It is known that scalar-tensor theory of gravity admits regular crossing of the phantom divide line w _ { { DE } } = -1 for dark energy , and existing viable models of present dark energy for its particular case – f ( R ) gravity – possess one such crossing in the recent past , after the end of the matter dominated stage .
It was recently noted that during the future evolution of these models the dark energy equation of state w _ { { DE } } may oscillate with an arbitrary number of phantom divide crossings .
In this paper we prove that the number of crossings can be infinite , present an analytical condition for the existence of this effect and investigate it numerically .
With the increase of the present mass of the scalaron ( a scalar particle appearing in f ( R ) gravity ) beyond the boundary of the appearance of such oscillations , their amplitude is shown to decrease very fast .
As a result , the effect quickly becomes small and its beginning is shifted to the remote future .