Entropy and isocurvature dynamics in multi-field and scalar–torsion models

Characterize the generation and evolution of entropy and isocurvature perturbations in multi-field modified gravity inflation and scalar–torsion coupled theories, and determine their observational imprints and constraints.

Background

Multi-field dynamics and torsion couplings can source isocurvature modes that are tightly constrained by CMB observations. The review highlights the lack of a comprehensive treatment within several modified gravity settings.

A complete framework is needed to assess viability, forecast signatures, and potentially use isocurvature as a discriminator among models.

References

In modified gravity, scalar and tensor perturbation theory is often underdeveloped. Some open questions: How do entropy and isocurvature modes behave in multi-field or scalar–torsion models?

— 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)

In fact, it is far from clear how the geometric anisotropy of the extrinsic geometry can manifest itself in the form of physical anisotropies of the inflaton field, or indeed whether it does so at all.

— How Inflation Might Explain Large-Scale Anisotropy  (2608.19765 - McInnes, 20 Aug 2026) in Section 3, immediately following equation (OOO)