  We discuss the production of massive relic coherent gravitons in a particular
class of f(R) gravity which arises from string theory and their possible
imprint in Cosmic Microwave Background. In fact, in the very early universe
these relic gravitons could have acted as slow gravity waves. They may have
then acted to focus the geodesics of radiation and matter. Therefore, their
imprint on the later evolution of the universe could appear as filaments and
domain wall in the Universe today. In that case, the effect on Cosmic Microwave
Background should be analogous to the effect of water waves, which, in focusing
light, create optical caustics which are commonly seen on the bottom of
swimming pools. We analyze this important issue by showing how relic massive
GWs perturb the trajectories of Cosmic Microwave Background photons
(gravitational lensing by relic GWs). The consequence of the type of physics
discussed is outlined by illustrating an amplification of what might be called
optical chaos.

General Relativity and Quantum Cosmology