Einstein’s general relativity describes gravity not as a force-at-a-distance, but as a curvature of spacetime. This means that objects on the surface, for example, must be accelerated out of their inertial or free-falling paths in order to be "at rest" in a gravitationally accelerated reference frame. A follow-up to my video “How Gravity Makes Things Fall.” Please watch them together. (The content of both videos is standard science, not my own research, theory, or thing I pulled out of my behind.) Strictly speaking, acceleration is a change in velocity. For an object revolving around a center of rotation, the velocity is changing because the direction is changing (velocity combines speed and direction). In the case of standing on the surface of the Earth, your velocity through spacetime is what’s changing. *General relativity does not claim that the acceleration is linear and that opposite sides of the Earth are moving away from each other.* In my other video, being accelerated by curved spacetime is like following one of the curved lines when the device is in its “stretched” configuration. Meanwhile, inertial objects are following straight lines in curved spacetime — their velocity through spacetime is constant. (Unstretching the device, which causes the straight lines to become curved and vice versa, corresponds to changing from an inertial reference frame to the accelerated frame that we experience under the influence of gravity, where things on the ground appear to be at rest and inertially traveling objects fall.)
The confusion between linear acceleration and curved-spacetime acceleration has led many, many amateur scientists to a "theory" that all matter is expanding outward, and that this is what produces gravitational effects. It is an okay introductory visualization, like the famous "rubber sheet" demonstration of gravity — but like that demo, it’s flawed and oversimplified to the point of being totally wrong. Expansion gravity cannot explain elliptical orbits, hyperbolic trajectories, and the relation between surface gravity, radius, and density. It’s also wildly in conflict with experiments and the accepted calculations of celestial bodies’ surface gravities. Just think of gravity as a kind of acceleration-in-place that turns into more familiar (upward) linear acceleration when you are free to move inertially.
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