Some aspects of brane inflation

The inflaton potential in four-dimensional theory is rather arbitrary, and fine-tuning is required generically. By contrast, inflation in the brane wo rld scenario has the interesting feature that the inflaton potential is mot ivated from higher-dimensional gravity, or more generally, from bulk modes or string theory. We emphasize this feature with examples. We also consider the impact on the spectrum of density perturbation from a velocity-depende nt potential between branes in the brane inflationary scenario. It is likel y that such a potential can have an observable effect on the ratio of tenso r to scalar perturbations. (C) 2001 Elsevier Science B.V. All rights reserv ed.