Fifth force, sixth force and all that

In the recent literature, a few claims appeared about possible deviations from the ordinary gravitational laws (both at the terrestrial and at the galactic level). The experimental evidence does not seem to be conclusive; nor its is clear if new forces are showing up, or if we have to accept actual deviations from Newton or Einstein gravitation (in the latter case, the validity of the very Equivalence Principle might be on the stage). In such a situation, the attempts by various authors at explaining the ''new effects'' just on the basis of the ordinary theory of General Relativity (for instance, in terms of quantum gravity) can be regarded as logically instance, in terms of quantum gravity) can be regarded as logically questionable. In this pedagogically oriented paper, the problem is approached within the classical realm, by exploring whether the possible new effects can be accounted for through minimal modifications of the standard formulatiaon of General Relativity: in particular, through exploitation and extension of the role of the cosmological constant