Parity violation in the Cosmic Microwave Background from a pseudoscalar inflaton

TL;DR

The paper demonstrates that a pseudoscalar inflaton with the coupling $\phi F_{\mu\nu}\tilde{F}^{\mu\nu}/f$ naturally generates helical gauge-field fluctuations that source chiral gravitational waves, producing parity-violating TB and EB correlations in the CMB. It derives the gauge-field amplification, shows the tensor spectra satisfy ${\cal P}^{t,+} \neq {\cal P}^{t,-}$ with a large exponential dependence on $\xi$, and quantifies the resulting chirality $\\Delta\chi$, while revealing stringent non-Gaussianity constraints that single-field realizations cannot satisfy for detectable parity. The work then presents two viable extensions—(i) a curvaton-dominated scalar sector and (ii) many gauge fields—where parity-violating CMB signals remain detectable within current bounds. This framework provides a first mechanism to imprint parity violation in the CMB without modifying gravity, offering concrete targets for upcoming CMB polarization experiments.

Abstract

If the inflaton is a pseudoscalar, then it naturally interacts with gauge fields via an axion-like coupling to $F_{μν} \tilde{F}^{μν}$. Through this coupling, the rolling inflaton produces quanta of the gauge field, that in their turn source the tensor components of the metric perturbations. Due to the parity-violating nature of the system, the right- and the left-handed tensor modes have different amplitudes. Such an asymmetry manifests itself in the form of non-vanishing TB and EB correlation functions in the Cosmic Microwave Background (CMB). We compute the amplitude of the parity-violating tensor modes and we discuss two scenarios, consistent with the current data, where parity-violating CMB correlation functions will be detectable in future experiments.

Figures (1)

• Figure 1: Parameter space for parity violating tensor modes. The choice of parameters in the plot corresponds to the case where 90% of the scalar power spectrum is provided by a curvaton (i.e. $\delta=0.1$, see section 4.1 for details). In the region to the left of the thicker line, marked by $\Delta\chi<\Delta\chi_{min}$, the effect is too small to be observed at the 95% confidence level in a cosmic-variance limited experiments. In the region to the right of the thinner line, marked by $r>0.24$, the amplitude of the tensor modes exceeds the current limits. The region between the two lines and below $H\simeq 5\times 10^{-5}\,M_P$ is allowed by current data and leads to observable parity violation in the CMB.