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)