Eh... an actual "grand unifying theory" might be hyperbole. The original concept drives from Einstein's desire to unify Gravity with the other three electromagnetic forces. The reason was that gravity stuck out like a sore thumb, behaving well when discussing large objects, but breaking down under quantum mechanics.
As for me, I really just figure you can eliminate the problem all together by redefining gravity as an accumulation of the quantum electric forces over extremely long distances over the agregate.
Under this system, a point in space x would have the gravitational value g, where g = sum(electromagnetic effect).
If we take into account that atomic particles may hold positive or negative charges(or both), then fluid dynamics may explain the formation of galaxies, without the need for inflation.
For example, we see that a magnet has all the positve and negative poles aligned in opposite directions. If we consider that magnetic waves reinforce each other according to quantum principles, then the ability of a magnet to attract metal is explained.
In normal matter, the electromagnetic poles are not so neatly organized, but we can be certain that the electromagnetic fields propagate throughout the fullness of space. The positive and negative elements would mean that atractive forces and repulsive forces are constant throughout the cosmos, which would explain why galaxies are separating, but structures within galaxies hold together.
Of course, the underlying mathematical concepts behind this theory are very simple. The actual requisite calculations to come up with a gravitational constant would be extremely complex, mixing fluid dynamics with wave mechanics, and taking into account realtime adjustments in the quantum polarities of quantum particles.
Assuming that someone could figure out how to do the math right, however, the prediction to test is relatively straightforward. Indeed, if the theory is correct, then the aggregate electrical forces over any one point in space should match the observed gravitational constant.
If such an observation could be verified, then occam's razor would basically slice off the need for any type of gravitational force. The trick is that you would have to take into account all quantum electrical forces throughout a very, very large universe. Obviously, direct measurement is impossible, but as more particles are taken into account, we'd expect to see the value for any given point to approach the value predicted by relativity.
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"To have friends, a man need only be good natured, but when one has no enemies there must be something mean about him." -- The Prince, Oscar Wilde