# Will a bouncing cosmology lose information

1. Oct 10, 2015

### wolram

I have been looking for literature on this question but can not find any, will a (bounce) retain all the information of the previous galaxy ?

Last edited: Oct 10, 2015
2. Oct 10, 2015

### wolram

http://arxiv.org/abs/1509.01147

I propose that the information loss paradox can be resolved by considering the supertranslation of the horizon caused by the ingoing particles. Information can be recovered in principle, but it is lost for all practical purposes.

It seems like information will be lost in a black hole so why would a universal bounce be different?

3. Oct 11, 2015

### wolram

No one for this question or are you all ignoring me

4. Oct 11, 2015

### Chronos

5. Oct 12, 2015

### wolram

Thank you for your reply Chonos, what i do not know is how dense a collapsing universe would become, would it have to go to a BH state or could it bounce at some other density, if it had to go the BH state would our particle zoo be the same , are they natural in any universe.?

6. Oct 12, 2015

### wolram

Last edited: Oct 12, 2015
7. Oct 12, 2015

### slatts

Most of the "bounce" cosmologies I've read about have been quantum mechanical, but a favorite of mine, Aguirre & Gratton's with arrows of time pointing in opposite directions, is relativistic. Vilenkin, who wrote a critique of it (available free on the web, and called "Arrows of Time and the Beginning of the Universe") and also influenced its formulation, refers to the three-spherical boundary between the arrows (which isn't quite a "singularity") as a "bounce surface".

As you'll see on its p.3, he also describes "the universe" as "bouncing" at that surface. (I've also skimmed through AG's original paper, linked to the bibliography entry of it in Vilenkin's critique, and, surprisingly, Aguirre & Gratton themselves do not use the "bounce" terminology!)

I'm rather lame in physics, and was very surprised to find out, just a couple of days ago, that the de Sitter space that most of the cosmologies I've seen described are supposed to occur in actually requires a contracting phase, which is supposed to precede an expanding phase! The expanding phase, and the inflation or near-exponential expansion, is all that most of the relativistic cosmologists I've read ever talk about, in English at least. (I've mostly stuck to them, because, although I can't follow much of their math, I know even less about QM notation.)

I believe that the entropy problem Chronos was mentioning just has to do with repeatedly-bouncing cosmologies, and you can zero in on them, in web searches, by googling "oscillating universes". The density of entropy increases with each bounce, so that, if that was the way things had always been, tracing them back in time would reach a point where the smallest density traceable would have left their size near the Planck limit. (I've never been able to pry a straight answer out of anyone as to why the Planck limit is such a sacred cow, but it's been occurring to me lately that it may have something to do with differences between Euclidean and elliptical geometries that prevent a simple increase in scale between a figure drawn in one and a figure drawn in the other, although the angles can be preserved through a "conformal diagram".) In its effects, this limitation is similar to the more general problem formulated in the Borde-Guth-Vilenkin Theorem of 2003, which requires a beginning for universes with an average expansion rate greater than zero.

Last edited: Oct 12, 2015
8. Oct 12, 2015

### Staff: Mentor

Do you have a reference for this?

One thing to bear in mind when talking about de Sitter spacetime is that it has a number of different possible coordinate charts on it, all of which look like FRW charts, i.e., the metric looks like the FRW metric with a scale factor multiplying the spatial part, but with different behavior of the scale factor with time. At least one of these charts, the "flat slicing" described on the Wikipedia page, has the scale factor exponential in time, which means it is always increasing (expanding), even as $t$ goes to minus infinity.

(Also note that another possible slicing of de Sitter spacetime, described on the Wiki page, is static--the metric is independent of time in this slicing! So one has to be very careful making statements about the physical meaning of de Sitter spacetime.)

9. Oct 12, 2015

### slatts

No, I couldn't find that particular reference, but, as I was flailing through a lot of stuff looking for something about a thread of my own, I'm sure it must've been out-of-context, like you're saying. The items in my search history did include one statement by an author (different from the one I'd been thinking of), at << http://online.kitp.ucsb.edu/online/strings-c03/guth/pdf/KITPGuth_2up.pdf >>, saying that the "inflating phase", in "the full de Sitter spacetime", "must be preceded by a contracting phase, which is not part of an inflationary model", but that statement was on a page headed by "De Sitter Space in Flat Coordinates".

This will definitely be something to watch out for. With relativistic theories, I can see how the difference between "space" and "spacetime" can be critical in the English version, but, with quantum theories, the theorists' absolute view of time may make it difficult for me to follow your advice.

Last edited: Oct 12, 2015
10. Oct 13, 2015

### Staff: Mentor

When put in context, what he's referring to is that the various possible charts on de Sitter spacetime don't all cover the entire manifold; in particular, the "flat slicing" chart does not. (Note that he says "the de Sitter spacetime can of course be extended beyond $t = - \infty$".) The "contracting" part of the spacetime is in the portion that the flat slicing does not cover. Mathematically, that portion is there; but whether it is physically meaningful is a different question.

I'm not sure what you mean by this. Quantum field theory in curved spacetime certainly does not require or assume an "absolute" view of time; in fact, key effects, such as the Unruh effect and Hawking radiation, depend on observers in different states of motion having different notions of time.

11. Oct 13, 2015

### slatts

Well, as a way around the implausibility of the macroscopic superpositions required by the Many Worlds Interpretation of QM, I'd figured that the physicality of MWI must depend on the unlimited number of iterations of every combination of particles that is itself required by the future-eternal versions of inflation. Then, along came an impressive paper at
arXiv.org > hep-th > arXiv:1405.0298,
showing that MWI and eternal inflation are probably incompatible, except in odd branches of the wavefunction. Its authors (Boddy, Carroll, and Pollack) detail that incompatibility but point out that use of the Ghirardi-Rimini-Weber interpretation, instead of MWI, might restore validity to eternal inflation. However, they also mention that, in GRW, "the state of each particle has a fixed probability per unit time of spontaneously collapsing to a localized position", and, since it's hard to see what "unit time" could possibly apply to a spacetime expanding at up to six times the speed of light, it had seemed to me that QM might have an inflexibility about time corresponding only to relativity's insistence on the reality of space.

If you could give me just a hint of how some analysis of the CMB (or something) might resolve this impasse, I'd appreciate it.

12. Oct 13, 2015

### Staff: Mentor

"Time" here means "the proper time along the particle's worldline", or some similar notion (strictly speaking it would be the "average" worldline of the particle's wave function--I don't know how much work has actually been done to devise a relativistic formulation of GRW, the one I'm most familiar with is non-relativistic so it couldn't be used in cosmology anyway), which is perfectly well-defined even in a spacetime undergoing inflation at "six times the speed of light" (which is itself a very sloppy way of putting it; the expansion of the universe does not have a "speed", and the notional "speed" you get when you multiply the Hubble constant by the coordinate distance between two comoving objects is not a physically meaningful "speed", it's just a convenient abstract number that doesn't describe anything physical).

13. Oct 13, 2015

### anorlunda

I would like to ask the OP's question slightly differently.

Do the bouncing cosmology theories include non-unitary time evolution?