How does SU(N) -natural inflation isotropize the Universe?

Abstract

We study the homogeneous and anisotropic dynamics of pseudoscalar inflation coupled to an $\mathrm{SU}(N)$ gauge field. To see how the initially anisotropic universe is isotropized in such an inflation model, we derive the equations to obtain axisymmetric $\mathrm{SU}(N)$ gauge field configurations in Bianchi type-I geometry and discuss a method to identify their isotropic subsets which are the candidates of their late-time attractor. Each isotropic solution is characterized by the corresponding SU(2) subalgebra of the $\mathrm{SU}(N)$ algebra. It is shown numerically that the isotropic universe is a universal late-time attractor in the case of the SU(3) gauge field. Interestingly, we find that a transition between the two distinct gauge-field configurations characterized by different SU(2) subalgebras can occur during inflation. We clarify the conditions for this to occur. This transition could leave an observable imprint on the cosmic microwave background and the primordial gravitational wave background.