Reliable computation of B-mode polarization in Gauss–Bonnet and Einstein–Cartan backgrounds

Develop a robust perturbation framework to compute CMB B-mode polarization in inflation driven by scalar–Gauss–Bonnet couplings and Einstein–Cartan gravity, ensuring consistent treatment of tensor propagation speed, anisotropic stress, and possible extra degrees of freedom.

Background

The review notes that extensions such as scalar–Gauss–Bonnet couplings and Einstein–Cartan torsion modify tensor dynamics and can affect B-modes, but a complete and reliable calculation framework is still lacking.

Accurate predictions are essential for interpreting B-mode searches (LiteBIRD, CMB-S4) and for testing the microphysics of the inflationary sector.

References

In modified gravity, scalar and tensor perturbation theory is often underdeveloped. Some open questions: Can we compute B-mode polarization reliably in Gauss–Bonnet and Einstein–Cartan backgrounds?

— From geometry to cosmology: a pedagogical review of inflation in curvature, torsion, and extended gravity theories (2509.14306 - Momeni, 17 Sep 2025) in Section: Open Problems and Future Directions in Inflationary Modified Gravity (Perturbations and Observables: What Needs Improvement)