I just don't think that e=mc2 comes from pure empirical evidence, I don't believe (I hate the word believe!!), I don't think inertia or momentum contributes to mass,
I can certainly accept that when an atom is in it's lowest energy state that is has less mass (common shared forces = lowest energy/mass). So I don't think a hot object is heavier than a cold object, or a fast moving object is heavier either, or a charged capacitor.
I just do not see any evidence that that is the case.
Plus I don't think it is needed, I don't think Einstein's 'greatest mistake' was a mistake at all, except if you think accepting it as a mistake was a mistake.. (damn now I'm confused).. I'll get back to you!
So, I think that e=mc2 is a stand alone and complete, I don't think it needs to be modified for some justification of having an open or closed universe, or for an expanding universe.
I don't it needs interference by quantum mechanics, I think relativity is a far more complete and established by observation than quantum mechanics, QM is in my opinion able or capable of usurping or replacing relativity.
I think that mass is energy distributed over (all of) space and time (c2), you could call that energy 'gravity' but I prefer to think of it as the length of space and time, c is space and time (an amount of space over an amount of time).
Mass and energy are not truly equivalent, for one thing mass can never go at c, and energy can only go at c, mass occupies space and time (it is its own reference frame), and energy does not occupy space and time (effectively decoupled from space and time) and has no reference frame at all. (Time and space are meaningless to light/energy).
m = energy / (space x time)2 I have been noodling with the idea that mass gives space a fundamental length property (that we tend to call gravity) the fundamental length property of space gives rise to a time length (the same length) that is why c is a constant. (length of space/length of time = 1).
Greater mass give longer space (why Black holes redshift, and why Gravitational shift works), but with less mass you have shorter space so in relative terms you have higher energy (same energy over a smaller volume/time).
So there is a universal balance, mass/matter makes space long in order to better distribute and 'flatten out' energy, being at the bottom of a hill (or in a higher gravity) is a lower energy state.
In a universe with no mass at all, that fundamental length property of space (and therefore time) is zero, and any amount of everyone expressed over zero space length and zero time duration is of course infinite.
As soon as you introduce matter/mass, that fundamental space length becomes non-zero, and the energy of the mass is not what has given rise to the length of space (and therefore the length of time).
What I find interesting about this model is that it provides a mechanism for 'gravity' as that length of space corresponds to 'gravity' (Big G), it gives a mechanism for why things fall and why we get orbits and such. If you are going at a constant speed (including zero) and you are in that gradient of length you will appear to accelerate (with feeling acceleration), because the fundamental length of space 'down' is getting longer, you are not going more meters per second, you are going longer meters per longer second. You are in free fall.
Down then becomes the direction of local length of space, and gravity is the gradient of that length. Making gravity a field potential model with every point in space having a number, that number is the sum total of all the length of space contributions from all the mass in the universe as a function of the mass and your distance from that mass.
The greatest contribution to that length is the 'gravity capture' or has the greatest influence on your motion, this makes space very flat, and of course 'gravity' (the length gradient) very shallow or 'weak'.
I might try to do a post on this one day, and see what you guys think, I will put on my systems engineer's cap and build a universe that looks and acts like ours (overall) by using just that wonderful equation e=mc2.
After all the only components we have to work with is matter and energy and space and time, so no surprise the all appear in one simple equation. You just don't need anything else.
Just like you don't need any more than a simple switch to build the biggest computer and run the most complex programs using a very basic and fundamental principles. (anyway, that's my daily quota of the use of the word fundamental, so I will end this here to save you all more pain..