f(R) gravity theory and CMBR constraints

We consider the large-scale cosmic structure generation by an inflation based on a pure f(R)-type gravity theory. Comparison with recent CMBR observations gives the following results: (1) The near Zelʹdovich spectral conditions uniquely choose R2-type gravity. (2) The R2 gravity predicts specific nearly scale-invariant Zelʹdovich spectra for both the scalar- and tensor-type perturbations. Thus, (3) the considered model survives current observational data. The COBE-DMR quadrupole data (4) give constraints on the coupling constant and the energy scale during inflation, and (5) require the gravitational wave contribution to be suppressed. (6) Therefore, future observations of (2) and (5) can provide strong tests of the inflation scenario based on R2 gravity. Parallel analyzes made in the conformally transformed Einstein frame give the observationally identical results.