Einstein's revelations are based upon the fact that one of the variables of relativity must be unity(1). c, or the speed of light in a vacuum, was chosen.
Why c?
Energy, mass (not matter) and c. Where is space in the equation?
A thought experiment?
Space in the equation comes as a velocity of light (c) moving through space, right? No one can go from point 'A' to point 'B' with no transpired time (instantly). To do this may imply exceeding the speed of light, but that can't be done by any means I know of.
Isn't it equally plausible that the expansion/contraction of space should be a non-unity factor included in the formula?
Space is black at night because light leaves the universe and doesn't reflect off anything out there, right? Infinity can't 'expand' or 'contract', yes? It's infinite. Does this hypothesis change your question(s) above?
If so, then there may be better explanations of things like black holes with no escaping light. In this case, the compression of space would constitute a much longer path for escaping light to travel. Since escaping light would then take longer to travel, the energy of that light would appear to us as a much lower wavelength. To put it simply, the amount of energy doesn't change but something must so frequency does.
Galaxies and black holes at the edge of the universe appear red and infrared---Redshift, obviously---because at that distance frequency does change to a lower value. Light is forced to change by the action of traveling through space and time to get here. Space is still infinite, but light isn't.
Isn't it just as plausible to say space has expanded or compressed and the speed of light is only unity in the view of the observer located in the same frame?
Space in nonphysical, so, assuming it's nothing but empty volume, especially---theoretically and arguably---at the edge (if there is an 'edge' to infinity), it itself can't change, right? Finite things in space change. 'C' is a constant, but that also makes it non-infinite. So, then, 'c' might be view by the observer as a finite constant, but that's all?
If space dilation/compression is a factor then there is no unity variable except the observer's reference frame and gravity may just be a result of mass and space.
Dilation is a function of time and velocity is as well, and they're not constant the faster you move. 'C' is a constant and not a variable. All else, other than 'c', isn't unity in this thought experiment. Gravity isn't acting on the object moving, is it? Other than acceleration?
I had to think my way through this as I went along in real time. That being so, I may not be able to experimentally prove any of this, so take it for what it's worth: an informal opinion.
--Lee