Since mass/energy is conserved (because of the time-translational symmetry of the universe),
the right hand side of our equation is constant (its derivative is zero).
So, the derivative of the left-hand side must also be zero. The left-hand side is geometry!
That is, if the Cosmological Constant Λ is geometrical.
Einstein did not know of the Bianchi identities, so it took him years to find the second term in parentheses.
The derivative of the sum in parentheses is zero — mission accomplished!
This brings out clearly why you can add a CONSTANT term (to either side of the equation!)
Why would you want to do so? Because such a term, it is easy to show, creates "anti-gravity."
Hey, no levity, please!
But is it levity? "The whole thing is a vicious circle. The law of gravitation is⎯a put-up job."
Eddington, "The Nature of the Physical World"
