Hello all. This question assumes String/M/Brane theory is correct (big assumption, I know).

Brane cosmology theorizes that gravity is not constrained to our brane. If that were true could the gravity effects we observe from at least a portion of dark matter really be from matter in another brane or in the bulk, meaning we will never truly be able to determine exactly what it is, just that it is reacting gravitationally to create the scaffolding of our galaxies? Has anyone seen this possibility been incorporated into any papers and if so could you point me to them?

Thanks

Hi Arituay,

Welcome to the BAUT forum.

I don't think you can say that String/M/Brane theory being correct says that it is certain that gravity leaks from brane to brane. There are many such models, and not all agree about visible multi-dimensional consequences.

I personally don't have links to papers on this, but since there is no shortage, I'm sure someone on the forum will drop by with some pointers soon.

__________________
Forming opinions as we speak

I also can't link you to papers, and frankly I'm not sure whether that would even help-- string theory is too difficult to sound like anything but wildly unconstrained speculation to anyone but an expert (and to many of them too). But I think what you are really asking is, is it possible that dark matter, whatever its explanation, might only interact with the known universe gravitationally, and in no other way? To that I would say, anything's possible. But note that if this is the case, it's a pretty major crisis for astronomy-- there's only so much you can do by mapping its gravitational influence, and after that it would never get past hypothetical possibilities. We'd like an independent way to explore the properties of that stuff, such as a laboratory detection, or some type of astrophysical observation that would be directly attributed to its decay into things we can detect.

I don´t know much about brane cosmologies but the notion of gravity acting across branes through extra dimensions is already disproved by experience.

If such a thing were possible, the Gauss´Law calls for the gravitational flux to spread over those extra dimensions. Since Newton discovered an inverse square law (three dimensions for gravity to act on) instead of inverse N-1 (N dimensions, being N>3), the conclusion is that if those extra dimensions exist, gravity does not act through them. And that holds for brane cosmologies and for whatever cosmology which calls for extra dimensions.

The extra dimensions are not ruled out by this reasoning, just their ability to transmit gravity (or the electromagnetic field).

Frankly I have no idea how brane theorists get gravity to be weakened so dramatically using extra dimensions, but you can be assured that they are quite adept with Gauss' law. It might be that the extra dimensions are "tangled up" in some way that they only siphon off gravity on small scales, and on longer scales you find the usual Gauss' law. I don't know that that's it.

Tempus,

The concept I was referring to was based on this paper:

http://arxiv.org/abs/gr-qc/0312059v1

Ken,

Ok, thanks for the insight.

No, that is not the case (see below).

The inverse N-1 law applies only for dimensions that are larger than the scale gravity is being measured. String theory basically implies that there are 6 additional dimensions that are small, with the Planck length a natural length scale for these dimensions. Current measurements only rule out extra large dimensions, ones with scales of mm's or more. All we know is that there are only three large spatial dimensions (and one time dimension); there is no constraint on the number of dimensions smaller than the mm scale.

Also in reply to antoniseb, gravity transmits through all dimensions in String Theory. This is because the graviton cannot be constrained to a brane like other force carriers.

Thanks Arituay. I´ll take a time to read it. I am afraid quite a time and I am not confident in grasping all the math there. But I already see in the abstract that the scheme hypotheses at least one extra dimension being of enough large scale, which would call for the Gauss´Law despite cmsavage objections concerning the subset of brane/string theories with all the extra dimensions small.

And not that I can see why the Gauss´Law would not apply to small dimensions, with the provision that the enclosing hypersurface would look like an hyperspaghetto. I hope I will find some reference to it in the paper and that it is not just something that brane/string theorists wish for.

If dark matter is uniformly dispersed and interacts only gravitationally, then the following proposal is of no consequence.

However, if dark matter isn't uniformly dispersed, and is instead particulated (such as in the form of numerous small black holes), then we might be able to measure the passage of one of these black holes, should one of them come our way.

It would not be difficult to build a 3D device to measure the local warping of space-time that would reveal a passing small black hole. Even precise measurements of the orbits of the planets would reveal this, albeit in retrospect.

Over time, if no such variations in movement exist, then it's likely that dark matter is very distant, that it is evenly disbursed, or that there's something amiss in our current understanding of the space-time continuum which raises a false assumption that dark matter might exist in the first place.

__________________
If I set the budget, we'd have Ares and more. Unfortunately, I don't set the budget, and Ares is just too expensive and too far out for us to accomplish our goals within the budget we were given.

If we halt the ISS, all versions of Ares, and transport Orion and Altair aboard DIRECTv3's Jupiter family of Shuttle-Derived Launch Vehicles, we just might make it back to the Moon by 2020.

As I said, I have yet to read the paper to see how the Gauss´Law and the inverse N-1 law are ruled out for small space dimensions. Other references on that would be appreciated. Has it to do with those dimensions being closed?

So string theories in general state that gravity leaks through the extra dimensions. Since the leakage from other branes into ours in an invisible elf, the way to prove that would be to measure the actual leakage from our brane out to the bulk. I don´t know if it has been tried in laboratory whether it has been looked for in planetary orbits, but from your words I would imply that no such shortage in the gravity field expected has been found whithin the current error margins. Am I right?

And that the milimetre order limit in scale for the extra dimensions would be the minimum that would make the leakage detectable with ours means. Since it has not been detected, there is only room left for smaller scales.

Then coming back to the effect of other branes into our brane, I wonder how that small scale effect adds up to the candidates as observed phenomena (galaxy rotation, clusters) which happen to be extralarge scale without affecting the moderate large scale (planetary orbits and lab measurements).

I will wait to read the full paper before commenting on that, since as I said in my earlier post the abstract seems to suggest that scale makes a difference for the ability to transmit gravity.

Yes, it is due to the extra dimensions being compact. Take a look at this link: the gravitational potential scales as 1/r^(d+1) for r << L and as 1/r for r >> L, for d compact dimensions of size L.

The difference from a non-Newtonian force becomes highly suppressed at larger distances, with corrections on the order of (L/r)^2. If there was a "large" extra dimension of size 1 micrometer, then the variation of the force between the Earth and Sun (150 million km apart) would be about one part in 10^34, well beyond what we are capable of measuring (and may ever be capable of measuring). To search for extra dimensions, you want to look at variations of Newton's Law at small distances. However, gravity is pretty weak and it is difficult to measure it on such small scales since electromagnetic forces have such a huge effect on objects small enough to put only 1 mm or less apart (and Earth's gravity is a huge background that must be dealt with). That is why we currently only have upper limits of around mm's, even though we can see (using photons) much smaller distances (photons can be constrained to a brane and do not have to behave the same way as gravity).

I put "large" extra dimensions in quotes above because, in extra dimension papers and discussions, the "large" is relative to the Planck scale and is not associated with how we view the traditional 3 non-compact dimensions (which are obviously large). That is, an extra dimension can be 1/1000 the size of a nucleus, far smaller than the experimental limits, and would still be considered "large" because it is much larger than the Planck scale.

Because the variation gets smaller with larger distances, as noted above, the non-Newtonian effect is negligible on planetary scales, much less galactic scales.

If you have other branes near our brane that have massive objects, that is a different story. I suppose you can have gravitational interactions between particles on two different nearby branes (this has nothing to do with non-Newtonian forces), but I don't know enough about branes to say much about this.

Few people think dark matter is in the form of black holes, a more widespread idea is that it is a type of types of particles with the minute masses that fundamental particles typically have. That would make a gravitational detection of a single dark matter particle very far from feasible.
That is already more or less assumed to be the possibilities.

Sorry for the delay.
OK, I had a look at the link, and this is my interpretation on it:

The Gauss´Law holds for N spacial dimensions. Ken G was right on that.

The inverse N-1 law for the field would hold for N spacial dimensions. An inverse N-2 law applies to the potential (as expected). Just N is ajusted on diferent scales for the number of spacial dimensions which still can count for something, as a result of the enclosing hypersurface the field has to cross being spaghetti-like.

If one or more (but not all) of the extra dimensions was bigger than Planck scale, we would have another different, intermediate variation on r between the extreme scales (Planck´s and macroscopic). Cute.

Gravity potential spreads across all the dimensions in the brane, but no reference is made of gravity leaking across branes, at least in the linked page about hypergravity. Would have to look at the rest of the site though. And I still have the paper Arituay mentioned to look at.



One thought: it looks like stringbranists are using this to justify why gravity looks diminished. Unstated but implied is that they are comparing it to the electromagnetic field. But this poses two problems:

1. Why should gravity be comparable to the electromagnetic field? For peace of mind? I imagine they are trying at TGU but have they succeded? Why not to think that gravity has just its own order of magnitude? Would TGU really need gravity to be of comparable value to the other long range field?

2. If other (small scale, curled) dimensions exist, why would not EM spread through them just as gravity would? The only thing I can think that might prevent EM from doing so would be the elemental concentrations of charge having a size (do they really have a size?) bigger than the Planck´s lenght (since this is the size that it is postulated for the extra dimensions) while the elemental concentrations of mass would be smaller than the size of the extra dimensions. I don´t want to suggest singularities, but it looks close. The curious thing is that every particle with both a charge and a mass would have two different sizes, one for charge, one for mass.

Thank you all for your thoughts on the matter

Alternatively such a situation could be looked at as a boon for astronomers, since if brane cosmology is right the only chance they would have to "observe" other branes would be via such gravitational effects.

Even if unintentional. As I have said elsewhere, we know for sure at present that string theory is the foundation of . . . a lot of careers.

I think I'll go look up Lisi again.

Yeah, I don't even bother to wade through it-- not because I think it's wrong, but because it would take me a solid week of study to even form an opinion if it's science, let alone if it's right.