[HakTaisanip]

It would be interesting to provide the most general form of the Einstein equation where the observable and unobservable forms of energy-momentum are clearly distinguished. I wonder if that is what the authors were trying to achieve and whether they succeeded.

I assume they were just initially concentrating on additions that weren't considered in Einstein's time, but if we continue to reduce the equation on this basis, then why can't we separate the observable from the unobservable. Any further simplification would be good. It's easy in hindsight to say why hasn't this been attempted before....but, why hasn't this been attempted before? :-))