String production at the level of effective field theory

TL;DR

This paper analyzes the production of excited string states in time-dependent backgrounds from an effective field theory perspective, focusing on cosmological implications such as preheating. It develops a steepest descent framework to estimate Bogoliubov coefficients and derive convergence criteria for finite total string production, incorporating both fundamental and non-perturbative (wrapped-brane) strings. Key results show that production can be finite or exponentially enhanced depending on the analytic structure of the time-dependent frequency $\omega(t)$ and the minimum string tension $\tau_-$ relative to the inflaton mass; non-perturbative strings can relax tension requirements, allowing copious production even when tensions are not deeply below the Planck scale. The findings suggest excited strings could play a role in post-inflationary cosmology under certain moduli and brane-wiring conditions, while also highlighting potential issues such as overproduction of heavy stable states and the need for explicit constructions and backreaction analyses.

Abstract

Pair creation of strings in time-dependent backgrounds is studied from an effective field theory viewpoint, and some possible cosmological applications are discussed. Simple estimates suggest that excited strings may have played a significant role in preheating, if the string tension as measured in four-dimensional Einstein frame falls a couple of orders of magnitude below the four-dimensional Planck scale.