Gravity question that has been puzzling me...

[Vic]

It is now currently believed that the expansion of the universe is accelerating. This means the expansion is cause by more than just the Big Bang (if it were, then the expansion would be constant or even slowly decelerating), but that another force must be involved. This force is sometimes called dark energy or the cosmological constant. It is also believed that the acceleration of the expansion is greatest in those largest areas of empty space, i.e. the voids between galaxies and superclusters.

So... with that in mind... my question. Does gravity really work through matter bending space? Or might it not be empty space that it is bent (in the opposite direction of course) with the presence of matter mitigating that bending process? In other words, might be not be possible that gravity is the manifestation of empty space "repulsing" rather than matter "attracting" as hs been though all along?

Then again, it might be both or it might not matter. I apologize if this was a stupid question.

 

[PowerEngineer]

If gravity was a result of empty space "repulsing" then I'm not sure how to explain why it's decided to push all of us down onto the earth, rather than to the moon or the sun. Or maybe on a trip to Alpha Centuri. It seems that the direction of the gravitational force is always towards matter. If gravity isn't tied to matter, then how would you explain its directionality?

I'm also thinking that there isn't much empty space inside the solar system compared to what lies outside. This would suggest (at least to me) that a "repulsing" gravitational force should be fairly uniform throughout the solar system. I wouldn't then expect that the gravitational force on the moon would be only one-sixth of that on earth (which matches the ratio of the bodies masses).

I also wouldn't expect that a "repulsing" force from empty space outside the solar system would support the orbits of the planets (which seem to be neatly tied to the masses of the objects and the distance between them).

I do share your interest in the theories around a cosmological constant for a replusive force tied to space itself (that might become measurably dominant over gravity in the "vast emptiness of space" between galaxies), and also wonder if it is another force that warps the space-time continuum as gravity does. I don't think that gravity is a manifestation of that force.

 

[Jeff7]

I think I get what you mean. Matter offers a reprieve or shield from the "antigravity" of empty space, and thus from our point of view it seems to have a pulling effect.


My thoughts on dark matter: It might simply be due to our models using Newtonian physics instead of general relativity. I'd imagine that when you're running a model of a galaxy, with huge objects moving at very high speeds, and you don't account for relatavistic effects, which includes slight changes in mass, that could seriously screw up compounded calculations.

 

[bsobel]

Originally posted by: Jeff7
I think I get what you mean. Matter offers a reprieve or shield from the "antigravity" of empty space, and thus from our point of view it seems to have a pulling effect.


My thoughts on dark matter: It might simply be due to our models using Newtonian physics instead of general relativity. I'd imagine that when you're running a model of a galaxy, with huge objects moving at very high speeds, and you don't account for relatavistic effects, which includes slight changes in mass, that could seriously screw up compounded calculations.

Even with some compounded errors you don't come out missing 90% of the mass in a galaxy. While we don't know what it is yet, there is something to account for all that matter

 

[PlasmaBomb]

Relatavistic effects affect inertial mass not the actual mass of an object iirc

 

[Biftheunderstudy]

Inertial mass and Gravitational mass are the same quantity any way you measure them. One of the fundamental things about it is that you can write F=ma and F=GMm/r^2 using the same mass regardless of how fast it is moving just with a factor of gamma in the mass.

Dark matter has been measured many times and most of them not only account for both relativities but actually use them.

1. Rotation curves of Galaxies:
Using a radio telescope you measure the 21cm emission lines representing a molecular hydrogen cloud and find their red/blueshift. Then find their velocity taking into account the relativistic velocities towards or away from you and plot it against the distance to the center of the galaxy. Rinse and repeat and you have a curve of velocity versus radial distance, if the motion of the galaxy were like that of the solar system the curve would be like 1/r (I think don't quote me on that one), however one finds that the curve is flat, ie constant velocity with distance. This implies that mass increases with distance!!!! Next using the data you can plot a mass versus distance and find that the mass of the galaxy from the rotation curve is much larger than the mass that we can see. This process is for our galaxy which is the hardest to measure cuz we're in it, similar processes yield almost exactly the same results for other galaxies. Nearly 70% of the mass of the galaxy we can't see or detect!!!

2. Gravitational Lensing:
Large galaxies have enormous mass, so much that the effects of GR become very apparent, if there happens to be a galaxy behind a large galaxy in our line of sight we can see that smaller galaxy as an image exactly like a lens. The problem is we know how much mass the galaxy has to have in order to create an image like the one we see and it is much smaller than the mass from the luminous matter we see.

3. X-ray gas halos:
Around some galaxies there is very hot gas which emits x-rays, we can see how much of this gas there is and for it to be gravitationally bound the mass of the galaxy would have to be mush larger.

All of these processes yeild similar 70-90 % dark matter content.

Dark energy is different than dark matter though. Dark energy comes from the fact that the universe is accelerating, and that predictions place the critical density at 1, to make that so there has to be 90% of the energy in the universe as an energy that we can't detect.

The current best proposed candidate is in string theory.

 

[f95toli]

Originally posted by: Jeff7
I think I get what you mean. Matter offers a reprieve or shield from the "antigravity" of empty space, and thus from our point of view it seems to have a pulling effect.


My thoughts on dark matter: It might simply be due to our models using Newtonian physics instead of general relativity. I'd imagine that when you're running a model of a galaxy, with huge objects moving at very high speeds, and you don't account for relatavistic effects, which includes slight changes in mass, that could seriously screw up compounded calculations.

All "serious" models of the whole universe are based on general relativity. Newtonian physics is often used when e.g. modelling galaxies simply because it speeds up the calculations and the error is negligable, adding SR effects is certainly possible but wouldn't really affect the result.