Cosmological constraints on string scale and coupling arising from tachyonic instability

We demonstrate that string motivated inflation ending via tachyonic instability leaves a detectable imprint on the cosmic microwave background (CMB) radiation by virtue of the excitation of non-Gaussian gravitational fluctuations. The present WMAP bound on non-Gaussianity is shown to constrain the string scale by $M_S\mathrm{/}{M}_P\le {10}^{-4}$ for string coupling $g_s<0.1$, hence improving the existing bounds. If tachyon fluctuations during inflation are not negligible, we find the stringent constraint $g_s\sim {10}^{-9}$ for $M_S\mathrm{/}{M}_P<{10}^{-3}$. This case may soon be ruled out by the forthcoming CMB non-Gaussinianity bounds.