D-term Inflation and M-theory

Models of supersymmetric D-term inflation require a new mass scale near 10^{15-16} GeV in order to match the density perturbation spectrum observed by COBE. Attempts to obtain such a scale from the anomalous U(1) of string theories fail in most string models. However there is hope that models based on non-standard embeddings in M-theory can solve the discrepancy. We will show that such models still suffer from the other drawback of D-inflationary models, namely that Planckian field values are required to drive inflation. Thus it is hard to understand why the inflaton potential remains so flat without imposing stringent symmetries on the superpotential. We also examine a fascinating quasi-fixed point behavior for the gauge coupling of the anomalous U(1) in these extra-dimensional models, and show that either the presence of large numbers of fields in the 5-dimensional bulk or strongly suppressed U(1) gauge couplings is required in order to restore naturality to the inflationary potential.