In a class of realistic four and five dimensional supersymmetric grand unified models , the scalar spectral index is found to be n _ { s } = 0.98 ( \pm 0.01 ) , in excellent agreement with the values determined by several previous experiments and most recently by the Wilkinson Microwave Anisotropy Probe ( WMAP ) .
The models predict dn _ { s } / d { ln } k \sim 10 ^ { -3 } and a negligible tensor-to-scalar ratio r \sim 10 ^ { -8 } .
A new five dimensional supersymmetric SO ( 10 ) model along these is presented in which inflation is associated with the breaking of SO ( 10 ) to SU ( 5 ) at scale M , with \delta T / T \propto ( M / M _ { Planck } ) ^ { 2 } , so that M \simeq 10 ^ { 16 } GeV .
The inflaton decay leads to the observed baryon asymmetry via leptogenesis .
We also discuss how the monopole problem is solved without the use of non-renormalizable terms .