Chinese Acad Sci, Inst High Energy Phys, Beijing 100039, Peoples R China
; Inst Adv Study, Sch Nat Sci, Princeton, NJ 08540 USA
; Chinese Acad Sci, Inst Theoret Phys, Beijing 100080, Peoples R China
Recent fits of cosmological parameters by a Wilkinson Microwave Anisotropy Probe (WMAP) measurement favor a primordial scalar spectrum with a varying index. This result, if it stands, could severely constrain inflation model building. 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.