It is shown that F(R)-modified gravitational theories lead to curvature oscillations in astrophysical systems with rising energy density. The frequency and the amplitude of such oscillations could be very high and would lead to noticeable production of energetic cosmic ray particles.
The Universe evolution during the radiation-dominated epoch in the R^2-extended gravity theory is considered. The equations of motion for R and H are solved analytically and numerically. The particle production rate by the oscillating curvature is ca
lculated in one-loop approximation and the back reaction of particle production on the evolution of R is taken into account. Possible implications of the model for cosmological creation of non-thermal dark matter is discussed.
