Hi,

It's been a while since I've thrown myself into the fray of the
forefront of theoretical physics, so this question may be trivial.

However, I seem to recall M-theory predicting gravitons being closed
loops and therefore not limited to motion on one particular brane.
Now, if we buy the hypothesis that our universe is a three-dimensional
brane in a higher-dimensional space, one among many such branes in
close proximity to each other (the "slices of bread" analogy),
couldn't it be supposed that gravitons are able to freely move between
these branes, thereby causing a gravitational force between branes?

Given that dark matter is invisible and is supposed to account for 90
percent of our universe's mass, isn't it possible that the
gravitational effect ascribed to dark matter is nothing more than the
mass in our universe interacting with mass in neighboring universes?

BrainMcGee wrote:

However, I seem to recall M-theory predicting gravitons being closed
loops and therefore not limited to motion on one particular brane.
Now, if we buy the hypothesis that our universe is a three-dimensional
brane in a higher-dimensional space, one among many such branes in
close proximity to each other (the "slices of bread" analogy),
couldn't it be supposed that gravitons are able to freely move between
these branes, thereby causing a gravitational force between branes?

Yes, certainly.

Given that dark matter is invisible and is supposed to account for 90
percent of our universe's mass, isn't it possible that the
gravitational effect ascribed to dark matter is nothing more than the
mass in our universe interacting with mass in neighboring universes?

Indeed. This was discussed in a famous paper by Arkani-Hamed,
Dimopoulos, and Dvali, Phys.Rev. D59 (1999) 086004, available at
http://arxiv.org/abs/hep-ph/9807344. To quote their abstract:
``Higher-dimensional gravitons produced on our brane and captured
on a different "fat" brane can provide a natural dark matter candidate.''
There's been a lot of work on this idea, but it's not my specialty, and I
don't know the current status.

Steve Carlip

Given that dark matter is invisible and is supposed to account for 90
percent of our universe's mass, isn't it possible that the
gravitational effect ascribed to dark matter is nothing more than the
mass in our universe interacting with mass in neighboring universes?

Indeed. This was discussed in a famous paper by Arkani-Hamed,
Dimopoulos, and Dvali, Phys.Rev. D59 (1999) 086004, available at
http://arxiv.org/abs/hep-ph/9807344. To quote their abstract:
``Higher-dimensional gravitons produced on our brane and captured
on a different "fat" brane can provide a natural dark matter candidate.''
There's been a lot of work on this idea, but it's not my specialty, and I
don't know the current status.

The problem is that it has been shown that most dark matter is in a
halo surrounding each galaxy. Why should it be that there is a halo of
matter is a brane right next to us such that it would exactly mirror the
location of our galaxies? I'm not expert but it seems that if there were
some number of branes interacting each having significant mass that
most of the time you wouldn't expect to see any (or small) effects
from mass in neighboring branes but occasionally you would see some
galaxy warped due to the invisible presence of a large amount of mass
in a neighboring brane. I haven't heard of a case like this but I do agree
that gravitons should operate in all 10/11 dimensions instead of the 3
of the other forces. Why don't we see effects of mass in parallel
universes/neighboring branes....I don't know...but it's interesting.