Fermionic Zero Modes of Supergravity Cosmic Strings

TL;DR

This paper investigates how supergravity influences fermion zero modes on cosmic strings, revealing that including the gravitino reduces zero modes and relaxes vorton constraints, especially for brane inflation D-strings.

Contribution

It derives a general index theorem for zero modes in supergravity and shows gravitino inclusion diminishes zero modes, impacting cosmic string stability and constraints.

Findings

Including gravitino reduces fermion zero modes on cosmic strings.

Vorton constraints are weakened for certain supergravity cosmic strings.

Winding number one D-strings evade vorton constraints in brane inflation models.

Abstract

Recent developments in string theory suggest that cosmic strings could be formed at the end of brane inflation. Supergravity provides a realistic model to study the properties of strings arising in brane inflation. Whilst the properties of cosmic strings in flat space-time have been extensively studied there are significant complications in the presence of gravity. We study the effects of gravitation on cosmic strings arising in supergravity. Fermion zero modes are a common feature of cosmic strings, and generically occur in supersymmetric models. The corresponding massless currents can give rise to stable string loops (vortons). The vorton density in our universe is strongly constrained, allowing many theories with cosmic strings to be ruled out. We investigate the existence of fermion zero modes on cosmic strings in supergravity theories. A general index theorem for the number of zero modes is derived. We show that by including the gravitino, some (but not all) zero modes disappear. This weakens the constraints on cosmic string models. In particular, winding number one cosmic D-strings in models of brane inflation are not subject to vorton constraints. We also discuss the effects of supersymmetry breaking on cosmic D-strings.