Could someone please settle something conceptually for me if possible? marcus, if you catch this, please help me out.(adsbygoogle = window.adsbygoogle || []).push({});

Consistent probabilities in loop quantum cosmology - Craig, Sing

http://arxiv.org/abs/1306.6142

"Consistent with the prediction that "all states bounce", as notedall states in sLQC look like a particular symmetric superposition of expanding and collapsing Wheeler-DeWitt universes at large volume. While this is certainly a necessary condition for a theory in which a bounce is a generic feature, one may be led to inquire whether the presence of this superposition is in some sense the reason for the bounce. The answer is definitely NO. In fact, it was shown in Ref. 10. that a superposition of expanding and contracting universes in the Wheeler-DeWitt quantization of this same physical model does not, and indeed cannot bounce: all states are sucked in to the singularity at large. If a physical reason for the bounce is to be sought, it is in the "quantum repulsion" generated at small volume in loop quantum gravity, and manifested in this model in the ultraviolet cutoff in the dynamical eigenfunctions, not in the superposition of large expanding and contracting classical universes."

From a related paper "Consistent probabilities in Wheeler-DeWitt quantum cosmology" also by Craig, Singh:

"Of some special interest is the case where the quantum state is a superposition of contracting and expanding universes. Such a state might be regarded as an analog of a "Schrodinger's cat" state in ordinary quantum mechanics - a quantum superposition of macroscopically distinct states."

First, can you summarize what the above means conceptually? I find it incredibly interesting, but I feel like I am being given two different ways of conceptualizing the model. Admittedly, I am not mathematically-inclined, but what I am looking for is a visual way of understanding this.

According to these papers, I am visualizing a Universe that expands (bounces) from a quantum state, reaches a maximum area and then contracts back to a quantum state in the same "space." The expanding and contracting "branches" extending from the quantum state share the same spatio-"temporal" area, i.e., they are macroscopicallysuperposed. I am seeing this as a sphere that inflates rapidly from a Planckian space to a maximum area, and then contractsinwardback toward the same Planckian space and bounces, over and over in the same "space" localized from the same quantum "point."

So I admittedly could be totally confused, because the concept has been referred to MUCH differently...

So is the Universe bouncing "up" and then "down" in the same "space" as I stated above? (creating new Universes with each bounce)... Or is the Universe bouncing from "left" to "right" (as much as these terms can make sense) creating "new" space with endless branches of expansion and contraction conjoined at endless quantum bridges?

This is how the model has been shown graphically:

^is this what is the bouncephysicallyrepresents, the "quantum bridge" literally between a contracting previous Universe (on the "left", in a different area) and an expanding "new" Universe on the "right" in a spatially different area?

"This includes, in particular, Schrodinger's Cat" states, generic (but possibly macroscopic) superpositions ofleft-moving(contracting) andright-moving(expanding) states."

^ from one of the above papers... Is this just mathematically speaking?

I created this image to help "ask" my question:

Is this how you visualize the idea? Universes being created endlessly in different "areas"? With each Universe connected to another former Universe ad infinitum via quantum bridges?

Or is it all "self contained" as the superposition paper led me to believe, as this:

....or, as below, does it bounce from "one side to the other" and create new Universes? (Would each side have a superposition of expansion and contraction as discussed above? ...one side has a physical Universe, while the other does not, and switches back and forth, forever?):

^This is the model that Alejandro Corichi uses to show the idea

How do you conceptualize the bounce? Why do they sometimes refer to this "Universe on the other side" as here:

“The significance of this concept is that it answers what happened to the universe before the Big Bang,” Singh told PhysOrg.com. “It has remained a mystery, for models that could resolve the Big Bang singularity, whether it is a quantum foam or a classical space-time on the other side. For instance, if it were a quantum foam, we could not speak about a space-time, a notion of time, etc. Our study shows that the universeon the other sideis very classical as ours.”

“This means that the twin universe will have the same laws of physics and, in particular, the same notion of time as in ours,” Singh said. “The laws of physics will not change because the evolution is always unitary, which is the nicest way a quantum system can evolve. In our analogy, it will look identical to its twin when seen from afar; one could not distinguish them.”

“In the universe before the bounce, all the general features will be the same,” said Singh. “It will follow the same dynamical equations, the Einstein’s equations when the universe is large. Our model predicts that this happens when the universe becomes of the order 100 times larger than the Planck size. Further, the matter content will be the same, and it will have the same evolution. Since the pre-bounce universe is contracting, it will look as if we were looking at ours backward in time.”

“Such a universe will have many bouncesfrom one branch to another,” Singh said. “It is also possible that universes indifferent brancheswill be identical.”

I am sorry for writing so much, but I am insanely curious.

Any and all help appreciated and again, I am sorry for being redundant and ignorant.

**Physics Forums - The Fusion of Science and Community**

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Conceptualizing the Big Bounce in LQC

Loading...

Similar Threads - Conceptualizing Bounce | Date |
---|---|

For myKK, in bounce models what drives prior contraction? | Sep 10, 2015 |

In big bounce models, what drives the contracting phase? | Sep 9, 2015 |

Time direction in LQC bounce | Feb 13, 2015 |

Conceptual ramifications if black holes do not exist. | Jan 2, 2011 |

**Physics Forums - The Fusion of Science and Community**