Can Space Warp Independently of Mass?

[Buckethead]

Last night while thinking about galaxies with seemingly large DM halos and me being the skeptic of DM as I am, I got a flash of an interesting idea. Thinking I may not be the only person in the world who has thought of this, I am curious to know if this is a previously discussed idea, or if it simply makes no sense, or if indeed I have come up with a truly original thought

Mass warps space and the effect is gravity. Some galaxies such as NGC 4555 have large halos of hot gas which simply do not have enough visible mass with which to keep such a halo of gas sticking around, thus DM.

Is is conceivably possible that space can warp for reasons other than the presence of mass (dark or otherwise) which would manifest as a gravitational force? A twist in space in an empty part of the universe could for example attract matter and offer a seed (a place in space and a strong gravity) with which to start forming a galaxy. The galaxy would contribute to the overall gravitational strength present, and the remainder, currently attributed to DM might just come from this local twist in space which would now lie at the center of the galaxy.

Sensible or ridiculous?

 

[Drakkith]

Sensible or ridiculous?

If you're asking whether there is anything else that causes gravity then it's a sensible question. However if you are suggesting that something does then that would go against PF rules for personal theories. To our knowledge there is nothing else that can cause gravity other than mass.

 

[Buckethead]

I was definitely not offering a theory and am glad to hear you think it's a sensible idea. What I'm fishing for is any information as to whether such an idea has ever been presented in academic circles and if so whether it has been dismissed and for what reasons. Or any speculative ideas or discussions about how space could be warped without the presence of mass.

 

[PatrickPowers]

I was definitely not offering a theory and am glad to hear you think it's a sensible idea. What I'm fishing for is any information as to whether such an idea has ever been presented in academic circles and if so whether it has been dismissed and for what reasons. Or any speculative ideas or discussions about how space could be warped without the presence of mass.

Sure. The new "standard model" assumes a trivial topology. The Universe could be twisted or warped in all sorts of exotic ways, but there are infinite possibilities and no reason to assume any of them. So keep it simple is the choice.

 

[Buckethead]

Sure. The new "standard model" assumes a trivial topology. The Universe could be twisted or warped in all sorts of exotic ways, but there are infinite possibilities and no reason to assume any of them. So keep it simple is the choice.

Thanks for the answer. I agree that keeping things simple is always a good policy, but in this case we already know that mass bends space and the expanding universe shows that space itself is also stretching. It's convenient to assume that space isolated from mass is flat, but it's not unreasonable to imagine that parts of space may have distortions caused by clusters of galaxies off on one side or standing gravity waves crossing the universe or some other event. The reason this idea might be of some use is that such a distortion in mid space could be an attractor of sorts, drawing in a small galaxy to it's center and adding to the gravitational pull of the galaxy in general. Since dark matter has not yet been confirmed and since it's presence in the galaxy I suggested was a surprise according to dark matter profiles, looking for alternatives sudh as an anomalous bend in space in this area might be an intersting and possibly fruitful exercise.

 

[Drakkith]

I don't believe gravititational waves work that way. They will stretch or bend spacetime as they travel through, but similar to a normal gravity wave in water it won't actually move anything around. (FYI gravity waves are the waves on the surface of water. Gravitational waves are waves of gravity propagating through space.)

Also, for space to be isolated from mass then there must not be any mass nearby. Not even galaxy clusters, since their great mass will affect space a large distance away. Although technically I don't think there is any isolated space in the universe, as gravity is an infinite range force.

 

[Buckethead]

I don't believe gravititational waves work that way. They will stretch or bend spacetime as they travel through, but similar to a normal gravity wave in water it won't actually move anything around. (FYI gravity waves are the waves on the surface of water. Gravitational waves are waves of gravity propagating through space.)

Ah yes, that makes sense. So a gravitational wave wouldn't actually attract anything, but at best could only make things "bob around"? So if something like an empty space attractor existed it would really have to be locally created, a local warp in space. Doesn't a worm hole fall into this category? Can a worm hole be a gravitational source?

 

[bapowell]

Sure. The new "standard model" assumes a trivial topology. The Universe could be twisted or warped in all sorts of exotic ways, but there are infinite possibilities and no reason to assume any of them. So keep it simple is the choice.

But it is the geometric -- not topological -- properties of spacetime that are manifested gravitationally. The global topology of the universe is not relevant when discussing local curvatures, such as those in the vicinity of galaxies and clusters.

 

[Markus Hanke]

If you're asking whether there is anything else that causes gravity then it's a sensible question. However if you are suggesting that something does then that would go against PF rules for personal theories. To our knowledge there is nothing else that can cause gravity other than mass.

I am afraid but this is not correct. Any form of energy is a source of the gravitational field, as described by the energy-momentum tensor :

http://en.wikipedia.org/wiki/Energy-momentum_tensor

 

[Drakkith]

I am afraid but this is not correct. Any form of energy is a source of the gravitational field, as described by the energy-momentum tensor :

http://en.wikipedia.org/wiki/Energy-momentum_tensor

You are correct. A poor explanation on my part.

 

[Nabeshin]

So if something like an empty space attractor existed it would really have to be locally created, a local warp in space. Doesn't a worm hole fall into this category? Can a worm hole be a gravitational source?

Well, if you read the Einstein Equations 'backwards' as it were, knowing whatever geometry you have, it has some corresponding stress energy tensor. You really can't have curvature without a source according to Einstein. Beyond Einstein, well... That's not for this forum.

 

[Edi]

Well, essentially, mass IS gravity and gravity IS mass .. anything that warps space is mass, if it is not visible or otherwise detectable easily, then, what do you know - dark matter.

 

[Markus Hanke]

Well, essentially, mass IS gravity and gravity IS mass .. anything that warps space is mass, if it is not visible or otherwise detectable easily, then, what do you know - dark matter.

This is not entirely correct - you are right in saying that, if mass is present, we get gravity by definition. On the other hand, the fact that gravity is present does not necessary mean you have mass, because any form of energy is a source of Riemann curvature, as explained in post 9. Mass is only one form of energy, it could also be an electromagnetic field for example, or any other configuration of energy

 

[Markus Hanke]

Well, if you read the Einstein Equations 'backwards' as it were, knowing whatever geometry you have, it has some corresponding stress energy tensor. You really can't have curvature without a source according to Einstein. Beyond Einstein, well... That's not for this forum.

For an outside observer a wormhole would indeed have a gravitational attraction due to the intrinsic curvature in its metric tensor; one should remember though that it is not possible for a wormhole to form in the first place without a very strong source of gravitation, e.g. a black hole. I do not believe that "free" wormholes ( the kind without singularities at each end ) can be stable, macroscopic objects.