted on 3 Mar 2015)

Last edited: Mar 26, 2015
5. Mar 26, 2015

### wabbit

I believe a time reversed big bang and a BH both crush everything, but not in the same way: for comoving observers at least, the first one is perfectly symmetric so you are compressed but you do not change shape - while the BH doesn't compress, it spaghettifies you.