Circularly polarized gravitational waves from parity-violating scalar-tensor theory

We study both primordial GWs and scalar-induced gravitational waves (SIGWs) in a class of the parity-violating scalar-tensor (PVST) theory, of which the Lagrangian is the linear combination of seven ghost-free parity-violating scalar-tensor monomials dubbed the ``Qi-Xiu’’ Lagrangians. At linear order, we obtain the quadratic action for tensor perturbations and show that parity-violating terms associated with $\mathcal{L}_{1,2,5,6,7}$ render the tensor propagation polarization dependent, leading to chiral primordial spectra and a nonvanishing degree of circular polarization. At second order, we derive the EOM for SIGWs and identify the explicit parity-violating source terms. In particular, $\mathcal{L}_3$ and $\mathcal{L}_4$ enter exclusively through the source term for SIGWs, allowing parity violation to arise even when the linear GWs propagation remains effectively GR-like. During the radiation-dominated era, we compute the fractional energy density of SIGWs for both monochromatic and lognormal curvature power spectra. We find that, around the peak frequency, SIGWs in PVST gravity exhibit characteristic deviations from those in GR, resulting in a nonzero degree of circular polarization.

💡 Research Summary

The paper investigates both primordial gravitational waves (GWs) generated during inflation and scalar‑induced gravitational waves (SIGWs) produced at second order in a parity‑violating scalar‑tensor (PVST) theory. The PVST model considered is built from the seven ghost‑free parity‑violating monomials identified in the “Qi‑Xiu” classification. These monomials, denoted L₁ through L₇, involve various combinations of the scalar field φ, its second derivatives, and curvature tensors. Their total derivative count d (either 3 or 4) determines the energy dimension of each term.

Linear tensor sector (primordial GWs)
The authors first expand the action to quadratic order in the tensor perturbation γ_{ij}. The parity‑conserving part reproduces the usual GR term, while the parity‑violating part introduces two time‑dependent coefficients, c₁(η) and c₂(η), which are linear combinations of the coupling functions b₁…b₇, the background scalar velocity φ′, and the Hubble parameter H. These coefficients generate a helicity‑dependent modification of the wave equation: \