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We study the spectrum of cosmological fluctuations in the D3/D7 braneinflationary universe with particular attention to the parametric excitation ofentropy modes during the reheating stage. The same tachyonic instability whichrenders reheating in this model very rapid leads to an exponential growth ofentropy fluctuations during the preheating stage which in turn may induce alarge contribution to the large-scale curvature fluctuations. We take intoaccount the effects of long wavelength quantum fluctuations in the matterfields. As part of this work, we perform an analytical analysis of thereheating process. We find that the initial stage of preheating proceeds by thetachyonic instability channel. An upper bound on the time it takes for theenergy initially stored in the inflaton field to convert into fluctuations isobtained by neglecting the local fluctuations produced during the period oftachyonic decay and analyzing the decay of the residual homogeneous fieldoscillations, which proceeds by parametric resonance. We show that in spite ofthe fact that the resonance is of narrow-band type, it is sufficientlyefficient to rapidly convert most of the energy of the background fields intomatter fluctuations.

A Study of Structure Formation and Reheating in the D3/D7 Brane Inflation Model