Marginally deformed Starobinsky gravity

We show that quantum-induced marginal deformations of the Starobinsky gravitational action of the form R2(1-alpha), with R the Ricci scalar and a a positive parameter smaller than one half, can generate sizable primordial tensor modes. We also suggest natural microscopic sources of these corrections and demonstrate that they generally lead to a nonzero and positive alpha. Furthermore we argue, that within this framework, the scalar spectral index and tensor modes probe theories of grand unification including theories not testable at the electroweak scale.