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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.

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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)