[PapaBear3625]

Above a certain critical velocity, Felber believes, any mass will gravitationally repel other masses, an effect that is twice as strong in the forward direction of motion, but also works in the backward direction.

"Above a certain critical velocity" RELATIVE TO WHAT?

From the viewpoint of the Earth, stars at the edge of the observable universe are moving away from us at close to the speed of light. From the viewpoint of somebody on one of those stars, WE are moving away from THEM at close to the speed of light. Do we observe the repulsion phenomenon here on Earth, keeping in mind that Earth can be considered to be moving at close to the speed of light in any direction relative to some observer somewhere?

The essence of Relativity was the realization that there is no absolute frame of reference, there is only motion relative to some particular observer.

[Quix]

I think I understand that passably well.

. . .

I also understand that Einstein didn’t have everything all figured out.

Or, that if he did, a lot has been witheld from putlic knowledge.