From: "Kebz"
Date: 9 Jul 2006

Q Is there any evidence that these things arise from the interaction of our universe with another? So in this scenario what we are observing are the effects of that interaction with gravity and perhaps other forces spilling over into our universe where the interactions are strongest. The interacting universes could exist in alternative dimensions and interact invisibly with our own time-space dimensions.

A Good question - ideas something like this have actually been put forward in recent years!

Some theorists have suggested that dark matter may not be "stuff" in our own universe, but the gravitational shadow of matter in other universes. The idea is suggested by models in which space has more than the usual four dimensions (three of space, one of time). One possibility is that the visible universe is a sort of four-dimensional sheet (technically called a "3-brane") embedded in a higher-dimensional bulk (picture the 2-dimensional surface of a soap bubble floating in three-dimensional space). We don't perceive the other dimensions in these models because the interactions responsible for particle physics (electromagnetism and the strong and weak nuclear forces) are constrained to operate only along the brane. For example, since light rays can only travel along the surface of the brane, particles in the bulk (off the surface of the "soap bubble") would be invisible to our eyes.

Gravity, however, could leak away into the bulk. This may be why gravity seems so much weaker than the other forces of nature - its strength along the brane gets diluted by leaking away into the extra dimensions. The force of gravity in 5-dimensions between two objects will fall off as the inverse-cube of the distance between them, for example, much faster than the inverse-square law of 4-dimensional spacetime. If gravitational forces can leak only a short distance into the bulk, gravity could seem to follow an inverse-square law at large distances but become much stronger at very short distances.

As for dark matter (similar models may be interesting for dark energy, though I don't immediately know of an example), it could be the result of several branes like ours lying extremely close together in the bulk. We could never see the matter in these other branes, but their gravitational pull would affect our visible matter, just as dark matter does!

This is a very interesting alternative to our usual ideas about dark matter. It's not currently the favored explanation of most cosmologists, however. For one thing it depends on a rather complicated setup (a sheaf of branes extremely near each other, gravity that can leak a little but not very much, etc.). It's also not clear whether this theory should produce the observed structure for galaxies - blobs of stars embedded in much larger clouds of invisible gravitating mass. It's a very interesting possibility that's still being pursued, however.

Jeff Filippini
UC Berkeley Cosmology Group