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.

• Received 4 March 2003

DOI:https://doi.org/10.1103/PhysRevD.68.103511