I've been trying to intuit what gravity is actually doing in respect to spacetime for a while. I'm familiar with the technical descriptions the relevant equations present but I'm trying to form more of an abstract intuition of what's exactly going on.

Which has bought me to the notion that, given the FLRW metric following from the geometric properties of homogeneity and isotropy that the cosmos is (more or less) uniformly experiencing expansion; can we consider the space-time curvature describing a gravitational field, as an area of (for lack of a better term) drag on the cosmic expansion?

If we understand that Mass is the factor that slows cosmological expansion and that Mass defines spacetime curvature at a given local scale. Is it correct to deduce that what we're perceiving as spacetime curvature is an artifact / outcome of a reduced rate of expansion in the local area of gravitational influence of a given mass, manifesting as the physical attributes of curvature?

If we take as a given that without Mass space is expanding uniformly at a given rate and then we add a given mass to a specific point and calculate the curvature created. Would it be correct to intuit the gravitational field as being a slowing down of the expansion in that local area?

It seems to intuitively make the most sense to me. The greater the mass, the greater the drag; the greater the delay in expansion, creating a tendency for inertial vectors to tend asymptotically toward a single point. Where parallel geodesics would tend toward a single point the further 'into' the well they tend. Or to look at it another way, the smaller the relative scale of space something tends to relative to an observer at an inertial rate of expansion.

I could probably word this a lot better so excuse that if you may. But does anyone have any input on this view point? Is there some credence to this? And is it worth putting more effort in to clarifying this concept?


  • $\begingroup$ While it seems correct that there would be aberrations from the cosmological expansion on a local scale due to inhomogeneous mass-distributions on a local scale, it seems incorrect that the gravity itself is just the effect of slowed down expansion. In a completely static universe with no expansion whatsoever, gravity would still persist and in almost exactly the same manner as it appears now at the scale of terrestrial or galactic scales. $\endgroup$
    – Dvij D.C.
    Jun 21 '17 at 12:25

@Djiv in his comment has it right. Even if no expansion, eg near us, there is still gravity due to local mass-energy creating local curvatures.

More, the universe's expansion, or cosmic expansion, doesn't happen by inertia. It was an initial Big Bang impulse and inflation, but the contributions to the mass-energy of the universe is what makes it expand the way it does. The average matter density, radiation density and dark energy density contribute differently to the expansion, and what we see are the contributions of the 3 sources of mass-energy (technically, the stress energy tensor and the cosmological constant representing the dark energy). If those were different it would expand differently; in fact in the past it has had times when it expanded faster, then slower, and now faster.

See the current accelerated expansion and the equations for the contributions of the different sources of mass energy described at https://en.m.wikipedia.org/wiki/Accelerating_expansion_of_the_universe

Moreover, the fact that the velocity of expansion is (more or less, it deviates some at larger distances in ways we understand and have measured) proportional to the distance from us (or relative velocity of two galaxies in the universe, in the large cosmic scales, is proportional to their distance apart) also cannot be explained without the General Relativistic theory of gravity. The same theory also explains the more local gravity on the scales of our galaxy and solar system (although Newtonian gravity approximates it, when we measure more carefully or we see the bending of light by gravity, we can only explain it with General Relativity). Also it explains the gravity of black holes (which have now been detected) independently of the cosmic expansion

All of this goes to say that it is not of great use to obtain an intuitive idea of something which is wrong. Follow the logic of the theory first, try to understand the math and its conclusions, and you can start forming an intuition without wrong preconceptions. This last paragraph is not an answer to the question, it is not physics, just opinion. It talks to something you refer to in your question, and which is not physics but how to best understand something new, in my opinion.


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