Hellfire's introduction to quantum cosmology

[marcus]

If you want an accessible overview of quantum cosmology try this TOC:

https://www.physicsforums.com/blogs/hellfire-3727/category/interesting.html

Hellfire covers different attempts made since the 1960s, and by comparing them (and showing historical development) makes the whole subject somewhat clearer.

During the past week (4 June - 11 June) he added two essays on Loop Quantum Cosmology, which I think are especially useful and could work for us as a reference for people who want an understandable introduction to LQC. Here are the links to those essays in particular:

Loop Quantum Cosmology I
https://www.physicsforums.com/blogs/hellfire-3727/loop-quantum-cosmology-i-982/

Loop Quantum Cosmology II
https://www.physicsforums.com/blogs/hellfire-3727/loop-quantum-cosmology-ii-984/

 

[marcus]

If it is OK with Hellfire, I would like to have the opportunity to discuss quantum cosmology (past and present) out here in Cosmology forum including references to his essays and source material.

From the start, with the early efforts of Wheeler and De Witt in the 1960s, one of the main concerns has been to resolve the big bang singularity. (Historically it has happened with other physics theories that quantizing a model gets rid of singularities and so there was the expectation that the same might happen here.) And I think it is interesting to see how the various QC attempts tried to do this.

Hellfire has some good graphics. For example diagrams illustrating Hawking and Hartle's proposal for getting rid of the big bang singularity.
===================

Even though a major motivation driving QC has been the fact that classical cosmology breaks down at start of expansion----and the expectation that quantizing might mend the glitch or clarify what really happened---it turns out that the way QC has developed it is offering solutions to other puzzles as well

This is the main point I wanted to make.
A recent Magueijo Singh paper suggests that LQC may provide a solution of the HORIZON PROBLEM that does not require an "inflaton field".

They also raise the possibility that LQC may explain the characteristic scale-invariant bumpiness of the microwave background and the observed STRUCTURE FORMATION in an way not requiring an inflaton scalar field. And they offer this as a way of TESTING loop quantum cosmology by pinning down what structure formation it predicts and falsifying it if the predicted signature is not found. Magueijo is serving as phenomenologist here rather than advocate.

Bojowald's recent paper explores ways in which LQC can explain the observed expansive acceleration WITHOUT DARK ENERGY. He also has papers on structure formation and on testing LQC by cosmological observation.

In other words, we have a 40 year effort starting with Wheeler-DeWitt in the 1960s aimed at constructing a QC, largely motivated by concern about the classical Gen Rel model breaking down at the bang (and other situations of importance) and this effort is beginning to bear fruit in other directions, in the form of some UNEXPECTED BONUSES.

Anyway, that is how I see it. And there is no need to start arguing about who is right, at this point! LQC could be wrong. And one of the many "inflaton" scenarios could turn out to be right! What I see as desirable is to have some alternative explanations of the various outstanding puzzles, including some that can be tested and falsified by observation.

We may not be there yet, but quantum cosmology is showing signs of providing us with some much-needed testable alternative explanations

 

[hellfire]

Thank you marcus. I got the impression that there are no complete introductions to the subject that mention all approaches to quantum cosmology. My knowledge about the topic is not deep, but I try to give a complete overview without touching complex topics, specially without discussing specific quantum gravity issues. I encourage you and others to add comments, references, elaborations or questions here or in my blog. I will be glad to discuss or comment any topic as far as I can. This is a very broad, deep and fascinating topic.

 

[MeJennifer]

Well the first (obvious) question in quantum cosmology would be that since each measuring apparatus is part of the cosmos how do we handle quantum state reductions?

 

[marcus]

Some links related to post #2:
*intrinsic inflation without an inflaton field
http://arxiv.org/abs/gr-qc/0407069
Genericness of inflation in isotropic loop quantum cosmology

http://arxiv.org/abs/gr-qc/0403106
Inflationary Cosmology and Quantization Ambiguities in Semi-Classical Loop Quantum Gravity

http://arxiv.org/abs/astro-ph/0311015
Loop quantum gravity effects on inflation and the CMB

http://arxiv.org/abs/gr-qc/0206054
Inflation from Quantum Geometry

*structure formation
http://arxiv.org/abs/astro-ph/0611685
Formation and Evolution of Structure in Loop Cosmology

http://arxiv.org/abs/astro-ph/0703566
Thermal fluctuations in loop cosmology (Magueijo and Singh)

*dark energy effects from LQC
http://arxiv.org/abs/0705.4398
The Dark Side of a Patchwork Universe

 

[hellfire]

Well the first (obvious) question in quantum cosmology would be that since each measuring apparatus is part of the cosmos how do we handle quantum state reductions?

This is a good question. The wavefunction of the universe may be a superposition of different basis states, but one has to explain that we currently observe a classical universe. One may consider that the Copenhaguen interpretation is not adequate for quantum cosmology since it requires of an external observer to the system. There are, however, some hypothesis in-line with the orthodox Copenhaguen interpretation that postulate that a decoherence process did take place somehow differentiating between some environment or long-wavelenght degrees of freedom. Current models usually do not address this issue, but it seams to me that this and the appropiate interpretations of quantum mechanics is an open issue in quantum cosmology. Some discussion and references here:

Quantum cosmology: how to interpret and obtain results

 

[marcus]

Well the first (obvious) question in quantum cosmology would be that since each measuring apparatus is part of the cosmos how do we handle quantum state reductions?

oops! after posting I see that Hellfire has already replied. This is what I had to say in partial response
================================
As you may know there has been quite a lot written about that and related issues.
What comes to mind immediately is the video of a talk by James Hartle at Perimeter. I will see if I can find a link. Hartle has tried to address the question of how to change the quantum theory framework so as to better handle quantum cosmology.

Earlier he planned to give one of the plenary talks at the Loops '07 conference, but unfortunately (as I just learned) he will be unable to attend. The title and abstract of his talk do not address your question specifically but illustrate the sort of thinking in progress about questions like the one you raised:

James Hartle: Generalizing Quantum Mechanics for Quantum Spacetime

"Familiar textbook quantum mechanics assumes a fixed background spacetime to define states on spacelike surfaces and their unitary evolution between them. Quantum theory has been generalized as our conceptions of space and time have evolved. But quantum mechanics needs to be generalized further for quantum gravity where spacetime geometry is fluctuating and without definite value. This talk will review a fully four-dimensional, sum-over-histories, generalized quantum mechanics of cosmological spacetime geometry. In this generalization, states of fields on spacelike surfaces and their unitary evolution are emergent properties appropriate when spacetime geometry behaves approximately classically. The principles of generalized quantum theory would allow for further generalization that would be necessary were spacetime not fundamental. Emergent spacetime phenomena are discussed in general and illustrated with the examples of the classical spacetime geometries with large spacelike surfaces that emerge from the 'no-boundary' wave function of the universe. These must be Lorentzian with one, and only one, time direction. The question will be raised as to whether quantum mechanics itself is emergent."

 

[Chronos]

I think any theory without inflation has a hill to climb - and new physics at the top. So where are the new physics?

 

[hellfire]

Every relevant model that I am aware of (besides of some brane-world scenarios) does not remove inflation, but it intends to explain inflation. (With inflation I mean an inflationary expansion phase). However, the mechanisms for such an inflationary expansion to take place are different in different models, and in some models these mechanisms may differ from the ones that are assumed in standard cosmology.

 

[marcus]

to elaborate on what hellfire said,
in Loop Cosmology inflation is not "put in by hand"---the matter field doesn't need to be special.
you automatically get a certain amount of exponential expansion right at the start
(for roughly the same reason that you often see a bounce---quantum corrections which at high density cause gravity to be repellent)

you might want to look back 3 posts or so to the links I gave under
"*intrinsic inflation without an inflaton field"

but the period of exponential expansion (without a special inflaton scalar field) that you get in Loop Cosmology is not ENOUGH to take care of structure formation.

the "horizon problem" LQC can take care of---according to Magueijo and Singh's argument---I think this is kind of obvious in cases where there is a bounce but they reinforce and generalize that conclusion. but there is also the riddle of where did the speckly puckerinng of the CMB come from that seeded the coagulation (curdling and clustering) of the galaxies

to explain the scale invariant spectrum of CMB pimples and blotches (if you want to do it by the magnification of quantum fluctuations) you apparently need SIXTY EEEFOLDINGS-----jargon for exponential expansion by a factor of e⁶⁰

If Loop Cosmology cannot somehow come up with the right pimple-blotch spectrum in some other way (like by progressively frozen thermal fluctuations which magueijo-singh consider) then it will require an INFLATON ADD-ON which would be a shame.

it does remarkably well without putting a special inflaton field in by hand, except in this regard. what a pity if in the end you still have to throw in an inflaton as an add-on, just to get the right blotches.

as for "new physics", read Magueijo-Singh, they remark explicitly about that.

 

[jal]

I'm reading and if I see something "new" I can always change my blog.
With all the new info and tools I think that Professor Sir Fred Hoyle would have had a stronger case.
http://www.astrobiology.cf.ac.uk/fredhoyle.html
Professor Sir Fred Hoyle [1915-2001]
Fred believed that, as a general rule, solutions to major unsolved problems had to be sought by exploring radical hypotheses, whilst at the same time not deviating from well-attested scientific tools and methods. For if such solutions did indeed lie in the realms of orthodox theory upon which everyone agreed, they would either have been discovered already, or they would be trivial.

jal

 

[Garth]

My hero!

As a boy I used to see Fred walking his dog across the Cambridge meadows.

Garth

 

[Nereid]

To what extent do the different quantum cosmology approaches yield predictions of observables within the reach of an advanced LIGO (VIRGO, etc)? LISA? more years of WMAP? the Planck mission? a full 10 years of high quality LSST and Pan-STARRS results?

My guess would be that they should have something to say in one or more of the following areas:
* gravitational radiation - some equivalent of the CMB
* footprints of inflation and other (very) early universe eras in the CMB and (galaxy) power spectrum
* "dark energy" - equation of state, time variation, etc ...

How likely is it that any of these cosmologies imply something quite exciting awaits us from the likes of AMANDA and LOFAR (and other 'big bucket' UHECR detectors)?

 

[hellfire]

The main problem with experimental verification of quantum cosmology models is that currently they do not provide unique and clear predictions.

Many loop quantum cosmology models predict a superinflationary phase in the early universe. The state parameter might have been a little less than -1 and this would imply a very specific deviation from scale invariance with a spectral index a little greater than 1. See for example gr-qc/0411012. For more detailed predictions inhomogeneous models would be needed. Loop quantum cosmology is probably on the verge of providing some detailed and concrete inhomogeneous models. (May be marcus can add some more information about this.)

In string quantum cosmology it is not easy to get a nearly scale-invariant spectrum due to the pre-big-bang phase, but the models are more precise and interesting with other predictions. The most important one is a very characteristic spectrum of gravitational waves. The possibility of detection depends however on parameters that cannot be estimated properly. For optimistic scenarios the detection might be at reach with the advanced LIGO. See for example http://arXiv/hep-th/9512091 .

Also large-scale primordial magnetic fields are a prediction of string quantum cosmology. However, it is not clear to me how this can be verified against other hipothesis for the formation of large-scale magnetic fields. For string models lots of important papers are available in the section D "Phenomenological aspects" of http://www.ba.infn.it/~gasperin/.

 

[marcus]

To what extent do the different quantum cosmology approaches yield predictions ...
...
* "dark energy" - equation of state, time variation, etc ...

for anyone interested in this, there is a new paper worth reading
http://arxiv.org/abs/0705.4398

LQC has an explanation for acceleration that is sufficiently different from the cosmological constant vanilla. clearly different time variation. detectable by more detailed acceleration measurement at higher redshift.
testability is discussed in the paper
very interesting---possibly the most interesting QG paper this quarter 2007

 

[marcus]

To what extent do the different quantum cosmology approaches yield predictions...
...
* footprints of inflation and other (very) early universe eras in the CMB and (galaxy) power spectrum
...

http://arxiv.org/astro-ph/0703566
this is to appear in Physical Review D.
It is a "phenomenology" style paper which speaks to LQC people from the outside about testing a structure formation hypothesis.
It lists some theoretical consequences that need to be nailed down so that one can get testability. It urges the LQC to work out details in 3 specific areas, so that by further measurement of what you are talking about (footprint of early history in CMB and galaxy power spectrum) the theory can be tested and also, if it is not outright falsified, certain parameters in the theory can be constrained.

I think it is a constructive paper and I hope it will get attention of Loop Cosmology theorists.

But in terms of the importance of the paper, before reading this one I would suggest reading what I talked about in the previous post, regarding a testable dark energy signature. I think this has higher priority as a response to Nereid's question:

To what extent do the different quantum cosmology approaches yield predictions ...
...
* "dark energy" - equation of state, time variation, etc ...

for anyone interested in this, there is a new paper worth reading
http://arxiv.org/abs/0705.4398

LQC has an explanation for acceleration that is sufficiently different from the cosmological constant vanilla. clearly different time variation. detectable by more detailed acceleration measurement at higher redshift.
testability is discussed in the paper
very interesting---possibly the most interesting QG paper this quarter 2007