  We point out that the temperature fluctuations of cosmic microwave background
(CMB) can be generated in a way that is different from the one usually assumed
in slow-roll inflation. Our mechanism is based on vacuum fluctuations of fields
which are at rest at the bottom of the potential, such as Kaluza-Klein modes or
string excited states. When there are a large number (typically of order $N\sim
10^{14}$) of fields with small mass in unit of Hubble parameter during the
inflationary era, this effect can give significant contributions to the CMB
temperature fluctuations. This number $N$ makes it possible to enhance scalar
perturbation relative to tensor perturbation. Comparison with the observed
amplitudes suggests that models with string scale of order $10^{-5}$ of 4D
Planck scale are favorable.
