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marcus

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## Main Question or Discussion Point

This thread is distinct from discussion of the mainstream media coverage of the letter that just appeared in Nature Physics.

I just want to get more precise about the actual issues.

So far, not having a subscription, I don't have access to the advance online publication in Nature Physics (NP), but there is a preprint of a longer article at arxiv which cites the letter and mentions some relevant conclusions.

http://arxiv.org/abs/0706.1057

Martin Bojowald, Hector Hernandez, Aureliano Skirzewski

42 pages

The NP letter is reference

==quote==

...In such a situation, it is important to know the precise state,

i.e. all parameters c1, c2, α1, α2, δ1 and δ2, in order to determine the quantum nature of

the bounce. However, the relevant squeezing parameters cannot be fully determined from

using the state at only one side of the bounce

of some of the integration constants in (29)–(32) if one restricts φ to a fixed sign. Thus,

the precise quantum nature of the bounce may always depend on what assumptions one

makes for a state...

...

At first perturbative order, there is no strong back-reaction for a Wheeler-DeWitt quantization,

or for most of a single collapsing or expanding branch in a loop quantization.

But quantum back-reaction effects are noticeable for squeezed states around

the bounce of loop quantum cosmology due to the influence of the growing branch on the

collapsing one. Since squeezing parameters of a bouncing state are not determined by

properties in one of the two large volume regime only

the presence of a semiclassical bounce is available. There are quantum parameters whose

influence is negligible at large volume but which will become important near a bounce and

determine its fate.

==endquote==

I just want to get more precise about the actual issues.

So far, not having a subscription, I don't have access to the advance online publication in Nature Physics (NP), but there is a preprint of a longer article at arxiv which cites the letter and mentions some relevant conclusions.

http://arxiv.org/abs/0706.1057

**Effective equations for isotropic quantum cosmology including matter**Martin Bojowald, Hector Hernandez, Aureliano Skirzewski

42 pages

The NP letter is reference

**[29]**in this paper. See for example pages 34 and 37==quote==

...In such a situation, it is important to know the precise state,

i.e. all parameters c1, c2, α1, α2, δ1 and δ2, in order to determine the quantum nature of

the bounce. However, the relevant squeezing parameters cannot be fully determined from

using the state at only one side of the bounce

**[29]**due to exponential suppression factorsof some of the integration constants in (29)–(32) if one restricts φ to a fixed sign. Thus,

the precise quantum nature of the bounce may always depend on what assumptions one

makes for a state...

...

At first perturbative order, there is no strong back-reaction for a Wheeler-DeWitt quantization,

or for most of a single collapsing or expanding branch in a loop quantization.

But quantum back-reaction effects are noticeable for squeezed states around

the bounce of loop quantum cosmology due to the influence of the growing branch on the

collapsing one. Since squeezing parameters of a bouncing state are not determined by

properties in one of the two large volume regime only

**[29]**, no general statement aboutthe presence of a semiclassical bounce is available. There are quantum parameters whose

influence is negligible at large volume but which will become important near a bounce and

determine its fate.

==endquote==

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