Quantum effects derived through conformal anomaly lead to an inflationary model that can be either stable or unstable .
The unstable version requires a large dimensionless coefficient of about 5 \times 10 ^ { 8 } in front of the R ^ { 2 } term that results in the inflationary regime in the R + R ^ { 2 } ( ‘ ‘ Starobinsky ’ ’ ) model being a generic intermediate attractor .
In this case the non-local terms in the effective action are practically irrelevant , and there is a ’ graceful exit ’ to a low curvature matter-like dominated stage driven by high-frequency oscillations of R – scalarons , which later decay to pairs of all particles and antiparticles , with the amount of primordial scalar ( density ) perturbations required by observations .
The stable version is a genuine generic attractor , so there is no exit from it .
We discuss a possible transition from stable to unstable phases of inflation .
It is shown that this transition is automatic if the sharp cut-off approximation is assumed for quantum corrections in the period of transition .
Furthermore , we describe two different quantum mechanisms that may provide a required large R ^ { 2 } -term in the transition period .