Perturbations in Chromo-Natural Inflation

Abstract

Chromo-Natural Inflation is the first worked example of a model of inflation in which slow-roll inflation is achieved by "magnetic drift" as opposed to Hubble friction.In this work, we give an account of the perturbations at linear order in this model. Our analysis uncovers two novel phenomena. First, the amplitude of scalar curvature perturbations is not directly tied to the shape of the inflationary potential. This allows the theory to violate naïve formulations of the Lyth bound. Second, the tensor sector of the theory is significantly altered from the usual case: the non-Abelian gauge field perturbations have a tensor degree of freedom. One chirality of the this tensor can be exponentially enhanced by a temporary instability near horizon crossing; this chiral instability exists because of the classical gauge field background, which violates parity. These tensor fluctuations of the gauge field also couple to gravitational waves at linear order in perturbation theory and source a chiral spectrum of gravitational waves. This spectrum can be exponentially enhanced over the usual inflationary spectrum due to the instability in the gauge sector.These new features cause the theory in its present form to be in significant tension with current observational data. This is because the new scalar physics leads to a significant reddening of the spectral tilt in the same region of parameter space where the exponential enhancement of the gravitational wave amplitude is small enough to satisfy current constraints on the tensor-to-scalar index. Hence, the model either predicts a spectral tilt that is too red, or it overproduces gravitational waves, or both.

Inflation and String Theory

“…Taken at face value, the original model (5.293) is in conflict with the CMB data [820]: it either predicts a spectral tilt that is too red, overproduces gravitational waves, or both. Nevertheless, it remains interesting to explore whether large Chern-Simons couplings can arise in consistent string compactifications and if models with viable phenomenology can be constructed.…”

Gravitational leptogenesis, reheating, and models of neutrino mass

“…Unfortunately, it has been recently shown that this mechanism does not generate a sufficient lepton asymmetry without spoiling inflation [28]. Other, more promising examples are inflationary scenarios that contain SU(2) gauge fields with classical vacuum expectation values, such as gauge-flation [29][30][31] and its variants [32,33], chromo-natural inflation (CNI) [34][35][36][37] and its variants [38][39][40], and models that include spectator chromo-natural-like sectors [41][42][43][44]. Gravitational leptogenesis has been studied within the context of these SU(2) models [45][46][47]; however, these works focused on the UV modes and suffer from similar criticisms regarding regularization and renormalization as the original proposal.…”

de Sitter symmetries and inflationary correlators in parity violating scalar-vector models

“…A natural option of the above modifications, is to include vector fields in the inflationary dynamics, because they naturally could be responsible of several interesting possibilities such as breaking of statistical isotropy, parity violating patterns, the origin of primordial magnetic fields, or even support an inflationary period, among others 1 (see for instance [27][28][29] and references therein). These new features are usually confronted with observations because they can become in an interesting discriminating tests of the proposed models [23,24,30,31].…”