The large scale cosmic microwave background cut-off and the tensor-to-scalar ratio

We show that if inflation lasts just longer than the required 60 or so e-folds (N), both scalar and tensor contributions to the cosmic microwave background power spectra are expected to show a cut-off. However the behaviour of the scalar-to-tensor ratio on large scales depends on whether inflation is preceded by a kinetic dominated (KD) or radiation dominated stage. Future experiments may be able to distinguish between the two behaviours and thus shed light on the nature of the low-l cut-off. In particular if the cut-off is due to a KD stage the ratio is expected to grow on large scales. If observed, this would challenge our current understanding of the overall probability of inflation lasting for N greater than 60.