Can Gravitational Waves Test String Theory?

[Schreiberdk]

http://arxiv.org/PS_cache/arxiv/pdf/1105/1105.5283v1.pdf

Abstract
We provide a simple transfer function that considers the impact of an early matter dominated era on the gravitational wave background and show that string theory can be tested by observations of the gravitational wave background from inflation.

 

[PAllen]

Exciting. Here is the link to the abstract rather than the pdf. This is how arxiv papers should be linked.

http://arxiv.org/abs/1105.5283

 

[Schreiberdk]

Oh sorry :) I am new to linking papers, but thanks :D

 

[marcus]

Schreiber,
about the www.physicsforums[/URL]... issue, just now someone kindly explained what my problem with it was. No need for you to bother about it, just do whatever is most convenient.

About the Durrer paper. Notice that she is pleading for the BBO mission to be funded.
The BBO (big bang observer) mission would be grav. wave. detection on a grand scale by a fleet of spacecraft , as proposed in 2004 by Phinney et al:

S. Phinney et al., [I]The Big Bang Observer: Direct detection of gravitational waves from the birth of the Universe to the Present[/I], NASA Mission Concept Study (2004).

You may have noticed that LISA, a single spaceborne interferometer mission, has been cut.
[PLAIN]http://scienceblogs.com/catdynamics/2011/04/nasa_wiping_the_slate_clean.php
http://lisa.nasa.gov/

The BBO would have been a FLEET OF LISAS! a coordinated array of triangular interferometers orbiting the sun, each more sensitive than LISA (which until recently was planned as a joint ESA-NASA mission.)

Here is something about BBO:
http://arxiv.org/abs/gr-qc/0512039
Detecting the Cosmic Gravitational Wave Background with the Big Bang Observer
Vincent Corbin, Neil J. Cornish

Here's a 2002 video talk at Caltech about the BBO. The speaker says it would involve a minimum of 12 spacecraft orbiting the sun, working as 4 separate but coordinated interferometers.
http://www.cosmolearning.com/video-lectures/the-big-bang-observatory-bbo-a-possible-follow-on-mission-to-lisa-8855/
Kip Thorne is asking him questions.

 

[marcus]

Anyway here is the final paragraph of the Ruth Durrer paper you pointed to:

In conclusion, we have shown that combining future CMB polarization measurements with very sensitive gravitational wave probes like, e.g., BBO can provide a crucial test for string theory. Since this is our best candidate for a fundamental theory of Nature, and there has not been proposed any other single experiment to ultimately test it, it is of uttermost importance that we realize a BBO–like experiment.​

http://arxiv.org/abs/1105.5283

So she pins her hopes on BBO. I am not sure that she is right about it really testing String because her argument depends on a claim made in one paper by Gordon Kane (and his related piece in Physics Today). But if you grant Durrer the Gordon Kane premise and accept her argument then you have a reason to wish that LISA had not been cut and that there was going to be a followup mission called BBO.

As the guy describes in his video talk the difficulty with trying to directly observe grav. waves from the start of expansion of the universe is how to distinguish those signals from similar frequency ones from binary systems. Binary dwarfs, binary neutron stars, binary supermassive black holes. They exist in the billions and their waves are all noise which would tend to obscure early universe GW..
=========================

The proposals to test Loop cosmology do not involve direct detection of grav. waves. Just CMB polarization measurements. CMB-pol mapping is more immediately feasible.

Direct detection of GW is an exciting prospect but AFAIK the large groundbased instruments have so far not detected any, which is very strange. The quest for GW has been going on for some years now. Surely they are there, why don't we see them? Their nondetection is something of a puzzle.

 

[Schreiberdk]

This explains a lot :) I was also quite shocked, that suddenly a testable prediction was comming from string theory. I was not aware that the BBO was such a large experiment, and that LISA had been cut, so this really explains the difficulty of the testing.

So if I am correct, because of the cuts on the LISA project the BBO-experiment will be delayed a lot / not even conducted? If so i see the problem of the test :)

 

[marcus]

Schreiber, a wise person (I don't remember who) said that to think in a productive/creative way one must be able to hold contradictory ideas in one's head.

What I'm trying to say does not necessarily have to do with String---I believe that a spaceborne GW interferometer detector like LISA will fly. I think that we will put instruments in space that can detect neutrinos from the big bang, and instruments that can study the planets orbiting other stars, detect their atmosphere etc. And also instruments that can study the GW background.
And also discover what the transparent matter clouds are made of, and why the expansion of distances is speeding up.

It is just a question of how long, and in what order? What will be the priorities?
Eventually all these great pure-science questions will be addressed.

But also there is a civil war going on between democratic socialism and the powers of unregulated wealth. A kind of tug-of-war which is going to disrupt things and cause a lot of economic hardship. Science budgets may suffer for a time. Worthwhile projects may be postponed.

If one cares about a theory, one must continue to believe it can and will be tested, but with a patient belief. I can't think of any other response.

 

[atyy]

Direct detection of GW is an exciting prospect but AFAIK the large groundbased instruments have so far not detected any, which is very strange. The quest for GW has been going on for some years now. Surely they are there, why don't we see them? Their nondetection is something of a puzzle.

The current non-detection limits threaten GR or some well-established scenario?

 

[marcus]

Atyy I fully expect the detection of GW to be announced next week The trouble is I'm just a bystander. My state of mind counts for very little, I almost wouldn't bother telling you what I think because it doesn't matter.

But no, as I see it, how could GR possibly be threatened? It is too beautiful and too well verified in other ways! Geometry is dynamic and undulates. How could there not be waves in geometry all through and around us? People just have to keep increasing the sensitivity until they detect them.

But I expect you know of people who think differently and puzzle over this and try to imagine different theories of gravity where waves would not exist or else be undetectable. I've heard talk of this, just don't recall specifics.

 

[atyy]

I am not sure that she is right about it really testing String because her argument depends on a claim made in one paper by Gordon Kane (and his related piece in Physics Today).

Yes, Durrer's argument at full strength doesn't seem obviously right. The supposed Kane claim is clearly stated by him to be based on a conjecture. http://arxiv.org/abs/1006.3272

It looks like he does claim to prove it in "well understood" cases, which doesn't show that the not well understood cases don't exist.
"In Appendix B we systematically discuss the moduli masses in all known (at least to us) examples where moduli stabilization is well understood."

Also, even if the conjecture applied to the not well-understood cases, it's only generic, and he gives a long discussion on non-generic cases.

 

[atyy]

Atyy I fully expect the detection of GW to be announced next week The trouble is I'm just a bystander. My state of mind counts for very little, I almost wouldn't bother telling you what I think because it doesn't matter.

But no, as I see it, how could GR possibly be threatened? It is too beautiful and too well verified in other ways! Geometry is dynamic and undulates. How could there not be waves in geometry all through and around us? People just have to keep increasing the sensitivity until they detect them.

But I expect you know of people who think differently and puzzle over this and try to imagine different theories of gravity where waves would not exist or else be undetectable. I've heard talk of this, just don't recall specifics.

Actually, I haven't kept track of what's happening here, and thought you might know. My impression was that they haven't been detected just because experiments haven't been good enough. I am open to GR being threatened by non-detection of gravitational waves, but of course, they'd really need to show that GR predicted that the waves would pass through the detector (ie. specific scenarios), and that they could have detected a wave.

I don't know how the statistics would go for a non-detection. Usually, it seems the excluded parts are done at 95% CL (3 sigma), but detection of a particle requires 99.98% CL (5 sigma). Now if the non-detection of the Higgs becomes the interesting thing, then don't they now need to go back and up the non-detection from 3 sigma to 5 sigma?