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Do Strings = Dark Matter?


Pinch Paxton

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If you are really really good at maths you should in principle be able to explain low energy dynamics in terms of string theory too, but it is not the way people usually think about it. It is like asking someone what they put in the sauce they served with dinner, and getting the reply 'strings'.

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Cross-posted there.

 

It is only appropriate to use string theory at energies where gravity is strong and therefore important (because otherwise you could use the Standard Model SU(3)xSU(2)xU(1) gauge theories much more easily). Gravity becomes stringer and stronger at high energies and is expected to become of a similar strenght to the other forces at around [math]10^{19}[/math] GeV, or in other words 100000000000000 times the energy of our biggest particle accelerators.

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Cross-posted there.

 

It is only appropriate to use string theory at energies where gravity is strong and therefore important (because otherwise you could use the Standard Model SU(3)xSU(2)xU(1) gauge theories much more easily). Gravity becomes stringer and stronger at high energies and is expected to become of a similar strenght to the other forces at around [math]10^{19}[/math] GeV' date=' or in other words 100000000000000 times the energy of our biggest particle accelerators.[/quote']

 

Hi Severian, do you happen to have a link to some case where string theory was actually useful in calculating or predicting anything?

I had the impression that it is not considered appropriate for real physical calculation perhaps because there are too many versions of stringy theories and no one knows how to calculate unambiguous predictions from them that can then be tested by experiment. Indeed there appeared some waffling recently by Michael Douglas and Leonard Susskind as to whether to expect SUSY at LHC and i got the impression of strong reluctance to predict.

 

So I would be glad to have links to some examples where stringy models are actually appropriate for calculating and predict something definite.

 

there is a comparable situation in Loop Quantum Gravity which has already had some relevant astronomical observations which have guided development by ruling out some alternatives. further predictions will be tested by gammaray burst observation over the next few years. this situation was reviewed at the February "Quantum Gravity Phenomenology" symposium and briefly summarized in Smolin's recent survey article

written for Reviews of Modern Physics called "Invitation to LQG".

If LQG can generate predictions testable by available means, then why don't stringy models also? It must be something besides the fact that Planck energy is 2 gigajoules---since that would seemingly apply equally to both approaches.

 

One simply does not rely on particle accelerators to test hypotheses! Their energies are too low. One relies on higher energy astronomical stuff that has traveled astronomical-scale distances over which predicted effects accumulate. hence the current interest in using UHECR and GRB (gamma ray burst).

 

I can supply some arXiv links if you would like.

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Here.

Severian, have a look at section 5 page 27 "The Near Term Experimental Situation" in

http://arxiv.org/hep-th/0408048

 

You will see ongoing and upcoming connection of LQG with experiment.

 

But both LQG and the stringy pictures are planck-scale

so you'd think that if one could generate predictions testable by available means then so could the other

 

or if one could only make predictions that you would have a chance of seeing with an impossibly huge accelerator, then the other would be likewise limited to predicting essentially unobservable effects.

 

but it does not seem to be turning out so---one has satellite observatories that can be used in place of accelerator-experiements and an interesting phenomenology is emerging. This February conference has its lectures online:

"Quantum Gravity Phenomenology" Feb 4-Feb14, 2004

http://ws2004.ift.uni.wroc.pl/html.html

It is mostly about the empirical testing of quantum gravity.

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As I said before it is entirely inappropriate to use string theory to calculate low energy phenomena, not because string theory isn't up to it, but simply because it is too hard. It is much better to us the lower energy models to calculate theings like dark matter relic densities. (An anology would be that one does not use quantum mechanics to calculate the trajectory of a football - one uses Newtonian mechanics. The QM would give the correct answer but is far to complicated to be prectical.)

 

I am aware of there being work done of what is euphamistically termed 'String phenomenology', which isn't phenomenology as an experimentalist would recognise, but is rather taking low energy limits of string theory to see which low energy effective models they reproduce. The most famous of these is the [math]E8 \times E8'[/math] heterotic string theory which breaks down into the Standard Model or other low energy theories.

 

Even then though, this only tells you what your DM candidates are, and what their annihilation cross-sections are.. You still have to solve the Boltzman equation to find the relic density itself.

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Here.

Severian' date=' have a look at section 5 page 27 "The Near Term Experimental Situation" in

http://arxiv.org/hep-th/0408048

[/quote']

 

This is interesting, but to my mind hopelessly impractical. You notice that in Eq.39 the extra terms in the Energy-momentum relation have factor of the planck length in them - this is exactly because the string theory results in an effective theory with deviations from our normal physics which would only be large at the planck scale (where gravity is strong). So the effective theory will contain extra higher dimensional operators which which are suppressed by inverse powers of the planck energy (or in Eq.39, powers of the planck length, which is the same thing in planck units). So assuming alpha (in Eq.39) to be or drder 1 the associated extra term would be an effect of one part in 10^16 for current collider energies. This is just unseeable I think.

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This is interesting, but to my mind hopelessly impractical. You notice that in Eq.39 the extra terms in the Energy-momentum relation have factor of the planck length in them ... This is just unseeable I think.

 

So you had a look at Smolin "Invitation to LQG"!

 

Observation has already been able to effectively dispose of one approach (see Smolin's scenario A and B page 27). The astronomy papers that have been published in the area of testing QG are in Smolin's bibliography.

See his references [130, 131]

 

Very restrictive contraints on alpha in case A.

http://arxiv.org/astro-ph/0212190 (synchrotron radiation from crab nebula)

http://arxiv.org/gr-qc/0402028 (experimental challenges for QG)

http://arxiv.org/hep-ph/0301124

these seem already to exclude case A so that one is not so interesting

 

In case B there are already some observational results constraining alpha.

But the first significant results are expected to be from looking at the peaks in Gamma Ray Bursts (GRB) by an satellite called GLAST scheduled for launch in 2007 (Gammaray Large Array Space Telescope).

 

A good summary of the testing that has been performed so far and is planned would be this list of talks----the talks are also available on line.

http://ws2004.ift.uni.wroc.pl/html.html

 

I have to go out now but will be back in a couple of hours and try to give more detail.

To give an example of why one can sometimes test a hypothesis with GRB or UHECR (ultrahigh energy cosmic ray) that one may not be able to test with accelerator, let's consider a GRB that is coming to us over 1 billion LY distance. If there is even a very tiny difference in speed of light depending by the "alpha" of Eq.39 on the energy, but suppressed by Planck scale (as you point out) then because the light gets to travel for 1 billion years even this very very small difference will cause a part of the pulse to arrive sooner. So with GLAST one wants to look at the profile of spikes comprising a gammaray burst and check for some dispersion. It is a way of getting at Eq. 39 that does not use accelerators.

Sorry I have to go without being able to edit this and make less wordy!

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If there is even a very tiny difference in speed of light depending by the "alpha" of Eq.39 on the energy' date=' but suppressed by Planck scale (as you point out) then because the light gets to travel for 1 billion years even this very very small difference will cause a part of the pulse to arrive sooner. So with GLAST one wants to look at the profile of spikes comprising a gammaray burst and check for some dispersion. It is a way of getting at Eq. 39 that does not use accelerators.

[/quote']

 

Yes - this makes sense. You need to have a very large distance scale to cancel against the very small Planck length ot give something or order 1 (or whatever is seeable). It makes sense that this has to be astophysical data from distant galaxies.

 

I somethimes think too accelerator based.

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