On chromonatural inflation in string theory

Abstract

Sourced gravitational waves in chromonatural inflation (CNI) can give rise to a chiral spectrum of tensor fluctuations that is considerably enhanced relative to the vacuum fluctuations. If the field content of CNI acts purely as a spectator (SCNI), the inflationary sector can be consistent with current data making SCNI very appealing in view of future observations. We investigate the prospects of embedding SCNI in string theory, in the framework of Kähler inflation in type IIB large volume string compactifications, with a spectator sector associated with gaugino condensation on multiply magnetised D7-branes. We first introduce a generalised field theory framework that describes non-trivial multifield inflation coupled to gauge fields of the form generically arising in supergravity and string theory. We then use these results to study numerically and analytically both the background evolution and the dynamics of cosmological perturbations. We show that a successful inflationary background evolution with a large enhancement of the gravitational wave spectrum and a controllable backreaction from the amplified tensor fluctuations can be achieved by considering suitable values of three parameters present in our scenario: the magnetic flux, the degree of the condensing gauge group and the wrapping number of the D7-brane. On the other hand, the required values for these quantities may present a challenge for its successful realisation within string theory. We also discuss these challenges and the model from the viewpoint of the weak gravity conjecture.