[N9u9ie4p]

It's not clear to me that there is anything fundamentally new here. Modifications to the Einstein equation are really about distinguishing observable energy-momentum from unobservable energy-momentum. It should be noted that whether or not a particular form of energy-momentum is observable is deducible from the symmetry of the energy-momentum. The cosmological constant does have the symmetry to be unobservable as energy-momentum, and also allows the observable energy-momentum to be conserved. On the other hand, there is energy-momentum with the symmetry to make it unobservable, but isn't a cosmological constant. In this case, neither forms of energy-momentum are separately conserved, though the total is conserved.

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.