The everpresent eta-problem: knowledge of all hidden sectors required

We argue that the ?-problem in supergravity inflation cannot be solved without knowledge of the
ground state of hidden sectors that are gravitationally coupled to the inflaton. If the hidden sector
breaks supersymmetry independently, its fields cannot be stabilized during cosmological evolution of the inflaton. We show that both the subsequent dynamical mixing between sectors as well as the
lightest mass of the hidden sector are set by the scale of supersymmetry breaking in the hidden sector. The true cosmological ?-parameter arises from a linear combination of the lightest mode of the hidden sector with the inflaton. Generically, either the true ? deviates considerably from the naïve ? implied by the inflaton sector alone, or one has to consider a multifield model. Only if the lightest mass in the hidden sector is much larger than the inflaton mass and if the inflaton mass is much larger than the scale of hidden sector supersymmetry breaking, is the effect of the hidden sector on the slow-roll dynamics of the inflaton negligible.