Cosmological constraints on string scale and coupling arising from tachyonic instability

TL;DR

This paper shows that tachyonic instability in string-inspired inflation models can produce observable non-Gaussian signals in the CMB, constraining the string scale and coupling with current and future data.

Contribution

It provides new bounds on the string scale and coupling based on CMB non-Gaussianity measurements, linking string theory parameters to observable cosmological signatures.

Findings

WMAP bounds constrain string scale to M_S/M_P ≤ 10^{-4} for g_s<0.1

String coupling g_s must be around 10^{-9} if tachyon fluctuations are significant

Future CMB data could rule out certain string inflation models based on these constraints

Abstract

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 $M_S/M_P\leq 10^{-4}$ 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 $M_{S}/M_{P}<10^{-3}$. This case may soon be ruled out by the forthcoming CMB non-Gaussinianity bounds.