Recent fits of cosmological parameters by the Wilkinson Microwave Anisotropy
Probe (WMAP) measurement favor a primordial scalar spectrum with varying index.
This result, if stands, could severely constrain inflation model buildings.
Most extant slow-roll inflation models allow for only a tiny amount of scale
variations in the spectrum. We propose in this paper an extra-dimensional
inflation model which is natural theoretically and can generate the required
variations of the spectral index as implied by the WMAP for suitable choices of
parameters.Comment: 5 pages, 3 figures, REVTeX 4. Comments on low CMB quadrupoles added;
Version accepted for publication in Phys. Rev.